ON THE MODIFYING INFLUENCE OF A HIGHER FORM OF LIFE. 421 



25. On the Influence of the Advent of a higher Form of Life in 

 modifying the Structure of an older and lower Form. By 

 Professor Bichard Owen, C.B., F.H.S., F.G.S., &c. (Bead 

 February 6, 1878.) 



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 (Palceotherium, e. g.), thence verging in simplifica- 

 tion, through ffipparion, to the existing single-hoofed Equines *. 

 Thus it has been observed : — " As the surface of the earth con- 

 solidated, 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 with- 

 drawn from use, shrank, and so on, according to the hardening of 

 the ground, until only the hidden rudiments of metapodials re- 

 mained, and one hoof became maximized for all the work "f . 



To this it may be objected that demonstration of such progres- 

 sive 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 demons- 

 trated, 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 evi- 

 dence, that cold-blooded aquatic animals formed the food of Croco- 

 diles 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, fre- 

 quently 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 pro- 

 coelian as compared with amphiccelian Crocodiles. The proccelian 



* Intermediate conditions are exemplified in the Eohippus, Orohipptts, 

 Mesohippus, of Marsh, from North -American Tertiaries. See his 'Introduc- 

 tion and Succession of Vertebrate Life in America.' 8vo. 1877. 



t 'Anatomy of Vertebrates,' vol. iii. pp. 792, 793, and cut 614, " Derivation 

 of Equines" (1868). 



