220 DARWIN AND PALEONTOLOGY 



ogy may however be said to lend support to this 

 hypothesis. Changes of proportion in long 

 periods of time, that is, in milhons of years, play 

 an enormous part in evolution, as seen by the fol- 

 lowing contrasts in certain well-known structures, 

 among herbivorous quadrupeds: — 



Short-toothed and long-toothed^ short-lived and long- 

 lived. 



Short-toothed and long-toothed^browsers and grazers. 



Short-footed and long-footed ^short rangers and 

 long rangers. 



Short-headed and long-headed :=browsers and grazers. 



Short-necked and long-necked =near feeders and far 

 feeders. 



Among the horses these very changes of pro- 

 portion in four important organs, the teeth, the 

 feet, the head, and the neck, constitute a very 

 large percentage of the total evolutionary 

 changes, and result finally in certain phyla of 

 horses becoming long-hved animals, capable of 

 travehng long distances, capable of grazing on 

 the harder kinds of food, and capable of reach- 

 ing food at a considerable distance from the 

 body. This joint action of heredity, ontogeny, 

 environment, and selection of congenital varia- 

 tions of proportion, appears to best explain the 

 transformation of round-headed or brachyceph- 

 alic into the long-headed or dohchocephaUc forms 

 of the horses as well as of other herbivora, in re- 

 lation to the browsing or grazing habit respect- 

 ively. The only explanation which has as yet 



