In tracing the modifications of structure in a series of animals so similar to one another as to be termed a 'natural group,' one is led to consider the relations to such modifications of concomitant changes in external influences during the geological period of the existence of such group. A difference in the density of the parts of the earth's surface habitually trodden by hoofed quadrupeds has been suggested, for example, as a concomitant, if not causal condition in the transmutation of a five-hoofed perissodactyle (Coryphodon, e.g.) to a three-hoofed one (Palæotherium, e.g.), thence verging in simplification, through Hipparion, to the existing single-hoofed Equines[1]. Thus it has been observed:—"As the surface of the earth consolidated, the larger and more produced mid hoof of the old three-toed pachyderms took a greater share in sustaining the animal's weight; and more blood being required to meet the greater demand of the more active middle toe, it grew; whilst the side toes, losing their share of nourishment, and becoming more and more withdrawn from use, shrank, and so on, according to the hardening of the ground, until only the hidden rudiments of metapodials remained, and one hoof became maximized for all the work"[2].
To this it may be objected that demonstration of such progressive gain of hardness and resistance in parts of the earth's surface, trodden by successive Tertiary forms of hoofed beasts, has not yet been had.
There is, however, another series of conditions which is demonstrated, and which may be legitimately taken into consideration in the relation defined at the outset of the present paper. I refer to the changes in the nature of the prey of certain carnivorous animals.
I assume, at least, a legitimacy of inference from negative evidence, that cold-blooded aquatic animals formed the food of Crocodiles in a much greater proportion in the Mesozoic than in the Neozoic period, and that terrestrial air-breathing animals seldom, if ever, were the prey of the Mesozoic Crocodiles, but were, as now, frequently the food of Neozoic ones.
On this assumption I appreciate, with a satisfaction not felt before, the well-marked distinction in certain parts of the structure of procœlian as compared with amphicœlian Crocodiles. The procœlian
- ↑ Intermediate conditions are exemplified in the Eohippus, Orohipptts, Mesohippus, of Marsh, from North-American Tertiaries. See his 'Introduction and Succession of Vertebrate Life in America.' 8vo. 1877.
- ↑ 'Anatomy of Vertebrates,' vol. iii. pp. 792, 793, and cut 614, "Derivation of Equines" (1868).
