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                                     350 BC

                            ON THE PARTS OF ANIMALS

                                  by Aristotle

                           translated by William Ogle

                              Book I

                                 1

  EVERY systematic science, the humblest and the noblest alike,
seems to admit of two distinct kinds of proficiency; one of which
may be properly called scientific knowledge of the subject, while
the other is a kind of educational acquaintance with it. For an
educated man should be able to form a fair off-hand judgement as to
the goodness or badness of the method used by a professor in his
exposition. To be educated is in fact to be able to do this; and
even the man of universal education we deem to be such in virtue of
his having this ability. It will, however, of course, be understood
that we only ascribe universal education to one who in his own
individual person is thus critical in all or nearly all branches of
knowledge, and not to one who has a like ability merely in some
special subject. For it is possible for a man to have this
competence in some one branch of knowledge without having it in all.

  It is plain then that, as in other sciences, so in that which
inquires into nature, there must be certain canons, by reference to
which a hearer shall be able to criticize the method of a professed
exposition, quite independently of the question whether the statements
made be true or false. Ought we, for instance (to give an illustration
of what I mean), to begin by discussing each separate species-man,
lion, ox, and the like-taking each kind in hand inde. pendently of the
rest, or ought we rather to deal first with the attributes which
they have in common in virtue of some common element of their
nature, and proceed from this as a basis for the consideration of them
separately? For genera that are quite distinct yet oftentimes
present many identical phenomena, sleep, for instance, respiration,
growth, decay, death, and other similar affections and conditions,
which may be passed over for the present, as we are not yet prepared
to treat of them with clearness and precision. Now it is plain that if
we deal with each species independently of the rest, we shall
frequently be obliged to repeat the same statements over and over
again; for horse and dog and man present, each and all, every one of
the phenomena just enumerated. A discussion therefore of the
attributes of each such species separately would necessarily involve
frequent repetitions as to characters, themselves identical but
recurring in animals specifically distinct. (Very possibly also
there may be other characters which, though they present specific
differences, yet come under one and the same category. For instance,
flying, swimming, walking, creeping, are plainly specifically
distinct, but yet are all forms of animal progression.) We must, then,
have some clear understanding as to the manner in which our
investigation is to be conducted; whether, I mean, we are first to
deal with the common or generic characters, and afterwards to take
into consideration special peculiarities; or whether we are to start
straight off with the ultimate species. For as yet no definite rule
has been laid down in this matter. So also there is a like uncertainty
as to another point now to be mentioned. Ought the writer who deals
with the works of nature to follow the plan adopted by the
mathematicians in their astronomical demonstrations, and after
considering the phenomena presented by animals, and their several
parts, proceed subsequently to treat of the causes and the reason why;
or ought he to follow some other method? And when these questions
are answered, there yet remains another. The causes concerned in the
generation of the works of nature are, as we see, more than one. There
is the final cause and there is the motor cause. Now we must decide
which of these two causes comes first, which second. Plainly, however,
that cause is the first which we call the final one. For this is the
Reason, and the Reason forms the starting-point, alike in the works of
art and in works of nature. For consider how the physician or how
the builder sets about his work. He starts by forming for himself a
definite picture, in the one case perceptible to mind, in the other to
sense, of his end-the physician of health, the builder of a
house-and this he holds forward as the reason and explanation of
each subsequent step that he takes, and of his acting in this or
that way as the case may be. Now in the works of nature the good end
and the final cause is still more dominant than in works of art such
as these, nor is necessity a factor with the same significance in them
all; though almost all writers, while they try to refer their origin
to this cause, do so without distinguishing the various senses in
which the term necessity is used. For there is absolute necessity,
manifested in eternal phenomena; and there is hypothetical
necessity, manifested in everything that is generated by nature as
in everything that is produced by art, be it a house or what it may.
For if a house or other such final object is to be realized, it is
necessary that such and such material shall exist; and it is necessary
that first this then that shall be produced, and first this and then
that set in motion, and so on in continuous succession, until the
end and final result is reached, for the sake of which each prior
thing is produced and exists. As with these productions of art, so
also is it with the productions of nature. The mode of necessity,
however, and the mode of ratiocination are different in natural
science from what they are in the theoretical sciences; of which we
have spoken elsewhere. For in the latter the starting-point is that
which is; in the former that which is to be. For it is that which is
yet to be-health, let us say, or a man-that, owing to its being of
such and such characters, necessitates the pre-existence or previous
production of this and that antecedent; and not this or that
antecedent which, because it exists or has been generated, makes it
necessary that health or a man is in, or shall come into, existence.
Nor is it possible to track back the series of necessary antecedents
to a starting-point, of which you can say that, existing itself from
eternity, it has determined their existence as its consequent. These
however again, are matters that have been dealt with in another
treatise. There too it was stated in what cases absolute and
hypothetical necessity exist; in what cases also the proposition
expressing hypothetical necessity is simply convertible, and what
cause it is that determines this convertibility.

  Another matter which must not be passed over without consideration
is, whether the proper subject of our exposition is that with which
the ancient writers concerned themselves, namely, what is the
process of formation of each animal; or whether it is not rather, what
are the characters of a given creature when formed. For there is no
small difference between these two views. The best course appears to
be that we should follow the method already mentioned, and begin
with the phenomena presented by each group of animals, and, when
this is done, proceed afterwards to state the causes of those
phenomena, and to deal with their evolution. For elsewhere, as for
instance in house building, this is the true sequence. The plan of the
house, or the house, has this and that form; and because it has this
and that form, therefore is its construction carried out in this or
that manner. For the process of evolution is for the sake of the thing
Anally evolved, and not this for the sake of the process.
Empedocles, then, was in error when he said that many of the
characters presented by animals were merely the results of
incidental occurrences during their development; for instance, that
the backbone was divided as it is into vertebrae, because it
happened to be broken owing to the contorted position of the foetus in
the womb. In so saying he overlooked the fact that propagation implies
a creative seed endowed with certain formative properties. Secondly,
he neglected another fact, namely, that the parent animal
pre-exists, not only in idea, but actually in time. For man is
generated from man; and thus it is the possession of certain
characters by the parent that determines the development of like
characters in the child. The same statement holds good also for the
operations of art, and even for those which are apparently
spontaneous. For the same result as is produced by art may occur
spontaneously. Spontaneity, for instance, may bring about the
restoration of health. The products of art, however, require the
pre-existence of an efficient cause homogeneous with themselves,
such as the statuary's art, which must necessarily precede the statue;
for this cannot possibly be produced spontaneously. Art indeed
consists in the conception of the result to be produced before its
realization in the material. As with spontaneity, so with chance;
for this also produces the same result as art, and by the same
process.

  The fittest mode, then, of treatment is to say, a man has such and
such parts, because the conception of a man includes their presence,
and because they are necessary conditions of his existence, or, if
we cannot quite say this, which would be best of all, then the next
thing to it, namely, that it is either quite impossible for him to
exist without them, or, at any rate, that it is better for him that
they should be there; and their existence involves the existence of
other antecedents. Thus we should say, because man is an animal with
such and such characters, therefore is the process of his
development necessarily such as it is; and therefore is it
accomplished in such and such an order, this part being formed
first, that next, and so on in succession; and after a like fashion
should we explain the evolution of all other works of nature.

  Now that with which the ancient writers, who first philosophized
about Nature, busied themselves, was the material principle and the
material cause. They inquired what this is, and what its character;
how the universe is generated out of it, and by what motor
influence, whether, for instance, by antagonism or friendship, whether
by intelligence or spontaneous action, the substratum of matter
being assumed to have certain inseparable properties; fire, for
instance, to have a hot nature, earth a cold one; the former to be
light, the latter heavy. For even the genesis of the universe is
thus explained by them. After a like fashion do they deal also with
the development of plants and of animals. They say, for instance, that
the water contained in the body causes by its currents the formation
of the stomach and the other receptacles of food or of excretion;
and that the breath by its passage breaks open the outlets of the
nostrils; air and water being the materials of which bodies are
made; for all represent nature as composed of such or similar
substances.

  But if men and animals and their several parts are natural
phenomena, then the natural philosopher must take into consideration
not merely the ultimate substances of which they are made, but also
flesh, bone, blood, and all other homogeneous parts; not only these,
but also the heterogeneous parts, such as face, hand, foot; and must
examine how each of these comes to be what it is, and in virtue of
what force. For to say what are the ultimate substances out of which
an animal is formed, to state, for instance, that it is made of fire
or earth, is no more sufficient than would be a similar account in the
case of a couch or the like. For we should not be content with
saying that the couch was made of bronze or wood or whatever it
might be, but should try to describe its design or mode of composition
in preference to the material; or, if we did deal with the material,
it would at any rate be with the concretion of material and form.
For a couch is such and such a form embodied in this or that matter,
or such and such a matter with this or that form; so that its shape
and structure must be included in our description. For the formal
nature is of greater importance than the material nature.

  Does, then, configuration and colour constitute the essence of the
various animals and of their several parts? For if so, what Democritus
says will be strictly correct. For such appears to have been his
notion. At any rate he says that it is evident to every one what
form it is that makes the man, seeing that he is recognizable by his
shape and colour. And yet a dead body has exactly the same
configuration as a living one; but for all that is not a man. So
also no hand of bronze or wood or constituted in any but the
appropriate way can possibly be a hand in more than name. For like a
physician in a painting, or like a flute in a sculpture, in spite of
its name it will be unable to do the office which that name implies.
Precisely in the same way no part of a dead body, such I mean as its
eye or its hand, is really an eye or a hand. To say, then, that
shape and colour constitute the animal is an inadequate statement, and
is much the same as if a woodcarver were to insist that the hand he
had cut out was really a hand. Yet the physiologists, when they give
an account of the development and causes of the animal form, speak
very much like such a craftsman. What, however, I would ask, are the
forces by which the hand or the body was fashioned into its shape? The
woodcarver will perhaps say, by the axe or the auger; the
physiologist, by air and by earth. Of these two answers the
artificer's is the better, but it is nevertheless insufficient. For it
is not enough for him to say that by the stroke of his tool this
part was formed into a concavity, that into a flat surface; but he
must state the reasons why he struck his blow in such a way as to
effect this, and what his final object was; namely, that the piece
of wood should develop eventually into this or that shape. It is
plain, then, that the teaching of the old physiologists is inadequate,
and that the true method is to state what the definitive characters
are that distinguish the animal as a whole; to explain what it is both
in substance and in form, and to deal after the same fashion with
its several organs; in fact, to proceed in exactly the same way as
we should do, were we giving a complete description of a couch.

  If now this something that constitutes the form of the living
being be the soul, or part of the soul, or something that without
the soul cannot exist; as would seem to be the case, seeing at any
rate that when the soul departs, what is left is no longer a living
animal, and that none of the parts remain what they were before,
excepting in mere configuration, like the animals that in the fable
are turned into stone; if, I say, this be so, then it will come within
the province of the natural philosopher to inform himself concerning
the soul, and to treat of it, either in its entirety, or, at any rate,
of that part of it which constitutes the essential character of an
animal; and it will be his duty to say what this soul or this part
of a soul is; and to discuss the attributes that attach to this
essential character, especially as nature is spoken of in two
senses, and the nature of a thing is either its matter or its essence;
nature as essence including both the motor cause and the final
cause. Now it is in the latter of these two senses that either the
whole soul or some part of it constitutes the nature of an animal; and
inasmuch as it is the presence of the soul that enables matter to
constitute the animal nature, much more than it is the presence of
matter which so enables the soul, the inquirer into nature is bound on
every ground to treat of the soul rather than of the matter. For
though the wood of which they are made constitutes the couch and the
tripod, it only does so because it is capable of receiving such and
such a form.

  What has been said suggests the question, whether it is the whole
soul or only some part of it, the consideration of which comes
within the province of natural science. Now if it be of the whole soul
that this should treat, then there is no place for any other
philosophy beside it. For as it belongs in all cases to one and the
same science to deal with correlated subjects-one and the same
science, for instance, deals with sensation and with the objects of
sense-and as therefore the intelligent soul and the objects of
intellect, being correlated, must belong to one and the same
science, it follows that natural science will have to include the
whole universe in its province. But perhaps it is not the whole
soul, nor all its parts collectively, that constitutes the source of
motion; but there may be one part, identical with that in plants,
which is the source of growth, another, namely the sensory part, which
is the source of change of quality, while still another, and this
not the intellectual part, is the source of locomotion. I say not
the intellectual part; for other animals than man have the power of
locomotion, but in none but him is there intellect. Thus then it is
plain that it is not of the whole soul that we have to treat. For it
is not the whole soul that constitutes the animal nature, but only
some part or parts of it. Moreover, it is impossible that any
abstraction can form a subject of natural science, seeing that
everything that Nature makes is means to an end. For just as human
creations are the products of art, so living objects are manifest in
the products of an analogous cause or principle, not external but
internal, derived like the hot and the cold from the environing
universe. And that the heaven, if it had an origin, was evolved and is
maintained by such a cause, there is therefore even more reason to
believe, than that mortal animals so originated. For order and
definiteness are much more plainly manifest in the celestial bodies
than in our own frame; while change and chance are characteristic of
the perishable things of earth. Yet there are some who, while they
allow that every animal exists and was generated by nature,
nevertheless hold that the heaven was constructed to be what it is
by chance and spontaneity; the heaven, in which not the faintest
sign of haphazard or of disorder is discernible! Again, whenever there
is plainly some final end, to which a motion tends should nothing
stand in the way, we always say that such final end is the aim or
purpose of the motion; and from this it is evident that there must
be a something or other really existing, corresponding to what we call
by the name of Nature. For a given germ does not give rise to any
chance living being, nor spring from any chance one; but each germ
springs from a definite parent and gives rise to a definite progeny.
And thus it is the germ that is the ruling influence and fabricator of
the offspring. For these it is by nature, the offspring being at any
rate that which in nature will spring from it. At the same time the
offspring is anterior to the germ; for germ and perfected progeny
are related as the developmental process and the result. Anterior,
however, to both germ and product is the organism from which the
germ was derived. For every germ implies two organisms, the parent and
the progeny. For germ or seed is both the seed of the organism from
which it came, of the horse, for instance, from which it was
derived, and the seed of the organism that will eventually arise
from it, of the mule, for example, which is developed from the seed of
the horse. The same seed then is the seed both of the horse and of the
mule, though in different ways as here set forth. Moreover, the seed
is potentially that which will spring from it, and the relation of
potentiality to actuality we know.

  There are then two causes, namely, necessity and the final end.
For many things are produced, simply as the results of necessity. It
may, however, be asked, of what mode of necessity are we speaking when
we say this. For it can be of neither of those two modes which are set
forth in the philosophical treatises. There is, however, the third
mode, in such things at any rate as are generated. For instance, we
say that food is necessary; because an animal cannot possibly do
without it. This third mode is what may be called hypothetical
necessity. Here is another example of it. If a piece of wood is to
be split with an axe, the axe must of necessity be hard; and, if hard,
must of necessity be made of bronze or iron. Now exactly in the same
way the body, which like the axe is an instrument-for both the body as
a whole and its several parts individually have definite operations
for which they are made-just in the same way, I say, the body, if it
is to do its work, must of necessity be of such and such a
character, and made of such and such materials.

  It is plain then that there are two modes of causation, and that
both of these must, so far as possible, be taken into account in
explaining the works of nature, or that at any rate an attempt must be
made to include them both; and that those who fail in this tell us
in reality nothing about nature. For primary cause constitutes the
nature of an animal much more than does its matter. There are indeed
passages in which even Empedocles hits upon this, and following the
guidance of fact, finds himself constrained to speak of the ratio
(olugos) as constituting the essence and real nature of things.
Such, for instance, is the case when he explains what is a bone. For
he does not merely describe its material, and say it is this one
element, or those two or three elements, or a compound of all the
elements, but states the ratio (olugos) of their combination. As
with a bone, so manifestly is it with the flesh and all other
similar parts.

  The reason why our predecessors failed in hitting upon this method
of treatment was, that they were not in possession of the notion of
essence, nor of any definition of substance. The first who came near
it was Democritus, and he was far from adopting it as a necessary
method in natural science, but was merely brought to it, spite of
himself, by constraint of facts. In the time of Socrates a nearer
approach was made to the method. But at this period men gave up
inquiring into the works of nature, and philosophers diverted their
attention to political science and to the virtues which benefit
mankind.

  Of the method itself the following is an example. In dealing with
respiration we must show that it takes place for such or such a
final object; and we must also show that this and that part of the
process is necessitated by this and that other stage of it. By
necessity we shall sometimes mean hypothetical necessity, the
necessity, that is, that the requisite antecedants shall be there,
if the final end is to be reached; and sometimes absolute necessity,
such necessity as that which connects substances and their inherent
properties and characters. For the alternate discharge and re-entrance
of heat and the inflow of air are necessary if we are to live. Here we
have at once a necessity in the former of the two senses. But the
alternation of heat and refrigeration produces of necessity an
alternate admission and discharge of the outer air, and this is a
necessity of the second kind.

  In the foregoing we have an example of the method which we must
adopt, and also an example of the kind of phenomena, the causes of
which we have to investigate.

                                 2

  Some writers propose to reach the definitions of the ultimate
forms of animal life by bipartite division. But this method is often
difficult, and often impracticable.

  Sometimes the final differentia of the subdivision is sufficient
by itself, and the antecedent differentiae are mere surplusage. Thus
in the series Footed, Two-footed, Cleft-footed, the last term is
all-expressive by itself, and to append the higher terms is only an
idle iteration. Again it is not permissible to break up a natural
group, Birds for instance, by putting its members under different
bifurcations, as is done in the published dichotomies, where some
birds are ranked with animals of the water, and others placed in a
different class. The group Birds and the group Fishes happen to be
named, while other natural groups have no popular names; for instance,
the groups that we may call Sanguineous and Bloodless are not known
popularly by any designations. If such natural groups are not to be
broken up, the method of Dichotomy cannot be employed, for it
necessarily involves such breaking up and dislocation. The group of
the Many-footed, for instance, would, under this method, have to be
dismembered, and some of its kinds distributed among land animals,
others among water animals.

                                 3

  Again, privative terms inevitably form one branch of dichotomous
division, as we see in the proposed dichotomies. But privative terms
in their character of privatives admit of no subdivision. For there
can be no specific forms of a negation, of Featherless for instance or
of Footless, as there are of Feathered and of Footed. Yet a generic
differentia must be subdivisible; for otherwise what is there that
makes it generic rather than specific? There are to be found
generic, that is specifically subdivisible, differentiae; Feathered
for instance and Footed. For feathers are divisible into Barbed and
Unbarbed, and feet into Manycleft, and Twocleft, like those of animals
with bifid hoofs, and Uncleft or Undivided, like those of animals with
solid hoofs. Now even with differentiae capable of this specific
subdivision it is difficult enough so to make the classification, as
that each animal shall be comprehended in some one subdivision and
in not more than one; but far more difficult, nay impossible, is it to
do this, if we start with a dichotomy into two contradictories.
(Suppose for instance we start with the two contradictories, Feathered
and Unfeathered; we shall find that the ant, the glow-worm, and some
other animals fall under both divisions.) For each differentia must be
presented by some species. There must be some species, therefore,
under the privative heading. Now specifically distinct animals
cannot present in their essence a common undifferentiated element, but
any apparently common element must really be differentiated. (Bird and
Man for instance are both Two-footed, but their two-footedness is
diverse and differentiated. So any two sanguineous groups must have
some difference in their blood, if their blood is part of their
essence.) From this it follows that a privative term, being
insusceptible of differentiation, cannot be a generic differentia;
for, if it were, there would be a common undifferentiated element in
two different groups.

  Again, if the species are ultimate indivisible groups, that is,
are groups with indivisible differentiae, and if no differentia be
common to several groups, the number of differentiae must be equal
to the number of species. If a differentia though not divisible
could yet be common to several groups, then it is plain that in virtue
of that common differentia specifically distinct animals would fall
into the same division. It is necessary then, if the differentiae,
under which are ranged all the ultimate and indivisible groups, are
specific characters, that none of them shall be common; for otherwise,
as already said, specifically distinct animals will come into one
and the same division. But this would violate one of the requisite
conditions, which are as follows. No ultimate group must be included
in more than a single division; different groups must not be
included in the same division; and every group must be found in some
division. It is plain then that we cannot get at the ultimate specific
forms of the animal, or any other, kingdom by bifurcate division. If
we could, the number of ultimate differentiae would equal the number
of ultimate animal forms. For assume an order of beings whose prime
differentiae are White and Black. Each of these branches will
bifurcate, and their branches again, and so on till we reach the
ultimate differentiae, whose number will be four or some other power
of two, and will also be the number of the ultimate species
comprehended in the order.

  (A species is constituted by the combination differentia and matter.
For no part of an animal is purely material or purely immaterial;
nor can a body, independently of its condition, constitute an animal
or any of its parts, as has repeatedly been observed.)

  Further, the differentiae must be elements of the essence, and not
merely essential attributes. Thus if Figure is the term to be divided,
it must not be divided into figures whose angles are equal to two
right angles, and figures whose angles are together greater than two
right angles. For it is only an attribute of a triangle and not part
of its essence that its angles are equal to two right angles.

  Again, the bifurcations must be opposites, like White and Black,
Straight and Bent; and if we characterize one branch by either term,
we must characterize the other by its opposite, and not, for
example, characterize one branch by a colour, the other by a mode of
progression, swimming for instance.

  Furthermore, living beings cannot be divided by the functions common
to body and soul, by Flying, for instance, and Walking, as we see them
divided in the dichotomies already referred to. For some groups,
Ants for instance, fall under both divisions, some ants flying while
others do not. Similarly as regards the division into Wild and Tame;
for it also would involve the disruption of a species into different
groups. For in almost all species in which some members are tame,
there are other members that are wild. Such, for example, is the
case with Men, Horses, Oxen, Dogs in India, Pigs, Goats, Sheep; groups
which, if double, ought to have what they have not, namely,
different appellations; and which, if single, prove that Wildness
and Tameness do not amount to specific differences. And whatever
single element we take as a basis of division the same difficulty will
occur.

  The method then that we must adopt is to attempt to recognize the
natural groups, following the indications afforded by the instincts of
mankind, which led them for instance to form the class of Birds and
the class of Fishes, each of which groups combines a multitude of
differentiae, and is not defined by a single one as in dichotomy.
The method of dichotomy is either impossible (for it would put a
single group under different divisions or contrary groups under the
same division), or it only furnishes a single ultimate differentia for
each species, which either alone or with its series of antecedents has
to constitute the ultimate species.

  If, again, a new differential character be introduced at any stage
into the division, the necessary result is that the continuity of
the division becomes merely a unity and continuity of agglomeration,
like the unity and continuity of a series of sentences coupled
together by conjunctive particles. For instance, suppose we have the
bifurcation Feathered and Featherless, and then divide Feathered
into Wild and Tame, or into White and Black. Tame and White are not
a differentiation of Feathered, but are the commencement of an
independent bifurcation, and are foreign to the series at the end of
which they are introduced.

  As we said then, we must define at the outset by multiplicity of
differentiae. If we do so, privative terms will be available, which
are unavailable to the dichotomist.

  The impossibility of reaching the definition of any of the
ultimate forms by dichotomy of the larger group, as some propose, is
manifest also from the following considerations. It is impossible that
a single differentia, either by itself or with its antecedents,
shall express the whole essence of a species. (In saying a single
differentia by itself I mean such an isolated differentia as
Cleft-footed; in saying a single differentia with antecedent I mean,
to give an instance, Manycleft-footed preceded by Cleft-footed. The
very continuity of a series of successive differentiae in a division
is intended to show that it is their combination that expresses the
character of the resulting unit, or ultimate group. But one is
misled by the usages of language into imagining that it is merely
the final term of the series, Manycleft-footed for instance, that
constitutes the whole differentia, and that the antecedent terms,
Footed, Cleft-footed, are superfluous. Now it is evident that such a
series cannot consist of many terms. For if one divides and
subdivides, one soon reaches the final differential term, but for
all that will not have got to the ultimate division, that is, to the
species.) No single differentia, I repeat, either by itself or with
its antecedents, can possibly express the essence of a species.
Suppose, for example, Man to be the animal to be defined; the single
differentia will be Cleft-footed, either by itself or with its
antecedents, Footed and Two-footed. Now if man was nothing more than a
Cleft-footed animal, this single differentia would duly represent
his essence. But seeing that this is not the case, more differentiae
than this one will necessarily be required to define him; and these
cannot come under one division; for each single branch of a
dichotomy ends in a single differentia, and cannot possibly include
several differentiae belonging to one and the same animal.

  It is impossible then to reach any of the ultimate animal forms by
dichotomous division.

                                 4

  It deserves inquiry why a single name denoting a higher group was
not invented by mankind, as an appellation to comprehend the two
groups of Water animals and Winged animals. For even these have
certain attributes in common. However, the present nomenclature is
just. Groups that only differ in degree, and in the more or less of an
identical element that they possess, are aggregated under a single
class; groups whose attributes are not identical but analogous are
separated. For instance, bird differs from bird by gradation, or by
excess and defect; some birds have long feathers, others short ones,
but all are feathered. Bird and Fish are more remote and only agree in
having analogous organs; for what in the bird is feather, in the
fish is scale. Such analogies can scarcely, however, serve universally
as indications for the formation of groups, for almost all animals
present analogies in their corresponding parts.

  The individuals comprised within a species, such as Socrates and
Coriscus, are the real existences; but inasmuch as these individuals
possess one common specific form, it will suffice to state the
universal attributes of the species, that is, the attributes common to
all its individuals, once for all, as otherwise there will be
endless reiteration, as has already been pointed out.

  But as regards the larger groups-such as Birds-which comprehend many
species, there may be a question. For on the one hand it may be
urged that as the ultimate species represent the real existences, it
will be well, if practicable, to examine these ultimate species
separately, just as we examine the species Man separately; to examine,
that is, not the whole class Birds collectively, but the Ostrich,
the Crane, and the other indivisible groups or species belonging to
the class.

  On the other hand, however, this course would involve repeated
mention of the same attribute, as the same attribute is common to many
species, and so far would be somewhat irrational and tedious. Perhaps,
then, it will be best to treat generically the universal attributes of
the groups that have a common nature and contain closely allied
subordinate forms, whether they are groups recognized by a true
instinct of mankind, such as Birds and Fishes, or groups not popularly
known by a common appellation, but withal composed of closely allied
subordinate groups; and only to deal individually with the
attributes of a single species, when such species, man, for
instance, and any other such, if such there be-stands apart from
others, and does not constitute with them a larger natural group.

  It is generally similarity in the shape of particular organs, or
of the whole body, that has determined the formation of the larger
groups. It is in virtue of such a similarity that Birds, Fishes,
Cephalopoda, and Testacea have been made to form each a separate
class. For within the limits of each such class, the parts do not
differ in that they have no nearer resemblance than that of
analogy-such as exists between the bone of man and the spine of
fish-but differ merely in respect of such corporeal conditions as
largeness smallness, softness hardness, smoothness roughness, and
other similar oppositions, or, in one word, in respect of degree.

  We have now touched upon the canons for criticizing the method of
natural science, and have considered what is the most systematic and
easy course of investigation; we have also dealt with division, and
the mode of conducting it so as best to attain the ends of science,
and have shown why dichotomy is either impracticable or
inefficacious for its professed purposes.

  Having laid this foundation, let us pass on to our next topic.

                                 5

  Of things constituted by nature some are ungenerated,
imperishable, and eternal, while others are subject to generation
and decay. The former are excellent beyond compare and divine, but
less accessible to knowledge. The evidence that might throw light on
them, and on the problems which we long to solve respecting them, is
furnished but scantily by sensation; whereas respecting perishable
plants and animals we have abundant information, living as we do in
their midst, and ample data may be collected concerning all their
various kinds, if only we are willing to take sufficient pains. Both
departments, however, have their special charm. The scanty conceptions
to which we can attain of celestial things give us, from their
excellence, more pleasure than all our knowledge of the world in which
we live; just as a half glimpse of persons that we love is more
delightful than a leisurely view of other things, whatever their
number and dimensions. On the other hand, in certitude and in
completeness our knowledge of terrestrial things has the advantage.
Moreover, their greater nearness and affinity to us balances
somewhat the loftier interest of the heavenly things that are the
objects of the higher philosophy. Having already treated of the
celestial world, as far as our conjectures could reach, we proceed
to treat of animals, without omitting, to the best of our ability, any
member of the kingdom, however ignoble. For if some have no graces
to charm the sense, yet even these, by disclosing to intellectual
perception the artistic spirit that designed them, give immense
pleasure to all who can trace links of causation, and are inclined
to philosophy. Indeed, it would be strange if mimic representations of
them were attractive, because they disclose the mimetic skill of the
painter or sculptor, and the original realities themselves were not
more interesting, to all at any rate who have eyes to discern the
reasons that determined their formation. We therefore must not
recoil with childish aversion from the examination of the humbler
animals. Every realm of nature is marvellous: and as Heraclitus,
when the strangers who came to visit him found him warming himself
at the furnace in the kitchen and hesitated to go in, reported to have
bidden them not to be afraid to enter, as even in that kitchen
divinities were present, so we should venture on the study of every
kind of animal without distaste; for each and all will reveal to us
something natural and something beautiful. Absence of haphazard and
conduciveness of everything to an end are to be found in Nature's
works in the highest degree, and the resultant end of her
generations and combinations is a form of the beautiful.

  If any person thinks the examination of the rest of the animal
kingdom an unworthy task, he must hold in like disesteem the study
of man. For no one can look at the primordia of the human frame-blood,
flesh, bones, vessels, and the like-without much repugnance. Moreover,
when any one of the parts or structures, be it which it may, is
under discussion, it must not be supposed that it is its material
composition to which attention is being directed or which is the
object of the discussion, but the relation of such part to the total
form. Similarly, the true object of architecture is not bricks,
mortar, or timber, but the house; and so the principal object of
natural philosophy is not the material elements, but their
composition, and the totality of the form, independently of which they
have no existence.

  The course of exposition must be first to state the attributes
common to whole groups of animals, and then to attempt to give their
explanation. Many groups, as already noticed, present common
attributes, that is to say, in some cases absolutely identical
affections, and absolutely identical organs,-feet, feathers, scales,
and the like-while in other groups the affections and organs are
only so far identical as that they are analogous. For instance, some
groups have lungs, others have no lung, but an organ analogous to a
lung in its place; some have blood, others have no blood, but a
fluid analogous to blood, and with the same office. To treat of the
common attributes in connexion with each individual group would
involve, as already suggested, useless iteration. For many groups have
common attributes. So much for this topic.

  As every instrument and every bodily member subserves some partial
end, that is to say, some special action, so the whole body must be
destined to minister to some Plenary sphere of action. Thus the saw is
made for sawing, for sawing is a function, and not sawing for the saw.
Similarly, the body too must somehow or other be made for the soul,
and each part of it for some subordinate function, to which it is
adapted.

  We have, then, first to describe the common functions, common,
that is, to the whole animal kingdom, or to certain large groups, or
to the members of a species. In other words, we have to describe the
attributes common to all animals, or to assemblages, like the class of
Birds, of closely allied groups differentiated by gradation, or to
groups like Man not differentiated into subordinate groups. In the
first case the common attributes may be called analogous, in the
second generic, in the third specific.

  When a function is ancillary to another, a like relation
manifestly obtains between the organs which discharge these functions;
and similarly, if one function is prior to and the end of another,
their respective organs will stand to each other in the same relation.
Thirdly, the existence of these parts involves that of other things as
their necessary consequents.

  Instances of what I mean by functions and affections are
Reproduction, Growth, Copulation, Waking, Sleep, Locomotion, and other
similar vital actions. Instances of what I mean by parts are Nose,
Eye, Face, and other so-called members or limbs, and also the more
elementary parts of which these are made. So much for the method to be
pursued. Let us now try to set forth the causes of all vital
phenomena, whether universal or particular, and in so doing let us
follow that order of exposition which conforms, as we have
indicated, to the order of nature.

                              Book II

                                 1

  THE nature and the number of the parts of which animals are
severally composed are matters which have already been set forth in
detail in the book of Researches about Animals. We have now to inquire
what are the causes that in each case have determined this
composition, a subject quite distinct from that dealt with in the
Researches.

  Now there are three degrees of composition; and of these the first
in order, as all will allow, is composition out of what some call
the elements, such as earth, air, water, fire. Perhaps, however, it
would be more accurate to say composition out of the elementary
forces; nor indeed out of all of these, but out of a limited number of
them, as defined in previous treatises. For fluid and solid, hot and
cold, form the material of all composite bodies; and all other
differences are secondary to these, such differences, that is, as
heaviness or lightness, density or rarity, roughness or smoothness,
and any other such properties of matter as there may be. second degree
of composition is that by which the homogeneous parts of animals, such
as bone, flesh, and the like, are constituted out of the primary
substances. The third and last stage is the composition which forms
the heterogeneous parts, such as face, hand, and the rest.

  Now the order of actual development and the order of logical
existence are always the inverse of each other. For that which is
posterior in the order of development is antecedent in the order of
nature, and that is genetically last which in nature is first.

  (That this is so is manifest by induction; for a house does not
exist for the sake of bricks and stones, but these materials for the
sake of the house; and the same is the case with the materials of
other bodies. Nor is induction required to show this. it is included
in our conception of generation. For generation is a process from a
something to a something; that which is generated having a cause in
which it originates and a cause in which it ends. The originating
cause is the primary efficient cause, which is something already
endowed with tangible existence, while the final cause is some
definite form or similar end; for man generates man, and plant
generates plant, in each case out of the underlying material.)

  In order of time, then, the material and the generative process must
necessarily be anterior to the being that is generated; but in logical
order the definitive character and form of each being precedes the
material. This is evident if one only tries to define the process of
formation. For the definition of house-building includes and
presupposes that of the house; but the definition of the house does
not include nor presuppose that of house-building; and the same is
true of all other productions. So that it must necessarily be that the
elementary material exists for the sake of the homogeneous parts,
seeing that these are genetically posterior to it, just as the
heterogeneous parts are posterior genetically to them. For these
heterogeneous parts have reached the end and goal, having the third
degree of composition, in which degree generation or development often
attains its final term.

  Animals, then, are composed of homogeneous parts, and are also
composed of heterogeneous parts. The former, however, exist for the
sake of the latter. For the active functions and operations of the
body are carried on by these; that is, by the heterogeneous parts,
such as the eye, the nostril, the whole face, the fingers, the hand,
and the whole arm. But inasmuch as there is a great variety in the
functions and motions not only of aggregate animals but also of the
individual organs, it is necessary that the substances out of which
these are composed shall present a diversity of properties. For some
purposes softness is advantageous, for others hardness; some parts
must be capable of extension, others of flexion. Such properties,
then, are distributed separately to the different homogeneous parts,
one being soft another hard, one fluid another solid, one viscous
another brittle; whereas each of the heterogeneous parts presents a
combination of multifarious properties. For the hand, to take an
example, requires one property to enable it to effect pressure, and
another and different property for simple prehension. For this
reason the active or executive parts of the body are compounded out of
bones, sinews, flesh, and the like, but not these latter out of the
former.

  So far, then, as has yet been stated, the relations between these
two orders of parts are determined by a final cause. We have, however,
to inquire whether necessity may not also have a share in the
matter; and it must be admitted that these mutual relations could
not from the very beginning have possibly been other than they are.
For heterogeneous parts can be made up out of homogeneous parts,
either from a plurality of them, or from a single one, as is the
case with some of the viscera which, varying in configuration, are
yet, to speak broadly, formed from a single homogeneous substance; but
that homogeneous substances should be formed out of a combination of
heterogeneous parts is clearly an impossibility. For these causes,
then, some parts of animals are simple and homogeneous, while others
are composite and heterogeneous; and dividing the parts into the
active or executive and the sensitive, each one of the former is, as
before said, heterogeneous, and each one of the latter homogeneous.
For it is in homogeneous parts alone that sensation can occur, as
the following considerations show.

  Each sense is confined to a single order of sensibles, and its organ
must be such as to admit the action of that kind or order. But it is
only that which is endowed with a property in posse that is acted on
by that which has the like property in esse, so that the two are the
same in kind, and if the latter is single so also is the former.
Thus it is that while no physiologists ever dream of saying of the
hand or face or other such part that one is earth, another water,
another fire, they couple each separate sense-organ with a separate
element, asserting this one to be air and that other to be fire.

  Sensation, then, is confined to the simple or homogeneous parts.
But, as might reasonably be expected, the organ of touch, though still
homogeneous, is yet the least simple of all the sense-organs. For
touch more than any other sense appears to be correlated to several
distinct kinds of objects, and to recognize more than one category
of contrasts, heat and cold, for instance, solidity and fluidity,
and other similar oppositions. Accordingly, the organ which deals with
these varied objects is of all the sense-organs the most corporeal,
being either the flesh, or the substance which in some animals takes
the place of flesh.

  Now as there cannot possibly be an animal without sensation, it
follows as a necessary consequence that every animal must have some
homogeneous parts; for these alone are capable of sensation, the
heterogeneous parts serving for the active functions. Again, as the
sensory faculty, the motor faculty, and the nutritive faculty are
all lodged in one and the same part of the body, as was stated in a
former treatise, it is necessary that the part which is the primary
seat of these principles shall on the one hand, in its character of
general sensory recipient, be one of the simple parts; and on the
other hand shall, in its motor and active character, be one of the
heterogeneous parts. For this reason it is the heart which in
sanguineous animals constitutes this central part, and in bloodless
animals it is that which takes the place of a heart. For the heart,
like the other viscera, is one of the homogeneous parts; for, if cut
up, its pieces are homogeneous in substance with each other. But it is
at the same time heterogeneous in virtue of its definite
configuration. And the same is true of the other so-called viscera,
which are indeed formed from the same material as the heart. For all
these viscera have a sanguineous character owing to their being
situated upon vascular ducts and branches. For just as a stream of
water deposits mud, so the various viscera, the heart excepted, are,
as it were, deposits from the stream of blood in the vessels. And as
to the heart, the very starting-point of the vessels, and the actual
seat of the force by which the blood is first fabricated, it is but
what one would naturally expect, that out of the selfsame nutriment of
which it is the recipient its own proper substance shall be formed.
Such, then, are the reasons why the viscera are of sanguineous aspect;
and why in one point of view they are homogeneous, in another
heterogeneous.

                                 2

  Of the homogeneous parts of animals, some are soft and fluid, others
hard and solid; and of the former some are fluid permanently, others
only so long as they are in the living body. Such are blood, serum,
lard, suet, marrow, semen, bile, milk when present, flesh, and their
various analogues. For the parts enumerated are not to be found in all
animals, some animals only having parts analogous to them. Of the hard
and solid homogeneous parts bone, fish-spine, sinew, blood-vessel, are
examples. The last of these points to a sub-division that may be
made in the class of homogeneous parts. For in some of them the
whole and a portion of the whole in one sense are designated by the
same term-as, for example, is the case with blood-vessel and bit of
blood-vessel-while in another sense they are not; but a portion of a
heterogeneous part, such as face, in no sense has the same designation
as the whole.

  The first question to be asked is what are the causes to which these
homogeneous parts owe their existence? The causes are various; and
this whether the parts be solid or fluid. Thus one set of
homogeneous parts represent the material out of which the
heterogeneous parts are formed; for each separate organ is constructed
of bones, sinews, flesh, and the like; which are either essential
elements in its formation, or contribute to the proper discharge of
its function. A second set are the nutriment of the first, and are
invariably fluid, for all growth occurs at the expense of fluid
matter; while a third set are the residue of the second. Such, for
instance, are the faeces and, in animals that have a bladder, the
urine; the former being the dregs of the solid nutriment, the latter
of the fluid.

  Even the individual homogeneous parts present variations, which
are intended in each case to render them more serviceable for their
purpose. The variations of the blood may be selected to illustrate
this. For different bloods differ in their degrees of thinness or
thickness, of clearness or turbidity, of coldness or heat; and this
whether we compare the bloods from different parts of the same
individual or the bloods of different animals. For, in the individual,
all the differences just enumerated distinguish the blood of the upper
and of the lower halves of the body; and, dealing with classes, one
section of animals is sanguineous, while the other has no blood, but
only something resembling it in its place. As regards the results of
such differences, the thicker and the hotter blood is, the more
conducive is it to strength, while in proportion to its thinness and
its coldness is its suitability for sensation and intelligence. A like
distinction exists also in the fluid which is analogous to blood. This
explains how it is that bees and other similar creatures are of a more
intelligent nature than many sanguineous animals; and that, of
sanguineous animals, those are the most intelligent whose blood is
thin and cold. Noblest of all are those whose blood is hot, and at the
same time thin and clear. For such are suited alike for the
development of courage and of intelligence. Accordingly, the upper
parts are superior in these respects to the lower, the male superior
to the female, and the right side to the left. As with the blood so
also with the other parts, homogeneous and heterogeneous alike. For
here also such variations as occur must be held either to be related
to the essential constitution and mode of life of the several animals,
or, in other cases, to be merely matters of slightly better or
slightly worse. Two animals, for instance, may have eyes. But in one
these eyes may be of fluid consistency, while in the other they are
hard; and in one there may be eyelids, in the other no such
appendages. In such a case, the fluid consistency and the presence
of eyelids, which are intended to add to the accuracy of vision, are
differences of degree. As to why all animals must of necessity have
blood or something of a similar character, and what the nature of
blood may be, these are matters which can only be considered when we
have first discussed hot and cold. For the natural properties of
many substances are referable to these two elementary principles;
and it is a matter of frequent dispute what animals or what parts of
animals are hot and what cold. For some maintain that water animals
are hotter than such as live on land, asserting that their natural
heat counterbalances the coldness of their medium; and again, that
bloodless animals are hotter than those with blood, and females than
males. Parmenides, for instance, and some others declare that women
are hotter than men, and that it is the warmth and abundance of
their blood which causes their menstrual flow, while Empedocles
maintains the opposite opinion. Again, comparing the blood and the
bile, some speak of the former as hot and of the latter as cold, while
others invert the description. If there be this endless disputing
about hot and cold, which of all things that affect our senses are the
most distinct, what are we to think as to our other sensory
impressions?

  The explanation of the difficulty appears to be that the term
'hotter' is used in several senses; so that different statements,
though in verbal contradiction with each other, may yet all be more or
less true. There ought, then, to be some clear understanding as to the
sense in which natural substances are to be termed hot or cold,
solid or fluid. For it appears manifest that these are properties on
which even life and death are largely dependent, and that they are
moreover the causes of sleep and waking, of maturity and old age, of
health and disease; while no similar influence belongs to roughness
and smoothness, to heaviness and lightness, nor, in short, to any
other such properties of matter. That this should be so is but in
accordance with rational expectation. For hot and cold, solid and
fluid, as was stated in a former treatise, are the foundations of
the physical elements.

  Is then the term hot used in one sense or in many? To answer this we
must ascertain what special effect is attributed to a hotter
substance, and if there be several such, how many these may be. A body
then is in one sense said to be hotter than another, if it impart a
greater amount of heat to an object in contact with it. In a second
sense, that is said to be hotter which causes the keener sensation
when touched, and especially if the sensation be attended with pain.
This criterion, however, would seem sometimes to be a false one; for
occasionally it is the idiosyncrasy of the individual that causes
the sensation to be painful. Again, of two things, that is the
hotter which the more readily melts a fusible substance, or sets on
fire an inflammable one. Again, of two masses of one and the same
substance, the larger is said to have more heat than the smaller.
Again, of two bodies, that is said to be the hotter which takes the
longer time in cooling, as also we call that which is rapidly heated
hotter than that which is long about it; as though the rapidity
implied proximity and this again similarity of nature, while the
want of rapidity implied distance and this again dissimilarity of
nature. The term hotter is used then in all the various senses that
have been mentioned, and perhaps in still more. Now it is impossible
for one body to be hotter than another in all these different
fashions. Boiling water for instance, though it is more scalding
than flame, yet has no power of burning or melting combustible or
fusible matter, while flame has. So again this boiling water is hotter
than a small fire, and yet gets cold more rapidly and completely.
For in fact fire never becomes cold; whereas water invariably does so.
Boiling water, again, is hotter to the touch than oil; yet it gets
cold and solid more rapidly than this other fluid. Blood, again, is
hotter to the touch than either water or oil, and yet coagulates
before them. Iron, again, and stones and other similar bodies are
longer in getting heated than water, but when once heated burn other
substances with a much greater intensity. Another distinction is this.
In some of the bodies which are called hot the heat is derived from
without, while in others it belongs to the bodies themselves; and it
makes a most important difference whether the heat has the former or
the latter origin. For to call that one of two bodies the hotter,
which is possessed of heat, we may almost say, accidentally and not of
its own essence, is very much the same thing as if, finding that
some man in a fever was a musician, one were to say that musicians are
hotter than healthy men. Of that which is hot per se and that which is
hot per accidens, the former is the slower to cool, while not rarely
the latter is the hotter to the touch. The former again is the more
burning of the two-flame, for instance, as compared with boiling
water-while the latter, as the boiling water, which is hot per
accidens, is the more heating to the touch. From all this it is
clear that it is no simple matter to decide which of two bodies is the
hotter. For the first may be the hotter in one sense, the second the
hotter in another. Indeed in some of these cases it is impossible to
say simply even whether a thing is hot or not. For the actual
substratum may not itself be hot, but may be hot when coupled witb
heat as an attribute, as would be the case if one attached a single
name to hot water or hot iron. It is after this manner that blood is
hot. In such cases, in those, that is, in which the substratum owes
its heat to an external influence, it is plain that cold is not a mere
privation, but an actual existence.

  There is no knowing but that even fire may be another of these
cases. For the substratum of fire may be smoke or charcoal, and though
the former of these is always hot, smoke being an uprising vapour, yet
the latter becomes cold when its flame is extinguished, as also
would oil and pinewood under similar circumstances. But even
substances that have been burnt nearly all possess some heat, cinders,
for example, and ashes, the dejections also of animals, and, among the
excretions, bile; because some residue of heat has been left in them
after their combustion. It is in another sense that pinewood and fat
substances are hot; namely, because they rapidly assume the
actuality of fire.

  Heat appears to cause both coagulation and melting. Now such
things as are formed merely of water are solidified by cold, while
such as are formed of nothing but earth are solidified by fire. Hot
substances again are solidified by cold, and, when they consist
chiefly of earth, the process of solidification is rapid, and the
resulting substance is insoluble; but, when their main constituent
is water, the solid matter is again soluble. What kinds of substances,
however, admit of being solidified, and what are the causes of
solidification, are questions that have already been dealt with more
precisely in another treatise.

  In conclusion, then, seeing that the terms hot and hotter are used
in many different senses, and that no one substance can be hotter than
others in all these senses, we must, when we attribute this
character to an object, add such further statements as that this
substance is hotter per se, though that other is often hotter per
accidens; or again, that this substance is potentially hot, that other
actually so; or again, that this substance is hotter in the sense of
causing a greater feeling of heat when touched, while that other is
hotter in the sense of producing flame and burning. The term hot being
used in all these various senses, it plainly follows that the term
cold will also be used with like ambiguity.

  So much then as to the signification of the terms hot and cold,
hotter and colder.

                                 3

  In natural sequence we have next to treat of solid and fluid.
These terms are used in various senses. Sometimes, for instance,
they denote things that are potentially, at other times things that
are actually, solid or fluid. Ice for example, or any other solidified
fluid, is spoken of as being actually and accidentally solid, while
potentially and essentially it is fluid. Similarly earth and ashes and
the like, when mixed with water, are actually and accidentally
fluid, but potentially and essentially are solid. Now separate the
constituents in such a mixture and you have on the one hand the watery
components to which its fluidity was due, and these are both
actually and potentially fluid, and on the other hand the earthy
components, and these are in every way solid; and it is to bodies that
are solid in this complete manner that the term 'solid' is most
properly and absolutely applicable. So also the opposite term
'fluld' is strictly and absolutely applicable to that only which is
both potentially and actually fluid. The same remark applies also to
hot bodies and to cold.

  These distinctions, then, being laid down, it is plain that blood is
essentially hot in so far as that heat is connoted in its name; just
as if boiling water were denoted by a single term, boiling would be
connoted in that term. But the substratum of blood, that which it is
in substance while it is blood in form, is not hot. Blood then in a
certain sense is essentially hot, and in another sense is not so.
For heat is included in the definition of blood, just as whiteness
is included in the definition of a white man, and so far therefore
blood is essentially hot. But so far as blood becomes hot from some
external influence, it is not hot essentially.

  As with hot and cold, so also is it with solid and fluid. We can
therefore understand how some substances are hot and fluid so long
as they remain in the living body, but become perceptibly cold and
coagulate so soon as they are separated from it; while others are
hot and consistent while in the body, but when withdrawn under a
change to the opposite condition, and become cold and fluid. Of the
former blood is an example, of the latter bile; for while blood
solidifies when thus separated, yellow bile under the same
circumstances becomes more fluid. We must attribute to such substances
the possession of opposite properties in a greater or less degree.

  In what sense, then, the blood is hot and in what sense fluid, and
how far it partakes of the opposite properties, has now been fairly
explained. Now since everything that grows must take nourishment,
and nutriment in all cases consists of fluid and solid substances, and
since it is by the force of heat that these are concocted and changed,
it follows that all living things, animals and plants alike, must on
this account, if on no other, have a natural source of heat. This
natural heat, moreover, must belong to many parts, seeing that the
organs by which the various elaborations of the food are effected
are many in number. For first of all there is the mouth and the
parts inside the mouth, on which the first share in the duty clearly
devolves, in such animals at least as live on food which requires
disintegration. The mouth, however, does not actually concoct the
food, but merely facilitates concoction; for the subdivision of the
food into small bits facilitates the action of heat upon it. After the
mouth come the upper and the lower abdominal cavities, and here it
is that concoction is effected by the aid of natural heat. Again, just
as there is a channel for the admission of the unconcocted food into
the stomach, namely the mouth, and in some animals the so-called
oesophagus, which is continuous with the mouth and reaches to the
stomach, so must there also be other and more numerous channels by
which the concocted food or nutriment shall pass out of the stomach
and intestines into the body at large, and to which these cavities
shall serve as a kind of manger. For plants get their food from the
earth by means of their roots; and this food is already elaborated
when taken in, which is the reason why plants produce no excrement,
the earth and its heat serving them in the stead of a stomach. But
animals, with scarcely an exception, and conspicuously all such as are
capable of locomotion, are provided with a stomachal sac, which is
as it were an internal substitute for the earth. They must therefore
have some instrument which shall correspond to the roots of plants,
with which they may absorb their food from this sac, so that the
proper end of the successive stages of concoction may at last be
attained. The mouth then, its duty done, passes over the food to the
stomach, and there must necessarily be something to receive it in turn
from this. This something is furnished by the bloodvessels, which
run throughout the whole extent of the mesentery from its lowest
part right up to the stomach. A description of these will be found
in the treatises on Anatomy and Natural History. Now as there is a
receptacle for the entire matter taken as food, and also a
receptacle for its excremental residue, and again a third
receptacle, namely the vessels, which serve as such for the blood,
it is plain that this blood must be the final nutritive material in
such animals as have it; while in bloodless animals the same is the
case with the fluid which represents the blood. This explains why
the blood diminishes in quantity when no food is taken, and
increases when much is consumed, and also why it becomes healthy and
unhealthy according as the food is of the one or the other
character. These facts, then, and others of a like kind, make it plain
that the purpose of the blood in sanguineous animals is to subserve
the nutrition of the body. They also explain why no more sensation
is produced by touching the blood than by touching one of the
excretions or the food, whereas when the flesh is touched sensation is
produced. For the blood is not continuous nor united by growth with
the flesh, but simply lies loose in its receptacle, that is in the
heart and vessels. The manner in which the parts grow at the expense
of the blood, and indeed the whole question of nutrition, will find
a more suitable place for exposition in the treatise on Generation,
and in other writings. For our present purpose all that need be said
is that the blood exists for the sake of nutrition, that is the
nutrition of the parts; and with this much let us therefore content
ourselves.

                                 4

  What are called fibres are found in the blood of some animals but
not of all. There are none, for instance, in the blood of deer and
of roes; and for this reason the blood of such animals as these
never coagulates. For one part of the blood consists mainly of water
and therefore does not coagulate, this process occurring only in the
other and earthy constituent, that is to say in the fibres, while
the fluid part is evaporating.

  Some at any rate of the animals with watery blood have a keener
intellect than those whose blood is of an earthier nature. This is due
not to the coldness of their blood, but rather to its thinness and
purity; neither of which qualities belongs to the earthy matter. For
the thinner and purer its fluid is, the more easily affected is an
animal's sensibility. Thus it is that some bloodless animals,
notwithstanding their want of blood, are yet more intelligent than
some among the sanguineous kinds. Such for instance, as already
said, is the case with the bee and the tribe of ants, and whatever
other animals there may be of a like nature. At the same time too
great an excess of water makes animals timorous. For fear chills the
body; so that in animals whose heart contains so watery a mixture
the way is prepared for the operation of this emotion. For water is
congealed by cold. This also explains why bloodless animals are, as
a general rule, more timorous than such as have blood, so that they
remain motionless, when frightened, and discharge their excretions,
and in some instances change colour. Such animals, on the other
hand, as have thick and abundant fibres in their blood are of a more
earthy nature, and of a choleric temperament, and liable to bursts
of passion. For anger is productive of heat; and solids, when they
have been made hot, give off more heat than fluids. The fibres
therefore, being earthy and solid, are turned into so many hot
embers in the blood, like the embers in a vapour-bath, and cause
ebullition in the fits of passion.

  This explains why bulls and boars are so choleric and so passionate.
For their blood is exceedingly rich in fibres, and the bull's at any
rate coagulates more rapidly than that of any other animal. If these
fibres, that is to say if the earthy constituents of which we are
speaking, are taken out of the blood, the fluid that remains behind
will no longer coagulate; just as the watery residue of mud will not
coagulate after removal of the earth. But if the fibres are left the
fluid coagulates, as also does mud, under the influence of cold. For
when the heat is expelled by the cold, the fluid, as has been
already stated, passes off with it by evaporation, and the residue
is dried up and solidified, not by heat but by cold. So long, however,
as the blood is in the body, it is kept fluid by animal heat.

  The character of the blood affects both the temperament and the
sensory faculties of animals in many ways. This is indeed what might
reasonably be expected, seeing that the blood is the material of which
the whole body is made. For nutriment supplies the material, and the
blood is the ultimate nutriment. It makes then a considerable
difference whether the blood be hot or cold, thin or thick, turbid
or clear.

  The watery part of the blood is serum; and it is watery, either
owing to its not being yet concocted, or owing to its having become
corrupted; so that one part of the serum is the resultant of a
necessary process, while another part is material intended to serve
for the formation of the blood.

                                 5

  The differences between lard and suet correspond to differences of
blood. For both are blood concocted into these forms as a result of
abundant nutrition, being that surplus blood that is not expended on
the fleshy part of the body, and is of an easily concocted and fatty
character. This is shown by the unctuous aspect of these substances;
for such unctuous aspect in fluids is due to a combination of air
and fire. It follows from what has been said that no non-sanguineous
animals have either lard or suet; for they have no blood. Among
sanguineous animals those whose blood is dense have suet rather than
lard. For suet is of an earthy nature, that is to say, it contains but
a small proportion of water and is chiefly composed of earth; and this
it is that makes it coagulate, just as the fibrous matter of blood
coagulates, or broths which contain such fibrous matter. Thus it is
that in those horned animals that have no front teeth in the upper jaw
the fat consists of suet. For the very fact that they have horns and
huckle-bones shows that their composition is rich in this earthy
element; for all such appurtenances are solid and earthy in character.
On the other hand in those hornless animals that have front teeth in
both jaws, and whose feet are divided into toes, there is no suet, but
in its place lard; and this, not being of an earthy character, neither
coagulates nor dries up into a friable mass.

  Both lard and suet when present in moderate amount are beneficial;
for they contribute to health and strength, while they are no
hindrance to sensation. But when they are present in great excess,
they are injurious and destructive. For were the whole body formed
of them it would perish. For an animal is an animal in virtue of its
sensory part, that is in virtue of its flesh, or of the substance
analogous to flesh. But the blood, as before stated, is not sensitive;
as therefore is neither lard nor suet, seeing that they are nothing
but concocted blood. Were then the whole body composed of these
substances, it would be utterly without sensation. Such animals,
again, as are excessively fat age rapidly. For so much of their
blood is used in forming fat, that they have but little left; and when
there is but little blood the way is already open for decay. For decay
may be said to be deficiency of blood, the scantiness of which renders
it liable, like all bodies of small bulk, to be injuriously affected
by any chance excess of heat or cold. For the same reason fat
animals are less prolific than others. For that part of the blood
which should go to form semen and seed is used up in the production of
lard and suet, which are nothing but concocted blood; so that in these
animals there is either no reproductive excretion at all, or only a
scanty amount.

                                 6

  So much then of blood and serum, and of lard and suet. Each of these
has been described, and the purposes told for which they severally
exist. The marrow also is of the nature of blood, and not, as some
think, the germinal force of the semen. That this is the case is quite
evident in very young animals. For in the embryo the marrow of the
bones has a blood-like appearance, which is but natural, seeing that
the parts are all constructed out of blood, and that it is on blood
that the embryo is nourished. But, as the young animal grows up and
ripens into maturity, the marrow changes its colour, just as do the
external parts and the viscera. For the viscera also in animals, so
long as they are young, have each and all a blood-like look, owing
to the large amount of this fluid which they contain.

  The consistency of the marrow agrees with that of the fat. For
when the fat consists of lard, then the marrow also is unctuous and
lard-like; but when the blood is converted by concoction into suet,
and does not assume the form of lard, then the marrow also has a suety
character. In those animals, therefore, that have horns and are
without upper front teeth, the marrow has the character of suet; while
it takes the form of lard in those that have front teeth in both jaws,
and that also have the foot divided into toes. What has ben said
hardly applies to the spinal marrow. For it is necessary that this
shall be continuous and extend without break through the whole
backbone, inasmuch as this bone consists of separate vertebrae. But
were the spinal marrow either of unctuous fat or of suet, it could not
hold together in such a continuous mass as it does, but would either
be too fluid or too frangible.

  There are some animals that can hardly be said to have any marrow.
These are those whose bones are strong and solid, as is the case
with the lion. For in this animal the marrow is so utterly
insignificant that the bones look as though they had none at all.
However, as it is necessary that animals shall have bones or something
analogous to them, such as the fish-spines of water-animals, it is
also a matter of necessity that some of these bones shall contain
marrow; for the substance contained within the bones is the
nutriment out of which these are formed. Now the universal
nutriment, as already stated, is blood; and the blood within the bone,
owing to the heat which is developed in it from its being thus
surrounded, undergoes concoction, and self-concocted blood is suet
or lard; so that it is perfectly intelligible how the marrow within
the bone comes to have the character of these substances. So also it
is easy to understand why, in those animals that have strong and
compact bones, some of these should be entirely void of marrow,
while the rest contain but little of it; for here the nutriment is
spent in forming the bones.

  Those animals that have fish-spines in place of bones have no
other marrow than that of the chine. For in the first place they
have naturally but a small amount of blood; and secondly the only
hollow fish-spine is that of the chine. In this then marrow is formed;
this being the only spine in which there is space for it, and,
moreover, being the only one which owing to its division into parts
requires a connecting bond. This too is the reason why the marrow of
the chine, as already mentioned, is somewhat different from that of
other bones. For, having to act the part of a clasp, it must be of
glutinous character, and at the same time sinewy so as to admit of
stretching.

  Such then are the reasons for the existence of marrow, in those
animals that have any, and such its nature. It is evidently the
surplus of the sanguineous nutriment apportioned to the bones and
fish-spines, which has undergone concoction owing to its being
enclosed within them.

                                 7

  From the marrow we pass on in natural sequence to the brain. For
there are many who think that the brain itself consists of marrow, and
that it forms the commencement of that substance, because they see
that the spinal marrow is continuous with it. In reality the two may
be said to be utterly opposite to each other in character. For of
all the parts of the body there is none so cold as the brain;
whereas the marrow is of a hot nature, as is plainly shown by its
fat and unctuous character. Indeed this is the very reason why the
brain and spinal marrow are continuous with each other. For,
wherever the action of any part is in excess, nature so contrives as
to set by it another part with an excess of contrary action, so that
the excesses of the two may counterbalance each other. Now that the
marrow is hot is clearly shown by many indications. The coldness of
the brain is also manifest enough. For in the first place it is cold
even to the touch; and, secondly, of all the fluid parts of the body
it is the driest and the one that has the least blood; for in fact
it has no blood at all in its proper substance. This brain is not
residual matter, nor yet is it one of the parts which are anatomically
continuous with each other; but it has a character peculiar to itself,
as might indeed be expected. That it has no continuity with the organs
of sense is plain from simple inspection, and is still more clearly
shown by the fact, that, when it is touched, no sensation is produced;
in which respect it resembles the blood of animals and their
excrement. The purpose of its presence in animals is no less than
the preservation of the whole body. For some writers assert that the
soul is fire or some such force. This, however, is but a rough and
inaccurate assertion; and it would perhaps be better to say that the
soul is incorporate in some substance of a fiery character. The reason
for this being so is that of all substances there is none so
suitable for ministering to the operations of the soul as that which
is possessed of heat. For nutrition and the imparting of motion are
offices of the soul, and it is by heat that these are most readily
effected. To say then that the soul is fire is much the same thing
as to confound the auger or the saw with the carpenter or his craft,
simply because the work is wrought by the two in conjunction. So far
then this much is plain, that all animals must necessarily have a
certain amount of heat. But as all influences require to be
counterbalanced, so that they may be reduced to moderation and brought
to the mean (for in the mean, and not in either extreme, lies the true
and rational position), nature has contrived the brain as a
counterpoise to the region of the heart with its contained heat, and
has given it to animals to moderate the latter, combining in it the
properties of earth and water. For this reason it is, that every
sanguineous animal has a brain; whereas no bloodless creature has such
an organ, unless indeed it be, as the Poulp, by analogy. For where
there is no blood, there in consequence there is but little heat.
The brain, then, tempers the heat and seething of the heart. In order,
however, that it may not itself be absolutely without heat, but may
have a moderate amount, branches run from both blood-vessels, that
is to say from the great vessel and from what is called the aorta, and
end in the membrane which surrounds the brain; while at the same time,
in order to prevent any injury from the heat, these encompassing
vessels, instead of being few and large, are numerous and small, and
their blood scanty and clear, instead of being abundant and thick.
We can now understand why defluxions have their origin in the head,
and occur whenever the parts about the brain have more than a due
proportion of coldness. For when the nutriment steams upwards
through the blood-vessels, its refuse portion is chilled by the
influence of this region, and forms defluxions of phlegm and serum. We
must suppose, to compare small things with great, that the like
happens here as occurs in the production of showers. For when vapour
steams up from the earth and is carried by the heat into the upper
regions, so soon as it reaches the cold air that is above the earth,
it condenses again into water owing to the refrigeration, and falls
back to the earth as rain. These, however, are matters which may be
suitably considered in the Principles of Diseases, so far as natural
philosophy has anything to say to them.

  It is the brain again-or, in animals that have no brain, the part
analogous to it-which is the cause of sleep. For either by chilling
the blood that streams upwards after food, or by some other similar
influences, it produces heaviness in the region in which it lies
(which is the reason why drowsy persons hang the head), and causes the
heat to escape downwards in company with the blood. It is the
accumulation of this in excess in the lower region that produces
complete sleep, taking away the power of standing upright from those
animals to whom that posture is natural, and from the rest the power
of holding up the head. These, however, are matters which have been
separately considered in the treatises on Sensation and on Sleep.

  That the brain is a compound of earth and water is shown by what
occurs when it is boiled. For, when so treated, it turns hard and
solid, inasmuch as the water is evaporated by the heat, and leaves the
earthy part behind. Just the same occurs when pulse and other fruits
are boiled. For these also are hardened by the process, because the
water which enters into their composition is driven off and leaves the
earth, which is their main constituent, behind.

  Of all animals, man has the largest brain in proportion to his size;
and it is larger in men than in women. This is because the region of
the heart and of the lung is hotter and richer in blood in man than in
any other animal; and in men than in women. This again explains why
man, alone of animals, stands erect. For the heat, overcoming any
opposite inclination, makes growth take its own line of direction,
which is from the centre of the body upwards. It is then as a
counterpoise to his excessive heat that in man's brain there is this
superabundant fluidity and coldness; and it is again owing to this
superabundance that the cranial bone, which some call the Bregma, is
the last to become solidified; so long does evaporation continue to
occur through it under the influence of heat. Man is the only
sanguineous animal in which this takes place. Man, again, has more
sutures in his skull than any other animal, and the male more than the
female. The explanation is again to be found in the greater size of
the brain, which demands free ventilation, proportionate to its
bulk. For if the brain be either too fluid or too solid, it will not
perform its office, but in the one case will freeze the blood, and
in the other will not cool it at all; and thus will cause disease,
madness, and death. For the cardiac heat and the centre of life is
most delicate in its sympathies, and is immediately sensitive to the
slightest change or affection of the blood on the outer surface of the
brain.

  The fluids which are present in the animal body at the time of birth
have now nearly all been considered. Amongst those that appear only at
a later period are the residua of the food, which include the deposits
of the belly and also those of the bladder. Besides these there is the
semen and the milk, one or the other of which makes its appearance
in appropriate animals. Of these fluids the excremental residua of the
food may be suitably discussed by themselves, when we come to
examine and consider the subject of nutrition. Then will be the time
to explain in what animals they are found, and what are the reasons
for their presence. Similarly all questions concerning the semen and
the milk may be dealt with in the treatise on Generation, for the
former of these fluids is the very starting-point of the generative
process, and the latter has no other ground of existence than
generative purposes.

                                 8

  We have now to consider the remaining homogeneous parts, and will
begin with flesh, and with the substance that, in animals that have no
flesh, takes its place. The reason for so beginning is that flesh
forms the very basis of animals, and is the essential constituent of
their body. Its right to this precedence can also be demonstrated
logically. For an animal is by our definition something that has
sensibility and chief of all the primary sensibility, which is that of
Touch; and it is the flesh, or analogous substance, which is the organ
of this sense. And it is the organ, either in the same way as the
pupil is the organ of sight, that is it constitutes the primary
organ of the sense; or it is the organ and the medium through which
the object acts combined, that is it answers to the pupil with the
whole transparent medium attached to it. Now in the case of the
other senses it was impossible for nature to unite the medium with the
sense-organ, nor would such a junction have served any purpose; but in
the case of touch she was compelled by necessity to do so. For of
all the sense-organs that of touch is the only one that has
corporeal substance, or at any rate it is more corporeal than any
other, and its medium must be corporeal like itself.

  It is obvious also to sense that it is for the sake of the flesh
that all the other parts exist. By the other parts I mean the bones,
the skin, the sinews, and the blood-vessels, and, again, the hair
and the various kinds of nails, and anything else there may be of a
like character. Thus the bones are a contrivance to give security to
the soft parts, to which purpose they are adapted by their hardness;
and in animals that have no bones the same office is fulfilled by some
analogous substance, as by fishspine in some fishes, and by
cartilage in others.

  Now in some animals this supporting substance is situated within the
body, while in some of the bloodless species it is placed on the
outside. The latter is the case in all the Crustacea, as the Carcini
(Crabs) and the Carabi (Prickly Lobsters); it is the case also in
the Testacea, as for instance in the several species known by the
general name of oysters. For in all these animals the fleshy substance
is within, and the earthy matter, which holds the soft parts
together and keeps them from injury, is on the outside. For the
shell not only enables the soft parts to hold together, but also, as
the animal is bloodless and so has but little natural warmth,
surrounds it, as a chaufferette does the embers, and keeps in the
smouldering heat. Similar to this seems to be the arrangement in
another and distinct tribe of animals, namely the Tortoises, including
the Chelone and the several kinds of Emys. But in Insects and in
Cephalopods the plan is entirely different, there being moreover a
contrast between these two themselves. For in neither of these does
there appear to be any bony or earthy part, worthy of notice,
distinctly separated from the rest of the body. Thus in the
Cephalopods the main bulk of the body consists of a soft flesh-like
substance, or rather of a substance which is intermediate to flesh and
sinew, so as not to be so readily destructible as actual flesh. I call
this substance intermediate to flesh and sinew, because it is soft
like the former, while it admits of stretching like the latter. Its
cleavage, however, is such that it splits not longitudinally, like
sinew, but into circular segments, this being the most advantageous
condition, so far as strength is concerned. These animals have also
a part inside them corresponding to the spinous bones of fishes. For
instance, in the Cuttle-fishes there is what is known as the os
sepiae, and in the Calamaries there is the so-called gladius. In the
Poulps, on the other hand, there is no such internal part, because the
body, or, as it is termed in them, the head, forms but a short sac,
whereas it is of considerable length in the other two; and it was this
length which led nature to assign to them their hard support, so as to
ensure their straightness and inflexibility; just as she has
assigned to sanguineous animals their bones or their fish-spines, as
the case may be. To come now to Insects. In these the arrangement is
quite different from that of the Cephalopods; quite different also
from that which obtains in sanguineous animals, as indeed has been
already stated. For in an insect there is no distinction into soft and
hard parts, but the whole body is hard, the hardness, however, being
of such a character as to be more flesh-like than bone, and more
earthy and bone-like than flesh. The purpose of this is to make the
body of the insect less liable to get broken into pieces.

                                 9

  There is a resemblance between the osseous and the vascular systems;
for each has a central part in which it begins, and each forms a
continuous whole. For no bone in the body exists as a separate thing
in itself, but each is either a portion of what may be considered a
continuous whole, or at any rate is linked with the rest by contact
and by attachments; so that nature may use adjoining bones either as
though they were actually continuous and formed a single bone, or, for
purposes of flexure, as though they were two and distinct. And
similarly no blood-vessel has in itself a separate individuality;
but they all form parts of one whole. For an isolated bone, if such
there were, would in the first place be unable to perform the office
for the sake of which bones exist; for, were it discontinuous and
separated from the rest by a gap, it would be perfectly unable to
produce either flexure or extension; nor only so, but it would
actually be injurious, acting like a thorn or an arrow lodged in the
flesh. Similarly if a vessel were isolated, and not continuous with
the vascular centre, it would be unable to retain the blood within
it in a proper state. For it is the warmth derived from this centre
that hinders the blood from coagulating; indeed the blood, when
withdrawn from its influence, becomes manifestly putrid. Now the
centre or origin of the blood-vessels is the heart, and the centre
or origin of the bones, in all animals that have bones, is what is
called the chine. With this all the other bones of the body are in
continuity; for it is the chine that holds together the whole length
of an animal and preserves its straightness. But since it is necessary
that the body of an animal shall bend during locomotion, this chine,
while it is one in virtue of the continuity of its parts, yet its
division into vertebrae is made to consist of many segments. It is
from this chine that the bones of the limbs, in such animals as have
these parts, proceed, and with it they are continuous, being
fastened together by the sinews where the limbs admit of flexure,
and having their extremities adapted to each other, either by the
one being hollowed and the other rounded, or by both being hollowed
and including between them a hucklebone, as a connecting bolt, so as
to allow of flexure and extension. For without some such arrangement
these movements would be utterly impossible, or at any rate would be
performed with great difficulty. There are some joints, again, in
which the lower end of the one bone and the upper end of the other are
alike in shape. In these cases the bones are bound together by sinews,
and cartilaginous pieces are interposed in the joint, to serve as a
kind of padding, and prevent the two extremities from grating
against each other.

  Round about the bones, and attached to them by thin fibrous bands,
grow the fleshy parts, for the sake of which the bones themselves
exist. For just as an artist, when he is moulding an animal out of
clay or other soft substance, takes first some solid body as a
basis, and round this moulds the clay, so also has nature acted in
fashioning the animal body out of flesh. Thus we find all the fleshy
parts, with one exception, supported by bones, which serve, when the
parts are organs of motion, to facilitate flexure, and, when the parts
are motionless, act as a protection. The ribs, for example, which
enclose the chest are intended to ensure the safety of the heart and
neighbouring viscera. The exception of which mention was made is the
belly. The walls of this are in all animals devoid of bones; in
order that there may be no hindrance to the expansion which
necessarily occurs in this part after a meal, nor, in females, any
interference with the growth of the foetus, which is lodged here.

  Now the bones of viviparous animals, of such, that is, as are not
merely externally but also internally viviparous, vary but very little
from each other in point of strength, which in all of them is
considerable. For the Vivipara in their bodily proportions are far
above other animals, and many of them occasionally grow to an enormous
size, as is the case in Libya and in hot and dry countries
generally. But the greater the bulk of an animal, the stronger, the
bigger, and the harder, are the supports which it requires; and
comparing the big animals with each other, this requirement will be
most marked in those that live a life of rapine. Thus it is that the
bones of males are harder than those of females; and the bones of
flesh-eaters, that get their food by fighting, are harder than those
of Herbivora. Of this the Lion is an example; for so hard are its
bones, that, when struck, they give off sparks, as though they were
stones. It may be mentioned also that the Dolphin, in as much as it is
viviparous, is provided with bones and not with fish-spines.

  In those sanguineous animals, on the other hand, that are oviparous,
the bones present successive slight variations of character. Thus in
Birds there are bones, but these are not so strong as the bones of the
Vivipara. Then come the Oviparous fishes, where there is no bone,
but merely fish-spine. In the Serpents too the bones have the
character of fish-spine, excepting in the very large species, where
the solid foundation of the body requires to be stronger, in order
that the animal itself may be strong, the same reason prevailing as in
the case of the Vivipara. Lastly, in the Selachia, as they are called,
the fish-spines are replaced by cartilage. For it is necessary that
the movements of these animals shall be of an undulating character;
and this again requires the framework that supports the body to be
made of a pliable and not of a brittle substance. Moreover, in these
Selachia nature has used all the earthy matter on the skin; and she is
unable to allot to many different parts one and the same superfluity
of material. Even in viviparous animals many of the bones are
cartilaginous. This happens in those parts where it is to the
advantage of the surrounding flesh that its solid base shall be soft
and mucilaginous. Such, for instance, is the case with the ears and
nostrils; for in projecting parts, such as these, brittle substances
would soon get broken. Cartilage and bone are indeed fundamentally the
same thing, the differences between them being merely matters of
degree. Thus neither cartilage nor bone, when once cut off, grows
again. Now the cartilages of these land animals are without marrow,
that is without any distinctly separate marrow. For the marrow,
which in bones is distinctly separate, is here mixed up with the whole
mass, and gives a soft and mucilaginous consistence to the
cartilage. But in the Selachia the chine, though it is
cartilaginous, yet contains marrow; for here it stands in the stead of
a bone.

  Very nearly resembling the bones to the touch are such parts as
nails, hoofs, whether solid or cloven, horns, and the beaks of
birds, all of which are intended to serve as means of defence. For the
organs which are made out of these substances, and which are called by
the same names as the substances themselves, the organ hoof, for
instance, and the organ horn, are contrivances to ensure the
preservation of the animals to which they severally belong. In this
class too must be reckoned the teeth, which in some animals have but a
single function, namely the mastication of the food, while in others
they have an additional office, namely to serve as weapons; as is
the case with all animals that have sharp interfitting teeth or that
have tusks. All these parts are necessarily of solid and earthy
character; for the value of a weapon depends on such properties. Their
earthy character explains how it is that all such parts are more
developed in four-footed vivipara than in man. For there is always
more earth in the composition of these animals than in that of the
human body. However, not only all these parts but such others as are
nearly connected with them, skin for instance, bladder, membrane,
hairs, feathers, and their analogues, and any other similar parts that
there may be, will be considered farther on with the heterogeneous
parts. There we shall inquire into the causes which produce them,
and into the objects of their presence severally in the bodies of
animals. For, as with the heterogeneous parts, so with these, it is
from a consideration of their functions that alone we can derive any
knowledge of them. The reason for dealing with them at all in this
part of the treatise, and classifying them with the homogeneous parts,
is that under one and the same name are confounded the entire organs
and the substances of which they are composed. But of all these
substances flesh and bone form the basis. Semen and milk were also
passed over when we were considering the homogeneous fluids. For the
treatise on Generation will afford a more suitable place for their
examination, seeing that the former of the two is the very
foundation of the thing generated, while the latter is its
nourishment.

                                10

  Let us now make, as it were, a fresh beginning, and consider the
heterogeneous parts, taking those first which are the first in
importance. For in all animals, at least in all the perfect kinds,
there are two parts more essential than the rest, namely the part
which serves for the ingestion of food, and the part which serves
for the discharge of its residue. For without food growth and even
existence is impossible. Intervening again between these two parts
there is invariably a third, in which is lodged the vital principle.
As for plants, though they also are included by us among things that
have life, yet are they without any part for the discharge of waste
residue. For the food which they absorb from the ground is already
concocted, and they give off as its equivalent their seeds and fruits.
Plants, again, inasmuch as they are without locomotion, present no
great variety in their heterogeneous parts. For, where the functions
are but few, few also are the organs required to effect them. The
configuration of plants is a matter then for separate consideration.
Animals, however, that not only live but feel, present a greater
multiformity of parts, and this diversity is greater in some animals
than in others, being most varied in those to whose share has fallen
not mere life but life of high degree. Now such an animal is man.
For of all living beings with which we are acquainted man alone
partakes of the divine, or at any rate partakes of it in a fuller
measure than the rest. For this reason, then, and also because his
external parts and their forms are more familiar to us than those of
other animals, we must speak of man first; and this the more fitly,
because in him alone do the natural parts hold the natural position;
his upper part being turned towards that which is upper in the
universe. For, of all animals, man alone stands erect.

  In man, then, the head is destitute of flesh; this being the
necessary consequence of what has already been stated concerning the
brain. There are, indeed, some who hold that the life of man-would
be longer than it is, were his head more abundantly furnished with
flesh; and they account for the absence of this substance by saying
that it is intended to add to the perfection of sensation. For the
brain they assert to be the organ of sensation; and sensation, they
say, cannot penetrate to parts that are too thickly covered with
flesh. But neither part of this statement is true. On the contrary,
were the region of the brain thickly covered with flesh, the very
purpose for which animals are provided with a brain would be
directly contravened. For the brain would itself be heated to excess
and so unable to cool any other part; and, as to the other half of
their statement, the brain cannot be the cause of any of the
sensations, seeing that it is itself as utterly without feeling as any
one of the excretions. These writers see that certain of the senses
are located in the head, and are unable to discern the reason for
this; they see also that the brain is the most peculiar of all the
animal organs; and out of these facts they form an argument, by
which they link sensation and brain together. It has, however, already
been clearly set forth in the treatise on Sensation, that it is the
region of the heart that constitutes the sensory centre. There also it
was stated that two of the senses, namely touch and taste, are
manifestly in immediate connexion with the heart; and that as
regards the other three, namely hearing, sight, and the centrally
placed sense of smell, it is the character of their sense-organs which
causes them to be lodged as a rule in the head. Vision is so placed in
all animals. But such is not invariably the case with hearing or
with smell. For fishes and the like hear and smell, and yet have no
visible organs for these senses in the head; a fact which demonstrates
the accuracy of the opinion here maintained. Now that vision, whenever
it exists, should be in the neighbourhood of the brain is but what one
would rationally expect. For the brain is fluid and cold, and vision
is of the nature of water, water being of all transparent substances
the one most easily confined. Moreover it cannot but necessarily be
that the more precise senses will have their precision rendered
still greater if ministered to by parts that have the purest blood.
For the motion of the heat of blood destroys sensory activity. For
these reasons the organs of the precise senses are lodged in the head.

  It is not only the fore part of the head that is destitute of flesh,
but the hind part also. For, in all animals that have a head, it is
this head which more than any other part requires to be held up.
But, were the head heavily laden with flesh, this would be impossible;
for nothing so burdened can be held upright. This is an additional
proof that the absence of flesh from the head has no reference to
brain sensation. For there is no brain in the hinder part of the head,
and yet this is as much without flesh as is the front.

  In some animals hearing as well as vision is lodged in the region of
the head. Nor is this without a rational explanation. For what is
called the empty space is full of air, and the organ of hearing is, as
we say, of the nature of air. Now there are channels which lead from
the eyes to the blood-vessels that surround the brain; and similarly
there is a channel which leads back again from each ear and connects
it with the hinder part of the head. But no part that is without blood
is endowed with sensation, as neither is the blood itself, but only
some one of the parts that are formed of blood.

  The brain in all animals that have one is placed in the front part
of the head; because the direction in which sensation acts is in
front; and because the heart, from which sensation proceeds, is in the
front part of the body; and lastly because the instruments of
sensation are the blood-containing parts, and the cavity in the
posterior part of the skull is destitute of blood-vessels.

  As to the position of the sense-organs, they have been arranged by
nature in the following well-ordered manner. The organs of hearing are
so placed as to divide the circumference of the head into two equal
halves; for they have to hear not only sounds which are directly in
line with themselves, but sounds from all quarters. The organs of
vision are placed in front, because sight is exercised only in a
straight line, and moving as we do in a forward direction it is
necessary that we should see before us, in the direction of our
motion. Lastly, the organs of smell are placed with good reason
between the eyes. For as the body consists of two parts, a right
half and a left, so also each organ of sense is double. In the case of
touch this is not apparent, the reason being that the primary organ of
this sense is not the flesh or analogous part, but lies internally. In
the case of taste, which is merely a modification of touch and which
is placed in the tongue, the fact is more apparent than in the case of
touch, but still not so manifest as in the case of the other senses.
However, even in taste it is evident enough; for in some animals the
tongue is plainly forked. The double character of the sensations is,
however, more conspicuous in the other organs of sense. For there
are two ears and two eyes, and the nostrils, though joined together,
are also two. Were these latter otherwise disposed, and separated from
each other as are the ears, neither they nor the nose in which they
are placed would be able to perform their office. For in such
animals as have nostrils olfaction is effected by means of
inspiration, and the organ of inspiration is placed in front and in
the middle line. This is the reason why nature has brought the two
nostrils together and placed them as the central of the three
sense-organs, setting them side by side on a level with each other, to
avail themselves of the inspiratory motion. In other animals than
man the arrangement of these sense-organs is also such as is adapted
in each case to the special requirements.

                                11

  For instance, in quadrupeds the ears stand out freely from the
head and are set to all appearance above the eyes. Not that they are
in reality above the eyes; but they seem to be so, because the
animal does not stand erect, but has its head hung downwards. This
being the usual attitude of the animal when in motion, it is of
advantage that its ears shall be high up and movable; for by turning
themselves about they can the better take in sounds from every
quarter.

                                12

  In birds, on the other hand, there are no ears, but only the
auditory passages. This is because their skin is hard and because they
have feathers instead of hairs, so that they have not got the proper
material for the formation of ears. Exactly the same is the case
with such oviparous quadrupeds as are clad with scaly plates, and
the same explanation applies to them. There is also one of the
viviparous quadrupeds, namely the seal, that has no ears but only
the auditory passages. The explanation of this is that the seal,
though a quadruped, is a quadruped of stunted formation.

                                13

  Men, and Birds, and Quadrupeds, viviparous and oviparous alike, have
their eyes protected by lids. In the Vivipara there are two of
these; and both are used by these animals not only in closing the
eyes, but also in the act of blinking; whereas the oviparous
quadrupeds, and the heavy-bodied birds as well as some others, use
only the lower lid to close the eye; while birds blink by means of a
membrane that issues from the canthus. The reason for the eyes being
thus protected is that nature has made them of fluid consistency, in
order to ensure keenness of vision. For had they been covered with
hard skin, they would, it is true, have been less liable to get
injured by anything falling into them from without, but they would not
have been sharp-sighted. It is then to ensure keenness of vision
that the skin over the pupil is fine and delicate; while the lids
are superadded as a protection from injury. It is as a still further
safeguard that all these animals blink, and man most of all; this
action (which is not performed from deliberate intention but from a
natural instinct) serving to keep objects from falling into the
eyes; and being more frequent in man than in the rest of these
animals, because of the greater delicacy of his skin. These lids are
made of a roll of skin; and it is because they are made of skin and
contain no flesh that neither they, nor the similarly constructed
prepuce, unite again when once cut.

  As to the oviparous quadrupeds, and such birds as resemble them in
closing the eye with the lower lid, it is the hardness of the skin
of their heads which makes them do so. For such birds as have heavy
bodies are not made for flight; and so the materials which would
otherwise have gone to increase the growth of the feathers are
diverted thence, and used to augment the thickness of the skin.
Birds therefore of this kind close the eye with the lower lid; whereas
pigeons and the like use both upper and lower lids for the purpose. As
birds are covered with feathers, so oviparous quadrupeds are covered
with scaly plates; and these in all their forms are harder than hairs,
so that the skin also to which they belong is harder than the skin
of hairy animals. In these animals, then, the skin on the head is
hard, and so does not allow of the formation of an upper eyelid,
whereas lower down the integument is of a flesh-like character, so
that the lower lid can be thin and extensible.

  The act of blinking is performed by the heavy-bodied birds by
means of the membrane already mentioned, and not by this lower lid.
For in blinking rapid motion is required, and such is the motion of
this membrane, whereas that of the lower lid is slow. It is from the
canthus that is nearest to the nostrils that the membrane comes. For
it is better to have one starting-point for nictitation than two;
and in these birds this starting-point is the junction of eye and
nostrils, an anterior starting-point being preferable to a lateral
one. Oviparous quadrupeds do not blink in like manner as the birds;
for, living as they do on the ground, they are free from the necessity
of having eyes of fluid consistency and of keen sight, whereas these
are essential requisites for birds, inasmuch as they have to use their
eyes at long distances. This too explains why birds with talons,
that have to search for prey by eye from aloft, and therefore soar
to greater heights than other birds, are sharpsighted; while common
fowls and the like, that live on the ground and are not made for
flight, have no such keenness of vision. For there is nothing in their
mode of life which imperatively requires it.

  Fishes and Insects and the hard-skinned Crustacea present certain
differences in their eyes, but so far resemble each other as that none
of them have eyelids. As for the hard-skinned Crustacea it is
utterly out of the question that they should have any; for an
eyelid, to be of use, requires the action of the skin to be rapid.
These animals then have no eyelids and, in default of this protection,
their eyes are hard, just as though the lid were attached to the
surface of the eye, and the animal saw through it. Inasmuch,
however, as such hardness must necessarily blunt the sharpness of
vision, nature has endowed the eyes of Insects, and still more those
of Crustacea, with mobility (just as she has given some quadrupeds
movable ears), in order that they may be able to turn to the light and
catch its rays, and so see more plainly. Fishes, however, have eyes of
a fluid consistency. For animals that move much about have to use
their vision at considerable distances. If now they live on land,
the air in which they move is transparent enough. But the water in
which fishes live is a hindrance to sharp sight, though it has this
advantage over the air, that it does not contain so many objects to
knock against the eyes. The risk of collision being thus small,
nature, who makes nothing in vain, has given no eyelids to fishes,
while to counterbalance the opacity of the water she has made their
eyes of fluid consistency.

                                14

  All animals that have hairs on the body have lashes on the
eyelids; but birds and animals with scale-like plates, being hairless,
have none. The Libyan ostrich, indeed, forms an exception; for, though
a bird, it is furnished with eyelashes. This exception, however,
will be explained hereafter. Of hairy animals, man alone has lashes on
both lids. For in quadrupeds there is a greater abundance of hair on
the back than on the under side of the body; whereas in man the
contrary is the case, and the hair is more abundant on the front
surface than on the back. The reason for this is that hair is intended
to serve as a protection to its possessor. Now, in quadrupeds, owing
to their inclined attitude, the under or anterior surface does not
require so much protection as the back, and is therefore left
comparatively bald, in spite of its being the nobler of the two sides.
But in man, owing to his upright attitude, the anterior and
posterior surfaces of the body are on an equality as regards need of
protection. Nature therefore has assigned the protective covering to
the nobler of the two surfaces; for invariably she brings about the
best arrangement of such as are possible. This then is the reason that
there is no lower eyelash in any quadruped; though in some a few
scattered hairs sprout out under the lower lid. This also is the
reason that they never have hair in the axillae, nor on the pubes,
as man has. Their hair, then, instead of being collected in these
parts, is either thickly set over the whole dorsal surface, as is
the case for instance in dogs, or, sometimes, forms a mane, as in
horses and the like, or as in the male lion where the mane is still
more flowing and ample. So, again, whenever there is a tail of any
length, nature decks it with hair, with long hair if the stem of the
tail be short, as in horses, with short hair if the stem be long,
regard also being had to the condition of the rest of the body. For
nature invariably gives to one part what she subtracts from another.
Thus when she has covered the general surface of an animal's body with
an excess of hair, she leaves a deficiency in the region of the
tail. This, for instance, in the case with bears.

  No animal has so much hair on the head as man. This, in the first
place, is the necessary result of the fluid character of his brain,
and of the presence of so many sutures in his skull. For wherever
there is the most fluid and the most heat, there also must necessarily
occur the greatest outgrowth. But, secondly, the thickness of the hair
in this part has a final cause, being intended to protect the head, by
preserving it from excess of either heat or cold. And as the brain
of man is larger and more fluid than that of any other animal, it
requires a proportionately greater amount of protection. For the
more fluid a substance is, the more readily does it get excessively
heated or excessively chilled, while substances of an opposite
character are less liable to such injurious affections.

  These, however, are matters which by their close connexion with
eyelashes have led us to digress from our real topic, namely the cause
to which these lashes owe their existence. We must therefore defer any
further remarks we may have to make on these matters till the proper
occasion arises and then return to their consideration.

                                15

  Both eyebrows and eyelashes exist for the protection of the eyes;
the former that they may shelter them, like the eaves of a house, from
any fluids that trickle down from the head; the latter to act like the
palisades which are sometimes placed in front of enclosures, and
keep out any objects which might otherwise get in. The brows are
placed over the junction of two bones, which is the reason that in old
age they often become so bushy as to require cutting. The lashes are
set at the terminations of small blood-vessels. For the vessels come
to an end where the skin itself terminates; and, in all places where
these endings occur, the exudation of moisture of a corporeal
character necessitates the growth of hairs, unless there be some
operation of nature which interferes, by diverting the moisture to
another purpose.

                                16

  Viviparous quadrupeds, as a rule, present no great variety of form
in the organ of smell. In those of them, however, whose jaws project
forwards and taper to a narrow end, so as to form what is called a
snout, the nostrils are placed in this projection, there being no
other available plan; while, in the rest, there is a more definite
demarcation between nostrils and jaws. But in no animal is this part
so peculiar as in the elephant, where it attains an extraordinary
and strength. For the elephant uses its nostril as a hand; this
being the instrument with which it conveys food, fluid and solid
alike, to its mouth. With it, too, it tears up trees, coiling it round
their stems. In fact it applies it generally to the purposes of a
hand. For the elephant has the double character of a land animal,
and of one that lives in swamps. Seeing then that it has to get its
food from the water, and yet must necessarily breathe, inasmuch as
it is a land animal and has blood; seeing, also, that its excessive
weight prevents it from passing rapidly from water to land, as some
other sanguineous vivipara that breathe can do, it becomes necessary
that it shall be suited alike for life in the water and for life on
dry land. just then as divers are sometimes provided with
instruments for respiration, through which they can draw air from
above the water, and thus may remain for a long time under the sea, so
also have elephants been furnished by nature with their lengthened
nostril; and, whenever they have to traverse the water, they lift this
up above the surface and breathe through it. For the elephant's
proboscis, as already said, is a nostril. Now it would have been
impossible for this nostril to have the form of a proboscis, had it
been hard and incapable of bending. For its very length would then
have prevented the animal from supplying itself with food, being as
great an impediment as the of certain oxen, that are said to be
obliged to walk backwards while they are grazing. It is therefore soft
and flexible, and, being such, is made, in addition to its own
proper functions, to serve the office of the fore-feet; nature in this
following her wonted plan of using one and the same part for several
purposes. For in polydactylous quadrupeds the fore-feet are intended
not merely to support the weight of the body, but to serve as hands.
But in elephants, though they must be reckoned polydactylous, as their
foot has neither cloven nor solid hoof, the fore-feet, owing to the
great size and weight of the body, are reduced to the condition of
mere supports; and indeed their slow motion and unfitness for
bending make them useless for any other purpose. A nostril, then, is
given to the elephant for respiration, as to every other animal that
has a lung, and is lengthened out and endowed with its power of
coiling because the animal has to remain for considerable periods of
time in the water, and is unable to pass thence to dry ground with any
rapidity. But as the feet are shorn of their full office, this same
part is also, as already said, made by nature to supply their place,
and give such help as otherwise would be rendered by them.

  As to other sanguineous animals, the Birds, the Serpents, and the
Oviparous quadrupeds, in all of them there are the nostril-holes,
placed in front of the mouth; but in none are there any distinctly
formed nostrils, nothing in fact which can be called nostrils except
from a functional point of view. A bird at any rate has nothing
which can properly be called a nose. For its so-called beak is a
substitute for jaws. The reason for this is to be found in the natural
conformation of birds. For they are winged bipeds; and this makes it
necessary that their heads and neck shall be of light weight; just
as it makes it necessary that their breast shall be narrow. The beak
therefore with which they are provided is formed of a bone-like
substance, in order that it may serve as a weapon as well as for
nutritive purposes, but is made of narrow dimensions to suit the small
size of the head. In this beak are placed the olfactory passages.
But there are no nostrils; for such could not possibly be placed
there.

  As for those animals that have no respiration, it has already been
explained why it is that they are without nostrils, and perceive
odours either through gills, or through a blowhole, or, if they are
insects, by the hypozoma; and how the power of smelling depends,
like their motion, upon the innate spirit of their bodies, which in
all of them is implanted by nature and not introduced from without.

  Under the nostrils are the lips, in such sanguineous animals, that
is, as have teeth. For in birds, as already has been said, the
purposes of nutrition and defence are fulfilled by a bonelike beak,
which forms a compound substitute for teeth and lips. For supposing
that one were to cut off a man's lips, unite his upper teeth together,
and similarly his under ones, and then were to lengthen out the two
separate pieces thus formed, narrowing them on either side and
making them project forwards, supposing, I say, this to be done, we
should