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Show 1880.] ON THE ARRANGEMENT OF THE MAMMALIA. 649 1. On the Application of the Laws of Evolution to the Arrangement of the Vertebrata, and more particularly of the Mammalia. By T. H. HUXLEY, F.R.S. [Eeceived November 23, 1880.] There is evidence, the value of which ha3 not been disputed, and which, in m y judgment, amounts to proof, that, between the commencement of the Tertiary epoch and the present time, the group of the Equidae has been represented by a series of forms, of which the oldest is that which departs least from the general type of structure of the higher Mammalia, while the latest is that which most widely differs from that type. In fact, the earliest known equine animal possesses four complete subequal digits on the fore foot, three on the hind foot; the ulna is complete and distinct from the radius ; the fibula is complete and distinct from the tibia; there are 44 teeth, the full number of canines being present, and the cheek-teeth having short crowns with simple patterns and early formed roots. The latest, on the other hand, has only one complete digit on each foot, the others being represented by larger or smaller rudiments ; the ulna is reduced and ankylosed with the radius ; the fibula is still more reduced and partially ankylosed with the tibia ; the canine teeth are partially or completely suppressed in the females ; the first cheek-teeth usually remain undeveloped, and when they appear are very small; the other cheekteeth have long crowns with highly complicated patterns aud late-formed roots. The Equidse of intermediate age exhibit intermediate characters. With respect to the interpretation of these facts, two hypotheses, and only two, appear to be imaginable. The one assumes that these successive forms of equine animals have come into existence independently of one another. The other assumes that they are the result of the gradual modification undergone by the successive members of a continuous line of ancestry. As I am not aware that any zoologist maintains the first hypothesis, I do not feel called upon to discuss it. The adoption of the second, however, is equivalent to the acceptance of the doctrine of evolution, so far as Horses are concerned; and, in the absence of evidence to the contrary, I shall suppose that it is accepted. Thus, since the commencement of the Eocene epoch, the animals which constitute the family of the Equidse have undergone processes of modification of three kinds :- 1. There has been an excess of development of some parts in relation to others. 2. Certain parts have undergone complete or partial suppression. 3. Certain parts, which were originally distinct, have coalesced. Employing the term "law" simply in the sense of a general statement of facts ascertained by observation, I shall speak of these PROC. ZOOL. Soc-1880, No. XLIII. 43 |
