(3) Aristotle's greatest service to morphology is his clear recognition of the unity of plan holding throughout each of the great groups.
He recognises this most clearly in the case of man and the viviparous quadrupeds, with whose structure he was best acquainted. In the Historia Animalium he takes man as a standard, and describes his external and internal parts in detail, then considers viviparous quadrupeds and compares them with man. "Whatever parts a man has before, a quadruped has beneath; those that are behind in man form the quadruped's back" (Cresswell, loc. cit., p. 26). Apes, monkeys, and Cynocephali combine the characteristics of man and quadrupeds. He notices that all viviparous quadrupeds have hair. Oviparous quadrupeds resemble the viviparous, but they lack some organs, such as ears with an external pinna, mammæ, hair. Oviparous bipeds, or birds, also "have many parts like the animals described above." He does not, however, seem to realise that a bird's wings are the equivalent of a mammal's arms or fore-legs. Fishes are much more divergent; they possess no neck, nor limbs, nor testicles (meaning a solid ovoid body such as the testis in mammals), nor mammæ. Instead of hair they have scales.
Speaking generally, the Sanguinea differ from man and from one another in their parts, which may be present or absent, or exhibit differences in "excess and defect," or in form. Unity of plan extends to all the principal systems of organs. "All sanguineous animals have either a bony or a spinous column. The remainder of the bones exist in some animals; but not in others, for if they have the limbs they have the bones belonging to them" (Cresswell, loc. cit., p. 60). "Viviparous animals with blood and feet do not differ much in their bones, but rather by analogy, in hardness, softness, and size" (Cresswell, loc. cit., p. 59). The venous system, too, is built upon the same general plan throughout the Sanguinea. "In all sanguineous animals, the nature and origin of the principal veins are the same, but the multitude of smaller veins is not alike in all, for neither are the parts of the same nature, nor do all possess the same parts" (Cresswell, loc. cit., p. 56). It will be noticed in the first and last of these three quotations that Aristotle recognises the fact of correlation between systems of organs—between limbs and bones, and between blood-vessels and the parts to which they go.
Sanguineous animals all possess certain organs—heart, liver, spleen, kidneys, and so on. Other organs occur in most of the classes—the œsophagus and the lungs. "The position which these parts occupy is the same in all animals [sc. Sanguinea]" (Cresswell, loc. cit., p. 39).
Unity of plan is observable not only in the Sanguinea, but also within each of the other large groups. Aristotle recognises that all his cuttlefish are alike in structure. Among his Malacostraca he compares point by point the external parts of the carabus (Palinurus), and the astacus (Homarus), and he compares also the general internal anatomy of the various "genera" he distinguishes. As regards Testacea, he writes, "The nature of their internal structure is similar in all, especially in the turbinated animals, for they differ in size and in the relations of excess; the univalves and bivalves do not exhibit many differences" (Cresswell, loc. cit., p. 83). There is an interesting remark about "the creature called carcinium" (hermit-crab), that it "resembles both the Malacostraca and the Testacea, for this in its nature is similar to the animals that are like carabi, and it is born naked" (Cresswell, loc. cit., p. 85). In the last phrase we may perhaps read the first recognition of the embryological criterion.
With the recognition of unity of plan within each group necessarily goes the recognition of what later morphology calls the homology of parts. The parts of a horse can be compared one by one with the parts of another viviparous quadruped; in all the animals belonging to the same class the parts are the same, only they differ in excess or defect—these remarks are placed in the forefront of the Historia Animalium. Generally speaking, parts which bear the same name are for Aristotle homologous throughout the class. But he goes further and notes the essential resemblance underlying the differences of certain parts. He classes together nails and claws, the spines of the hedgehog, and hair, as being homologous structures. He says that teeth are allied to bones, whereas horns are more nearly allied to skin (Hist. Anim., iii.). This is an astonishingly happy guess, considering that all he had to go upon was the observation that in black animals the horns are black but the teeth white. One cannot but admire the way in which Aristotle fixes upon apparently trivial and commonplace facts, and draws from them far-reaching consequences. He often goes wrong, it is true, but he always errs in the grand manner.
While Aristotle certainly recognised the existence of homologies, and even had a feeling for them, he did not clearly distinguish homology from analogy. He comes pretty near the distinction in the following passage. After explaining that in animals belonging to the same class the parts are the same, differing only in excess or defect, he says, "But some animals agree with each other in their parts neither in form nor in excess and defect, but have only an analogous likeness, such as a bone bears to a spine, a nail to a hoof, a hand to a crab's claw, the scale of a fish to the feather of a bird, for that which is a feather in the bird is a scale in the fish" (Cresswell, loc. cit., p. 2). One of these comparisons is, however, a homology not an analogy, and the last phrase throws a little doubt upon the whole question, for it is not made clear whether it is position or function that determines what are equivalent organs.
In the De Partibus Animalium there occurs the following passage:—"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."[8] It is thus similarity in form and structure which determines the formation of the main groups. Within each group the parts differ only in degree, in largeness or smallness, softness and hardness, smoothness or roughness, and the like (loc. cit., i., 4, 644b). These passages show that Aristotle had some conception of homology as distinct from analogy. He did not, however, develop the idea. What Aristotle sought in the variety of animal structure, and what he found, were not homologies, but rather communities of function, parts with the same attributes. His interest was all in organs, in functioning parts, not in the mere spatial relationship of parts.
This comes out clearly in his treatise On the Parts of Animals, which is subsequent to, and the complement of, his History of Animals. The latter is a description of the variety of animal form, the former is a treatise on the functions of the parts. He describes the plan of the De Partibus Animalium as follows:—"We have, then, first to describe the common functions, common, that is, to the whole animal kingdom, or to certain large groups, or to 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" (i, 5, 645b, trans. Ogle). The alimentary canal is a good example of a part which is "analogous" throughout the animal kingdom, for "all animals possess in common those parts by which they take in food, and into which they receive it" (Cresswell, loc. cit., p. 6).
The De Partibus Animalium becomes in form a comparative organography, but the emphasis is always on function and community of function. Thus he treats of bone, "fish-spine," and cartilage together (De Partibus, ii., 9, 655a), because they have the same function, though he says elsewhere that they are only analogous structures (ii., 8, 653b). In the same connection he describes also the supporting tissues of Invertebrates—the hard exoskeleton of Crustacea and Insects, the shell of Testacea, the "bone" of Sepia (ii., 8, 654a). Aristotle took much more interest in analogies, in organs of similar function, than in homologies. He did recognise the existence of homologies, but rather malgré lui, because the facts forced it upon him.