‘The term life is used in various senses, and, if life is present in but a single one of these senses, we speak of a thing as living. Thus there is intellect, sensation, motion from place to place and rest, the motion concerned with nutrition, and, further, [there are the processes of] decay and growth,’ all various meanings or at least exhibitions of some form of life. Hence even ‘plants are supposed to have life, for they have within themselves a faculty and principle whereby they grow and decay.... They grow and continue to live so long as they are capable of absorbing nutriment. This form of life can be separated from the others ... and plants have no other faculty of soul at all,’ but only this lowest vegetative soul. ‘It is then in virtue of this principle that all living things live, whether animals or plants. But it is sensation which primarily constitutes the animal. For, provided they have sensation, even those creatures that are devoid of movement and do not change their place are called animals.... As the nutritive faculty may exist without touch or any form of sensation, so also touch may exist apart from other senses.’[46] Apart from these two lower forms of soul, the vegetative or nutritive and reproductive and the animal or sensitive, stands the rational or intellectual soul peculiar to man, a form of soul with which we shall here hardly concern ourselves.[47]
The possession of one or more of the three types of soul, vegetative, sensitive, and rational, provides in itself a basis for an elementary form of arrangement of living things in an ascending scale. We have already seen that Aristotle certainly describes something resembling a ‘Scala Naturae’ and that such a scheme can easily be drawn up from passages in his works. It may, however, be doubted whether his phraseology is capable of extension so as to include a true classification of animals in any modern sense. It is true that he repeatedly divides animals into classes, Sanguineous and Non-sanguineous, Oviparous and Viviparous, Terrestrial and Aquatic, &c., but his divisions are for the most part simply dichotomic. He certainly defines a few groups of animals as the Lophura (Equidae), the Cete (Cetacea), and the Selache (Elasmobranchiae together with the Lophiidae) in a way that fairly corresponds to similar groups in later systems. In most cases, however, his definitions are not exact enough for modern needs, for the same animal may fall into more than one of his classes and widely different animals into the same class. Thus he invents a category Carcharodonta for animals with sharp interlocking teeth and includes in it carnivores, reptiles, and fish; again, the horse kind must be included both among his Anepallacta or animals having flat crowned teeth as well as among the Amphodonta or animals with front teeth in both jaws. Such words as these are really terms of description, not of classification in the modern biological sense of that word.
There are, however, scattered through the biological works, certain terms which are applied to animal groups and organs and are defined in such a way as to suggest that they might ultimately have been developed for classificatory purposes. Thus his lowest group is the species. ‘The individuals comprised within a single species (εîδος) ... 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 and for all.’[48] This is surely not very far removed from the modern biological conception of a species.
‘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, 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 genus (γένος). For within the limits of each such genus, 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 they 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.’[49]
The Aristotelian genus thus differs widely from the term as used in modern biology. In another passage he comes nearer to defining it and the analogy of parts which extends from genus to genus.
‘Groups that differ only in the degree, and in the more or less of an identical element that they possess are aggregated together under a single genus; 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.’[50]
Aristotle nowhere gives to his term genus a rigid application that can be applied throughout the animal kingdom. He uses the word in fact much as we should use the conveniently flexible term group, now for a larger and less definite, now for a smaller and more definite collection of species. This varying use of a technical word makes it impossible to draw up a classification based on his genera or indeed with any consistent use of the terms which he actually employs.
The difficulty or impossibility of drawing up a satisfactory classificatory system from the Aristotelian writings has not, however, deterred numerous naturalists and scholars from making the attempt, and the subject has in itself a considerable history and literature[51] extending from the days of Edward Wotton (1492-1555) downward.[52] The more recent efforts at drawing up an Aristotelian classificatory system have been based on the methods of reproduction to which he certainly attached very great importance.[53] Provided that it be remembered that Aristotle does not himself detail any such system there can be no harm in constructing one from his works. At worst it will serve as a memoria technica for the extent and character of his knowledge of natural history, and at best it may represent a scheme to which he was tending.