About the time that Hippocrates died, Aristotle, who may be regarded as the founder of the science of "Natural History," was born (B.C. 384) in Stagira, an unimportant Hellenic colony in Thrace, near the Macedonian frontier. His father was a distinguished physician, and, like Hippocrates, boasted descent from the AsclepiadÆ. The importance attached by the Asclepiads to the habit of physical observation, which has been already referred to in the life of Hippocrates, secured for Aristotle, from his earliest years, that familiarity with biological studies which is so clearly evident in many of his works.

Both parents of Aristotle died when their son was still a youth, and in consequence of this he went to reside with Proxenus, a native of Atarneus, who had settled at Stagira. Subsequently he went to Athens and joined the school of Plato. Here he remained for about twenty years, and applied himself to study with such energy that he became pre-eminent even in that distinguished band of philosophers. He is said to have been spoken of by Plato as "the intellect" of the school, and to have been compared by him to a spirited colt that required the application of the rein to restrain its ardour.

Aristotle probably wrote at this time some philosophical works, the fame of which reached the ears of Philip, King of Macedonia, and added to the reputation which the young philosopher had already made with that monarch; for Philip is said to have written to him on the occasion of Alexander's birth, B.C. 356: "King Philip of Macedonia to Aristotle, greeting. Know that a son has been born to me. I thank the gods not so much that they have given him to me, as that they have permitted him to be born in the time of Aristotle. I hope that thou wilt form him to be a king worthy to succeed me and to rule the Macedonians."

After the death of Plato, which occurred in 347 B.C., Aristotle quitted Athens and went to Atarneus, where he stayed with Hermias, who was then despot of that town. Hermias was a remarkable man, who, from being a slave, had contrived to raise himself to the supreme power. He had been at Athens and had heard Plato's lectures, and had there formed a friendship for Aristotle. With this man the philosopher remained for three years, and was then compelled suddenly to seek refuge in Mitylene, owing to the perfidious murder of Hermias. The latter was decoyed out of the town by the Persian general, seized and sent prisoner to Artaxerxes, by whom he was hanged as a rebel. On leaving Atarneus, Aristotle took with him a niece of Hermias, named Pythias, whom he afterwards married. She died young, leaving an infant daughter.

Two or three years after this, Aristotle became tutor to Alexander, who was then about thirteen years old. The philosopher seems to have been a favourite with both the king and the prince, and, in gratitude for his services, Philip rebuilt Stagira and restored it to its former inhabitants, who had either been dispersed or carried into slavery. The king is said also to have established there a school for Aristotle. The high respect in which Alexander held his teacher is expressed in his saying that he honoured him no less than his own father, for while to one he owed life, to the other he owed all that made life valuable.

In 336 B.C. Alexander, who was then only about twenty years of age, became king, and Aristotle soon afterwards quitted Macedonia and took up his residence in Athens once more, after an absence of about twelve years. Here he opened a school in the LycÆum, a gymnasium on the eastern side of the city, and continued his work there for about twelve years, during which time Alexander was making his brilliant conquests. The lectures were given for the most part while walking in the garden, and in consequence, perhaps, of this, the sect received the name of the Peripatetics. The discourses were of two kinds—the esoteric, or abstruse, and the exoteric, or familiar; the former being delivered to the more advanced pupils only. During the greater part of this time Aristotle kept up correspondence with Alexander, who is said[3] to have placed at his disposal thousands of men, who were busily employed in collecting objects and in making observations for the completion of the philosopher's zoological researches. Alexander is, moreover, said to have given the philosopher eight hundred talents for the same purpose.

In spite of these marks of friendship and respect, Alexander, who was fast becoming intoxicated with success, and corrupted by Asiatic influences, gradually cooled in his attachment towards Aristotle. This may have been hastened by several causes, and among others by the freedom of speech and republican opinions of Callisthenes, a kinsman and disciple of Aristotle, who had been, by the latter's influence, appointed to attend on Alexander. Callisthenes proved so unpopular, that the king seems to have availed himself readily of the first plausible pretext for putting him to death, and to have threatened his former friend and teacher with a similar punishment. The latter, for his part, probably had a deep feeling of resentment towards the destroyer of his kinsman.

Meanwhile the Athenians knew nothing of these altered relations between Aristotle and Alexander, but continued to regard the philosopher as thoroughly imbued with kingly notions (in spite of his writings being quite to the contrary); so that he was an object of suspicion and dislike to the Athenian patriots. Nevertheless, as long as Alexander was alive, Aristotle was safe from molestation. As soon, however, as Alexander's death became known, the anti-Macedonian feeling of the Athenians burst forth, and found a victim in the philosopher. A charge of impiety was brought against him. It was alleged that he had paid divine honours to his wife Pythias and to his friend Hermias. Now, for the latter, a eunuch, who from the rank of a slave had raised himself to the position of despot over a free Grecian community, so far from coupling his name (as Aristotle had done in his hymn) with the greatest personages of Hellenic mythology, the Athenian public felt that no contempt was too bitter. To escape the storm the philosopher retired to Chalcis, in Euboea, then under garrison by Antipater, the Governor of Macedonia, remarking in a letter, written afterwards, that he did so in order that the Athenians might not have the opportunity of sinning a second time against philosophy (the allusion being, of course, to the fate of Socrates).

He probably intended to return to Athens again so soon as the political troubles had abated, but in September, 322 B.C., he died at Chalcis. An overwrought mind, coupled with indigestion and weakness of the stomach, from which he had long suffered, was most probably the cause of death. Some of his detractors, however, have asserted that he took poison, and others that he drowned himself in the Euboean Euripus.

It is not easy to arrive at a just estimate of the character of Aristotle. By some of his successors he has been reproached with ingratitude to his teacher, Plato; with servility to Macedonian power, and with love of costly display. How far these two last charges are due to personal slander it is impossible to say. The only ground for the first charge is, that he criticised adversely some of Plato's doctrines.

The manuscripts of Aristotle's works passed through many vicissitudes. At the death of the philosopher they were bequeathed to Theophrastus, who continued chief of the Peripatetic school for thirty-five years. Theophrastus left them, with his own works, to a philosophical friend and pupil, Neleus, who conveyed them from Athens to his residence at Scepsis, in Asia Minor. About thirty or forty years after the death of Theophrastus, the kings of Pergamus, to whom the city of Scepsis belonged, began collecting books to form a library on the Alexandrian plan. This led the heirs of Neleus to conceal their literary treasures in a cellar, and there the manuscripts remained for nearly a century and a half, exposed to injury from damp and worms. At length they were sold to Apellicon, a resident at Athens, who was attached to the Peripatetic sect. Many of the manuscripts were imperfect, having become worm-eaten or illegible. These defects Apellicon attempted to remedy; but, being a lover of books rather than a philosopher, he performed the work somewhat unskilfully. When Athens was taken by Sylla, 86 B.C., the library of Apellicon was transported to Rome. There various literary Greeks obtained access to it; and, among others, Tyrannion, a grammarian and friend of Cicero, did good service in the work of correction. Andronicus of Rhodes afterwards arranged the whole into sections, and published the manuscripts with a tabulated list.

The three principal works on biology which are extant are: "The History of Animals;" "On the Parts of Animals;" "On the Generation of Animals." The other biological works are: "On the Motion of Animals;" "On Respiration;" "Parva Naturalia;"—a series of essays which are planned to form an entire work on sense and the sensible.

"The History of Animals" is the largest and most important of Aristotle's works on biology. It contains a vast amount of information, not very methodically arranged, and spoiled by the occurrence here and there of very gross errors. It consists of nine books.

The first book opens with a division of the body into similar and dissimilar parts. Besides thus differing in their parts, animals also differ in their mode of life, their actions and dispositions. Thus some are aquatic, others terrestrial; of the former, some breathe water, others air, and some neither. Of aquatic animals, some inhabit the sea, and others rivers, lakes, or marshes. Again, some animals are locomotive, and others are stationary. Some follow a leader, others act independently. Various differences are in this way pointed out, and there is no lack of illustration and detail, but a suspicion is excited that the generalizations are sometimes based upon insufficient facts. The book closes with a description of the different parts of the human body, both internal and external. In speaking of the ear, Aristotle seems to have been aware of what we now call the Eustachian tube, for he says, "There is no passage from the ear into the brain, but there is to the roof of the mouth."[4]

In the second book he passes on to describe the organs of animals. The animals are dealt with in groups—viviparous and oviparous quadrupeds, fish, serpents, birds, etc. The ape, elephant, chameleon, and some others are especially noticed.

The third book continues the description of the internal organs. References which are made to a diagram by letters, a, b, c, d, show that the work was originally illustrated. At the close of this book Aristotle has some remarks on milk, and mentions the occasional appearance of milk in male animals. He speaks of a male goat at Lemnos which yielded so much that cakes of cheese were made from it. Similar instances of this phenomenon have been recorded by Humboldt, Burdach, Geoffroy St. Hilaire, and others.

In the first four chapters of the fourth book the anatomy of the invertebrata is dealt with, and the accounts given of certain mollusca and crustacea are very careful and minute. The rest of the book is devoted to a description of the organs of sense and voice; of sleep, and the distinctions of sex. The accurate knowledge which Aristotle exhibits of the anatomy and habits of marine animals, such as the Cephalopoda and the larger Crustacea, leaves no doubt that he derived it from actual observation. Professor Owen says, "Respecting the living habits of the Cephalopoda, Aristotle is more rich in detail than any other zoological author." What is now spoken of as the hectocotylization of one or more of the arms of the male cephalopod did not escape Aristotle's eye. And while he speaks of the teeth and that which serves these animals for a tongue, it is plain from the context that he means in the one case the two halves of the parrot-like beak, and in the other the anterior end of the odontophore.

Books five to seven deal with the subject of generation.

The eighth book contains a variety of details respecting animals, their food, migrations, hibernation, and diseases; with the influence of climate and locality upon them.

The ninth book describes the habits and instincts of animals. The details are interesting; but there is, as usual, very little attempt at classification. Disjointed statements and sudden digressions occur, the subjects being treated in the order in which they presented themselves to the author. Such curious statements as the following are met with: "The raven is an enemy to the bull and the ass, for it flies round them and strikes their eyes." "If a person takes a goat by the beard, all the rest of the herd stand by, as if infatuated, and look at it." "Female stags are captured by the sound of the pipe and by singing. When two persons go out to capture them, one shows himself, and either plays upon a pipe or sings, and the other strikes behind, when the first gives him the signal." "Swans have the power of song, especially when near the end of their life; for they then fly out to sea, and some persons sailing near the coast of Libya have met many of them in the sea singing a mournful song, and have afterwards seen some of them die." "Of all wild animals, the elephant is the most tame and gentle; for many of them are capable of instruction and intelligence, and they have been taught to worship the king."

In the work "On the Parts of Animals," the author considers not only the phenomena of life exhibited by each species, but also the cause or causes to which these phenomena are attributable. After a general introduction, he proceeds to enumerate the three degrees of composition, viz.:—

These divisions correspond roughly to the threefold study of structure which we nowadays recognize as chemical, histological, and anatomical.

As examples of Aristotle's method of treatment, his descriptions of blood, the brain, the heart, and the lung may be considered.

Of the blood he says, "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.... Too great an excess of water makes animals timorous.... Such animals, on the other hand, as have thick and abundant fibres in their blood are of a more choleric temperament, and liable to bursts of passion.... Bulls and boars are choleric, 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 are taken out of the blood, the fluid that remains will no longer coagulate."

From these quotations it will be noted that Aristotle attributed the coagulum to the presence of fibres, and in this he anticipated Malpighi's discovery made in the seventeenth century. His remarks on the proportion of coagulum and serum in different animals, which is enlarged upon in the "History of Animals,"[5] harmonize with modern observations. In another of his works[6] he remarks that the blood in certain diseased conditions will not coagulate. This is known to be the case in cholera, certain fevers, asphyxia, etc.; and the fact was probably obtained from Hippocrates. Although Aristotle speaks here of entire absence of coagulation in the blood of the deer and the roe, in the "History of Animals" he admits an imperfect coagulation, for he says, "so that their blood does not coagulate like that of other animals." The animals named are commonly hunted, and it was probably after they had been hunted to death that he examined them. Now, it is generally admitted that coagulation under such circumstances is imperfect and even uncommon. The statement as to the richness in fibres of the blood of bulls and boars has been confirmed by some modern investigations, which have shown that the clot bears a proportion to the strength and ferocity of the animal. The remarks, however, as to the relative rapidity of coagulation would appear to be contradicted by later observations, for Thackrah came to the conclusion that coagulation commenced sooner in small and weak animals than in strong.

Of the brain Aristotle makes the following among other assertions: "Of all parts of the body there is none so cold as the brain.... Of all the fluids of the body it is the one that has the least blood, for, in fact, it has no blood at all in its proper substance.... That it has no continuity with the organs of sense is plain from simple inspection, and still more closely shown by the fact that when it is touched no sensation is produced.... The brain tempers the heat and seething of the heart.... In order that it may not itself be absolutely without heat, blood-vessels from the aorta end in the membrane which surrounds the brain.... 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.... Man again has more sutures in his skull than any other animal, and the male more than the female. The explanation is to be found in the greater size of the brain, which demands free ventilation proportionate to its bulk.... There is no brain in the hinder part of the head.... The brain in all animals that have one is placed in the front part of the head ... because the heart, from which sensation proceeds, is in the front part of the body."

Although it would perhaps be difficult to find anywhere as many errors in as few words, yet it should be observed that Aristotle here shows himself to have been aware of the existence of the membranes of the brain—the pia mater and the dura mater; and elsewhere[7] he says more explicitly, "Two membranes enclose the brain; that about the skull is the stronger; the inner membrane is slighter than the outer one." And further, it should be noted that he describes the latter membrane as a vascular one. The fact of the brain substance being insensible to mechanical irritation was known to Aristotle, and may have been learnt from the practice of Hippocrates. Lastly, it should be remembered that—though this may have been but a lucky guess on Aristotle's part—the relative weight of brain to the entire body has been shown, with few exceptions, to be greater in man than in any other animal.

In describing the heart Aristotle says: "The heart lies about the centre of the body, but rather in its upper than in its lower half, and also more in front than behind.... In man it inclines a little towards the left, so that it may counterbalance the chilliness of that side. It is hollow, to serve for the reception of the blood; while its wall is thick, that it may serve to protect the source of heat. For here, and here alone, in all the viscera, and in fact in all the body, there is blood without blood-vessels, the blood elsewhere being always contained within vessels. The heart is the first of all the parts of the body to be formed, and no sooner is it formed than it contains blood.... For no sooner is the embryo formed than its heart is seen in motion like a living creature, and this before any of the other parts. The heart is abundantly supplied with sinews.... In no animal does the heart contain a bone, certainly in none of those that we ourselves have inspected, with the exception of the horse and a certain kind of ox. In animals of great size the heart has three cavities; in smaller animals it has two; and in all it has at least one."

It will be observed that here Aristotle so correctly describes the position of the human heart as to render it probable that he is speaking from actual inspection; although man is not the only animal in which the heart is turned towards the left. In contrasting the heart with the other viscera he appears to have overlooked the existence of the coronary vessels, and to have imagined that the nutrition of the heart was effected directly by the blood in its cavities. Although the heart is not really the first part to appear, the observation of its very early appearance in the embryo, which he treats more fully elsewhere,[8] is alone enough to establish his reputation as an original observer. It is remarkable that Aristotle should have overlooked the presence of the valves of the heart, the structure and functions of which were fully investigated within thirty years of his death by the anatomists of the Alexandrian school. This is the more remarkable, as he calls attention here, and in the "History of Animals," to the sinews or tendons (?e??a) with which, he says, the heart is supplied, and by which he probably meant chiefly the chordÆ tendineÆ. The "bone in the heart" of which he speaks was probably the cruciform ossification which is normally found in the ox and the stag below the origin of the aorta. It is found in the horse only in advanced age, or under abnormal conditions. The statement that the heart contains no more than three chambers has always been considered as a very gross blunder on the part of Aristotle. Even Cuvier, who generally lavishes upon the philosopher the most extravagant praise, sneers at this. Professor Huxley,[9] however, has shown, by a comparison of several passages from the "History of Animals," that what we now call the right auricle was regarded by the author as a venous sinus, as being a part not of the heart, but of the great vein (i.e. the superior and the inferior venÆ cavÆ).

Aristotle speaks of the lung as a single organ, sub-divided, but having a common outlet—the trachea. Elsewhere[10] he says, "Canals from the heart pass to the lung and divide in the same fashion as the windpipe does, closely accompanying those from the windpipe through the whole lung." His theory of respiration, as explained in his treatise on the subject, is that it tempers the excessive heat produced in the heart. The lung is compared to a pair of bellows. When the lung is expanded, air rushes in; when it is contracted, the air is expelled. The heat from the heart causes the lung to expand—cold air rushes in, the heat is reduced, the lung collapses, and the air is expelled. The cold air drawn into the lung reaches the bronchial tubes, and as the vessels containing hot blood run alongside these tubes, the air cools it and carries off its superfluous heat. Some of the air which enters the lung gets from the bronchial tubes into the blood-vessels by transudation, for there is no direct communication between them; and this air, penetrating the body, rapidly cools the blood throughout the vessels. But Aristotle did not consider the "pneuma," which thus reached the interior of the blood-vessels, to be exactly the same thing as air—it was "a subtilized and condensed air."[11] And this we now know to be oxygen.

The treatise "On the Generation of Animals" is an extraordinary production. "No ancient and few modern works equal it in comprehensiveness of detail and profound speculative insight. We here find some of the obscurest problems of biology treated with a mastery which, when we consider the condition of science at that day, is truly astounding. That there are many errors, many deficiencies, and not a little carelessness in the admission of facts, may be readily imagined; nevertheless at times the work is frequently on a level with, and occasionally even rises above, the speculations of many advanced embryologists."[12]

It commences with the statement that the present work is a sequel to that "On the Parts of Animals;" and first the masculine and feminine principles are defined. The masculine principle is the origin of all motion and generation; the feminine principle is the origin of the material generated. Aristotle's philosophy of nature was teleological, and the imperfect character of his anatomical knowledge often gives him occasion to explain particular phenomena by final causes. Thus animals producing soft-shelled eggs (e.g. cartilaginous fish and vipers) are said to do so because they have so little warmth that the external surface of the egg cannot be dried.

Among insects, some (e.g. grasshopper, cricket, ant, etc.) produce young in the ordinary way, by the union of the sexes; in other cases (e.g. flies and fleas) this union of the sexes results in the production of a skolex; while others have no parents, nor do they have congress—such are the ephemera, tipula, and the like. Aristotle discusses and rejects the theory that the male reproductive element is derived from every part of the body. He concludes that "instead of saying that it comes from all parts of the body, we should say that it goes to them. It is not the nutrient fluid, but that which is left over, which is secreted. Hence the larger animals have fewer young than the smaller, for by them the consumption of nutrient material will be larger and the secretion less. Another point to be noticed is, that the nutrient fluid is universally distributed through the body, but each secretion has its separate organ.... It is thus intelligible why children resemble their parents, since that which makes all the parts of the body, resembles that which is left over as secretion: thus the hand, or the face, or the whole animal pre-exists in the sperm, though in an undifferentiated state (?d????st??); and what each of these is in actuality (??e??e??), such is the sperm in potentiality (d???e?)."

In later times the two great rival theories put forward to account for the development of the embryo have been—

(a) The theory of Evolution, which makes the embryo pre-existent in the germ, and only rendered visible by the unfolding and expansion of its organs.

(b) The theory of Epigenesis, which makes the embryo arise, by a series of successive differentiations, from a simple homogeneous mass into a complex heterogeneous organism.