inous fishes [in reference to the viviparity of certain sharks]. (Book I., chap. V.) Of quadrupeds which have blood and are viviparous, some are (as to their extremities) many-cloven, as the hands and feet of man. For some are many-toed, as the lion, the dog, the panther; some are bifid, and have hoofs instead of nails, as the sheep, the goat, the elephant, the hippotamus; and some have undivided feet, as the solid hoofed animals, the horse and ass. The swine kind share both characters. An allusion to the "mule footed" swine, monstrosities in which the median digits are fused and terminate in a solid composite hoof. (Bood II., chap. V.)
Ray and later writers probably had this passage in mind when they used the descriptive terms "multifido," "bifido," "solidungula," "ungulata," "unguicnlata," "fissipedes." Here also attention is directed to the feet as exhibiting characteristic differences.
Animals have also great differences in the teeth both when compared with each other and with man. For all quadrupeds which have blood and are viviparous have teeth. And in the first place some are ambidenta[1] (having teeth in both jaws); and some are not so, wanting the front teeth in the upper jaw.[2] Some have neither front teeth nor horns, as the camel; some have tusks, as the boar; some have not. Some have serrated teeth,[3] as the lion, the panther, the dog; some have the teeth unvaried,[4] as the horse and the ox; for the animals which vary their cutting teeth have all serrated teeth. No animal has both tusks and horns; nor has any animal with serrated teeth either of those weapons. The greater part have the front teeth cutting, and those within broad (chap. II.).
This passage evidently directed the attention of later writers to the importance of the teeth as a means of distinguishing and hence of classifying mammals, and we shall see that Ray and, later, Linnaeus were quick to avail themselves of the suggestion.
Although Whewell[5] proves that Aristotle was quite unconscious of the classification that has been ascribed to him, he also admits that "Aristotle does show, as far as could be done at his time, a perception of the need of groups, and of names of groups, in the study of the animal kingdom; and thus may justly be held up as the great figure in the Prelude to the Formation of Systems which took place in more advanced scientific times."
Whewell quotes passages that show recognition of the lack of generic names to denominate natural groups. Aristotle says that "Of the class of viviparous quadrupeds there are many genera,[6] but these again are without names, except specific names, such as man. lion, stag, horse, dog, and the like. Yet there is a genus of animals that have manes, as the horse, the ass, the oreus, the ginnus, the innus, and the animal which in Syria is called heminus (mule). . . . Wherefore," he
