Part 6

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.’

‘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.’

De generatione animalium, i. 22; 730ᵇ 10.

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.’

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’.

A little later he observed that the body had become distinguishable, and was at first very small and white.

Historia animalium, vi. 3; 561ᵅ 4.

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.’

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.’

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.’

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.’

De generatione animalium, iii. 9; 758ᵅ 37.

Historia animalium, i. 5; 489ᵅ 35.

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. 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. 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.

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.

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.’

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. 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).

Metaphysics, i. 4. De generatione et corruptione, ii. 1.