standpoint of his opponents. ‘Why, however, it must be asked, should
we look on the operations of Nature as dictated by a final cause, and
intended to realize some desirable end? Why may they not be merely the
results of necessity, just as the rain falls of necessity, and not
that the corn may grow? For though the rain makes the corn grow, it no
more occurs in order to cause that growth, than a shower which spoils
the farmer’s crop at harvest-time occurs in order to do that mischief.
Now, why may not this, which is true of the rain, be true also of the
parts of the body? Why, for instance, may not the teeth grow to be such
as they are merely of necessity, and the fitness of the front ones
with their sharp edge for the comminution of the food, and of the hind
ones with their flat surface for its mastication, be no more than an
accidental coincidence, and not the cause that has determined their
development?’[24]
[23] _De partibus animalium_, i. 1; 641ᵇ 12.
[24] _Physics_, ii. 8, 3; 198ᵇ 6. This passage is considerably
abbreviated and slightly paraphrased.
The answers to these questions form a considerable part of Aristotle’s
philosophy where we are unable to follow him. For the limited field of
biology, however, the question is on somewhat narrower lines. ‘What,’
he asks, ‘are the forces by which the hand or the body was fashioned
into shape? The wood carver will perhaps say, by the axe or the
auger.... But 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 ... [similarly] the true
method [of biological science] is to state what the definite 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.... 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;) ... 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.’[25] Thus
in the Aristotelian writings the discussion of the nature and orders of
‘soul’ is almost inseparable from the subjects now included under the
term Biology.
[25] _De partibus animalium_, i. 1; 641ᵅ 7.
There can be no doubt that through much of the Aristotelian writings
runs a belief in a _kinetic_ as distinct from a static view of
existence. It cannot be claimed that he regarded the different kinds of
living things as actually passing one into another, but there can be no
doubt that he fully realized that the different kinds can be arranged
in a series in which the gradations are easy. His scheme would be
something like that represented on p. 30 (Fig. 7 a).
‘Nature,’ he says, ‘proceeds little by little from things lifeless to
animal life in such a way that it is impossible to determine the exact
line of demarcation, nor on which side thereof an intermediate form
should lie. Thus, next after lifeless things in the upward scale comes
the plant, and of plants one will differ from another as to its amount
of apparent vitality; and, in a word, the whole _genus_ of plants,
whilst it is devoid of life as compared with an animal, is endowed
with life as compared with other corporeal entities. Indeed, there is
observed in plants a continuous scale of ascent towards the animal. So,
in the sea, there are certain objects concerning which one would be at
a loss to determine whether they be animal or vegetable.’[26]
[26] _Historia animalium_, viii. 1; 588ᵇ 4.
[Illustration: Fig. 7a. The Order of Living Things according to
Aristotle.]
‘A sponge, in these respects completely resembles a plant, in that ...
it is attached to a rock, and that when separated from this it dies.
Slightly different from the sponges are the so-called Holothurias ...
as also sundry other sea-animals that resemble them. For these are free
and unattached, yet they have no feeling, and their life is simply
that of a plant separated from the ground. For even among land-plants
there are some that are independent of the soil—or even entirely free.
Such, for example, is the plant which is found on Parnassus, and which
some call the Epipetrum [probably _Sempervivum tectorum_, the common
houseleek]. This you may hang up on a peg and it will yet live for a
considerable time. Sometimes it is a matter of doubt whether a given
organism should be classed with plants or with animals. The Tethya,
for instance, and the like, so far resemble plants as that they never
live free and unattached, but, on the other hand, inasmuch as they have
a certain flesh-like substance, they must be supposed to possess some
degree of sensibility.’[27]
‘The Acalephae or Sea-nettles, ... lie outside the recognized groups.
Their constitution, like that of the Tethya, approximates them on the
one side to plants, on the other side to animals. For seeing that some
of them can detach themselves and can fasten on their food, and that
they are sensible of objects which come in contact with them, they must
be considered to have an animal nature.... On the other hand, they
are closely allied to plants, firstly by the imperfection of their
structures, secondly by their being able to attach themselves to the
rocks, which they do with great rapidity, and lastly by their having no
visible residuum notwithstanding that they possess a mouth.’[28]
Thus ‘Nature passes from lifeless objects to animals in such unbroken
sequence, interposing between them beings which live and yet are not
animals, that scarcely any difference seems to exist between two
neighbouring groups owing to their close proximity.’[29]
[27] _De partibus animalium_, iv. 5; 681ᵅ 15.
[28] _De partibus animalium_, iv. 5; 681ᵅ 36.
[29] _De partibus animalium_, iv. 5; 681ᵅ 10.
Some approach to evolutionary doctrine is also foreshadowed by
Aristotle in his theories of the development of the individual. This is
obscured, however, by his peculiar view of the nature of procreation.
On this topic his general conclusion is that the material substance
of the embryo is contributed by the female, but that this is mere
passive formable material, almost as though it were the soil in which
the embryo grows. The male by giving the principle of life, the soul,
contributes the essential generative agency. But this _soul_ is not
material and it is, therefore, not theoretically necessary for anything
material to pass from male to female. The material which does in fact
so pass with the seed of the male is an accident, not an essential, for
the essential contribution of the male is not matter but _form_ and
_principle_. The female provides the _material_, the male the _soul_,
the _form_, the _principle_, that which makes life. Aristotle was thus
prepared to accept instances of fertilization without material contact.
‘The female does not contribute semen to generation but does contribute
something ... for there must needs be that which generates and that
from which it generates.... If, then, the male stands for the effective
and active, and the female, considered as female, for the passive, it
follows that what the female would contribute to the semen of the male
would not be semen but material for the semen to work upon....
‘How is it that the male contributes to generation, and how is it
that the semen from the male is the cause of the offspring? Does [the
semen] exist in the body of the embryo as a part of it from the first,
mingling with the material which comes from the female? Or does the
semen contribute nothing to the material body of the embryo but only to
the power and movement in it?... The latter alternative appears to be
the right one both _a priori_ and in view of the facts.’[30]
[30] _De generatione animalium_, i. 21; 729ᵅ 21.
This discussion leads to the question of the natural process of
generation itself. It is a topic that we have seen discussed by an
earlier writer who had set forth a sort of doctrine of pangenesis (see
p. 14). His view Aristotle declines to share. ‘We must’, he says, ‘say
the opposite of what the ancients said. For whereas they said that
semen is that which _comes from all_ the body, we shall say that it
is that whose nature is to _go to all_ of it, and what they thought a
waste-product seems rather to be a secretion.’ According to Aristotle
semen is derived from the same nutritive material in the blood vessels
that is distributed to the rest of the body. The semen, however, is
strained or secreted off from this nutritive material—as being its most
essential and representative portion—before the distribution actually
takes place.[31] But why, it may be asked, if the semen does not come
from the various parts of the body, is it yet able to reproduce those
various parts? The answer, on the Aristotelian view, seems to be that
the semen contains special and peculiar fractions of the nutritive
fluid which have been so modified and adapted that, if not secreted
off as semen, they would be distributed to the different parts of the
body to nourish each of these various parts. These substances have
been elaborated by the _soul_ or vital principle in a manner that is
specifically suited for each organ, hand, liver, face, heart, &c., and
from each of these specific substances a specific essence is separated
off into the semen corresponding to hand, liver, face, heart, &c., of
the offspring.
The next question that arises is the mechanism by which the offspring
come to resemble their parents. The mechanism in the case of inheritance
from the father is comprehensible when we consider the origin and
nature of the semen, but the inheritance from the mother requires
further explanation. The view of Aristotle is based upon the nature
of the catamenia and their disappearance during gestation. ‘The
catamenia’, in his view, ‘are a secretion as the semen is.’[32] The
female contributes the material by which the embryo grows and she does
this through the catamenia which are suspended during gestation for
this very purpose. The matter is thus summed up by Aristotle.
[31] _De generations animalium_, i. 18; 725ᵅ 22.
[32] _De generatione animalium_, i. 19; 727ᵅ 31.
‘The male does not emit semen at all in some animals, and where he
does, this is no part of the resulting embryo; just so no material part
comes from the carpenter to the material, i.e. to the wood in which he
works, nor does any part of the carpenter’s art exist within what he
makes, but the shape and the form are imparted from him to the material
by means of the motion he sets up. It is his hands that move his tools,
his tools that move the material; it is his knowledge of his art, and
his _soul_, in which is the form, that move his hands or any other
part of him with a motion of some definite kind, a motion varying with
the varying nature of the object made. In like manner, in the male of
those animals which emit semen, Nature uses the semen as a tool and as
possessing motion in actuality, just as tools are used in the products
of any art, for in them lies in a certain sense the motion of the
art.’[33]
‘For the same reason the development of the embryo takes place in the
female; neither the male himself nor the female emits semen into the
female, but the female receives within herself the share contributed
by both, because in the female is the material from which is made
the resulting product. Not only must the mass of material from which
the embryo is in the first instance formed exist there, but further
material must constantly be added so that the embryo may increase
in size. Therefore the birth must take place in the female. For the
carpenter must keep in close connexion with his timber and the potter
with his clay, and generally all workmanship and the ultimate movement
imparted to matter must be connected with the material concerned, as,
for instance, architecture is _in_ the buildings it makes.’[34]
[33] _De generatione animalium_, i. 22; 730ᵇ 10.
[34] _De generatione animalium_, i. 22; 730ᵅ 34.
The problem of the nature of generation is one in which Aristotle never
ceased to take an interest, and among the methods by which he sought to
solve it was embryological investigation. In his ideas on the methods
of reproduction we must seek also the main bases of such classification
of animals as he exhibits. His most important embryological researches
were made upon the chick. He asserts that the first signs of
development are noticeable on the third day, the heart being visible
as a palpitating blood-spot whence, as it develops, two meandering
blood vessels extend to the surrounding tunics.
‘Generation from the egg’, he says, ‘proceeds in an identical manner
with all birds.... With the common hen after three days and nights
there is the first indication of the embryo.... The heart appears like
a speck of blood in the white of the egg. This point beats and moves as
though endowed with life, and from it two vessels with blood in them
trend in a convoluted course ... and a membrane carrying bloody fibres
now envelops the yolk, leading off from the vessels.’[35]
Aristotle lays considerable stress on the early appearance of the heart
in the embryo. Corresponding to the general gradational view that he
had formed of Nature, he held that the most primitive and fundamentally
important organs make their appearance before the others. Among the
organs all give place to the heart, which he considered ‘the first to
live and the last to die’.[36]
A little later he observed that the body had become distinguishable,
and was at first very small and white.
[35] _Historia animalium_, vi. 3; 561ᵅ 4.
[36] _Cor primum movens ultimum moriens._ This famous sentence is the
sense though not the phrasing of _De generatione animalium_, ii. 1 and
4.
‘The head is clearly distinguished and in it the eyes, swollen out to
a great extent.... At the outset the under portion of the body appears
insignificant in comparison with the upper portion....
‘When an egg is ten days old the chick and all its parts are distinctly
visible. The head still is larger than the rest of the body and the
eyes larger than the head. At this time also the larger internal
organs are visible, as also the stomach and the arrangement of the
viscera; and the vessels that seem to proceed from the heart are now
close to the navel. From the navel there stretch a pair of vessels,
one [vitelline vein] towards the membrane that envelops the yolk,
and the other [allantoic vein] towards that membrane which envelops
collectively the membrane wherein the chick lies, the membrane of the
yolk and the intervening liquid.... About the twentieth day, if you
open the egg and touch the chick, it moves inside and chirps; and it is
already coming to be covered with down when, after the twentieth day,
the chick begins to break the shell.’[37]
[37] _Historia animalium_, vi. 3; 561ᵅ 18.
Aristotle recognized a distinction in the mode of development of
mammals from that of all other viviparous creatures. Having divided the
apparently viviparous animals into two groups, one of which is truly
and internally and the other only externally viviparous, he pointed
out that in the mammalia, the group regarded by him as internally
viviparous, the foetus is connected until birth with the wall of
the mother’s womb by the navel-string. These animals, in his view,
produce their young without the intervention of an ovum, the embryo
being ‘living from the first’. Such non-mammals, on the other hand, as
are viviparous are so in the external sense only, that is, the young
which he considered to arise in this group from ova may indeed develop
within the mother’s womb and be born alive, but they go through their
development without organic connexion with the mother’s body, so that
her womb acts but as a nursery or incubator for her eggs. It was indeed
a sort of accident among the ovipara whether in any particular species
the ovum went through its development inside or outside the mother’s
body. ‘Some of the ovipara’, he says, ‘produce the egg in a perfect,
others in an imperfect state, but it is perfected outside the body as
has been stated of fish.’[38]
Yet though Aristotle regarded fish as an oviparous group, he knew
also of kinds of fish that were externally viviparous. It is most
interesting to observe, moreover, that he was acquainted with one
particular instance among fish in which matters were less simple and
in which the development bore an analogy to that of the mammalia, his
true internal vivipara. ‘Some animals’, he says, ‘are viviparous,
others oviparous, others vermiparous. Some are viviparous, such as man,
the horse, the seal and all other animals that are hair-coated, and,
of marine animals, the Cetaceans, as the dolphin, _and the so-called
Selachia_.’[39]
Aristotle tells us elsewhere that a species of these Selachia which
he calls _galeos_—a name still used for the dog-fish by Greek
fishermen—‘has its eggs in betwixt the [two horns of the] womb; these
eggs shift into each of the two horns of the womb and descend, and the
young develop with the navel-string attached to the womb, so that,
as the egg-substance gets used up, the embryo is sustained to all
appearances just as in quadrupeds. The navel-string is ... attached
as it were by a sucker, and also to the centre of the embryo in the
place where the liver is situated.... Each embryo, as in the case of
quadrupeds, is provided with a chorion and separate membranes.’[40]
[38] _De generatione animalium_, iii. 9; 758ᵅ 37.
[39] _Historia animalium_, i. 5; 489ᵅ 35.
[40] _Historia animalium_, vi. 10; 565ᵇ 2.
The remarkable anatomical relationship of the embryo of _Galeus_
(_Mustelus_) _laevis_ to its mother’s womb was little noticed by
naturalists until the whole matter was taken up by Johannes Müller
about 1840.[41] That great observer demonstrated the complete accuracy
of Aristotle’s description and the justice of his comparison to and
contrast with the mammalian mode of development.[42] The work of
Johannes Müller at once had the effect of drawing the attention of
naturalists to the importance and value of the Aristotelian biological
observations.
[41] The history of this discovery is given by Charles Singer, _Studies
in the History and Method of Science_, vol. ii, Oxford, 1921, pp. 32 ff.
[42] Johannes Müller, _Ueber den glatten Hai des Aristoteles_, Berlin,
1842.
Aristotle attempts to explain the viviparous character of the
Selachians. His explanation has perhaps little meaning for the modern
biologist, just as many of our scientific explanations will seem
meaningless to our successors. But such explanations are often worth
consideration not only as stages in the historical development of
scientific thought, but also as illustrating the fact that while the
ultimate object of science is a _description_ of nature, the immediate
motive of the best scientific work is usually an _explanation_ of
nature. Yet it is usually the descriptive, not the explanatory element
that bears the test of time.
‘Birds and scaly reptiles’, says Aristotle, ‘because of their heat
produce a perfect egg, but because of their dryness it is only an
egg. The cartilaginous fishes have less heat than these but more
moisture, so that they are intermediate, for they are both oviparous
and viviparous within themselves, the former because they are cold, the
latter because of their moisture; for moisture is vivifying, whereas
dryness is farthest removed from what has life. Since they have neither
feathers nor scales such as either reptiles or other fishes have, all
of which are signs rather of a dry and earthy nature, the egg they
produce is soft; for the earthy matter does not come to the surface in
their eggs any more than in themselves. That is why they lay eggs in
themselves, for if the egg were laid externally it would be destroyed,
having no protection.’[43]
[43] _De generatione animalium_, ii. 1; 733ᵅ 6.
This explanation is based on Aristotle’s fundamental doctrine of the
opposite _qualities_, heat, cold, wetness, and dryness, that are found
combined in pairs in the four _elements_, earth, air, fire, and water.
The theory was of the utmost importance for the whole subsequent
development of science and was not displaced until quite modern
times. It was not an original conception of Aristotle, for something
resembling it had been set forth long before his time in figurative
language by Empedocles (_c._ 500-_c._ 430 B. C.), as Aristotle himself
tells us.[44] The same view had been foreshadowed by Pythagoras (_c._
580-_c._ 490 B. C.) at an even earlier date and was perhaps of much
greater antiquity. But Aristotle developed the doctrine and was the
main channel for its conveyance to later ages, so that his name will
always be associated with it. Matter in general and living matter in
particular was held by him to be composed of these four essential
so-called _elements_ (στοιχεῑ), each of which is in turn compounded
from two of the primary _qualities_ (δυνάμεις) which Aristotle brought
into relation with the elements. Thus earth was cold and dry, water
cold and wet, air hot and wet, and fire hot and dry (Fig. 7b).
[44] _Metaphysics_, i. 4. _De generatione et corruptione_, ii. 1.
[Illustration: Fig. 7b. The Four Elements and the Four Qualities.]
The theory of the elements and qualities is applicable to all matter
and not specially to living things. The distinction between the
living and not-living is to be sought not so much in its material
constitution, but in the presence or absence of ‘soul’, and his
teaching on that topic is to be found in his great work περὶ ψυχῆς,
_On Soul_. He does not think of matter as organic or inorganic—that
is a distinction of the seventeenth century physiologists—nor does he
think of things as divided into animal, vegetable, and mineral—that is
a distinction of the mediaeval alchemists,—but he thinks of things as
either with soul or without soul (ἔμψυχα or ἄαψυχα).
His belief as to the relationship of this soul to material things is a
difficult and complicated subject which would take us far beyond the
topics included in biological writings to-day, but he tells us that
‘there is a class of existent things which we call substance, including
under that term, firstly, matter, which in itself is not this nor that;
secondly, shape or form, in virtue of which the term this or that is
at once applied; thirdly, the whole made up of matter and form. Matter
is identical with potentiality, form with actuality,’ the soul being,
in living things, that which gives the form or actuality. ‘Of natural
bodies’, he continues, ‘some possess life and some do not: where by
life we mean the power of self-nourishment and of independent growth
and decay’.[45] It should here be noted that in the Aristotelian sense
the ovum is not at first a living thing, for in its earliest stage
and before fertilization it does not possess soul even in its most
elementary form.
[45] _De anima_, ii. 1, ii.