420 ZOOLOGY 



was. laid out for it in the beginning. Thus, the horse 

 group was to increase in size, decrease the number of its 

 toes, etc. It actually behaved as if following out a 

 program planned in advance. The idea is not inher- 

 ently absurd, since this is the course of individual devel- 

 opment ; and it may well be imagined that there is some- 

 thing in the nature of a particular kind of protoplasm, 

 that will lead it to vary in a certain direction. Indeed, 

 we know that it does not varv in all directions ; thus we 







cannot get a genuinely blue rose. It is to be noted, 

 however, that the evolution of the horse group is also 

 strictly along the lines of adaptation. The climate be- 

 came cooler and drier ; the animal became an inhabitant 

 of the plains. The solid hoof is adapted for running on 

 hard ground, for receiving the impact of the heavier 

 body ; also for kicking the carnivorous enemies which 

 had in the meanwhile evolved to prey upon the horse. 

 The long-crowned, hard teeth are adapted for feeding on 

 the vegetation to be found in open, dry places, and what 

 might be regarded as a difficulty has been so completely 

 overcome that the animal now needs the type of food 

 for which it is specially fitted. The whole history is one 

 of adjustment to conditions, and the evolutionary 

 process could not have taken place in the Eohippus en- 

 vironment, for the simple reason that the changes would 

 all have been detrimental, leading eventually to ex- 

 tinction. In the case of the elephant group, as we shall 

 presently see, there was the same apparent orthogenesis, 

 until a certain structure became useless, when the whole 

 process was reversed. 



American ^' Eventually the horse group reached the Old World, 



horses undoubtedly by way of the land bridge to Asia which 



then existed in the north. During much earlier times 



the primitive horselike types had existed on both sides 



