THEORIES OF EVOLUTION 121 



association of individuals of all degrees of variation 

 there can be no maintenance of the original char- 

 acteristics of the species. This is equally true of 

 all characters, useful, harmful, or indifferent, so far 

 as the direct results of isolation are concerned. 



The question arises, are such geological phe- 

 nomena as those mentioned necessary to the com- 

 plete isolation of conspecific groups .^^ Obviously 

 not, but in contrast to spatial isolation of other 

 degrees they accomplish such clean-cut separation 

 that they are striking examples. Every species, 

 be it plant or animal, tends to be dispersed from 

 its center of distribution. Animals, through their 

 powers of locomotion, may spread more rapidly 

 than plants, but no more surely. The individual 

 animal may migrate long distances, while the 

 individual plant must usually stay in one place, 

 but the seeds of the plant are scattered through 

 many effective adaptations, sometimes over in- 

 credible distances. The animal may direct its own 

 dispersal into favorable regions, while the scatter- 

 ing of seeds is random, but the seeds may germi- 

 nate and grow only in favorable places so the result 

 is, in the end, the same. Ultimately both plants and 

 animals extend their distribution over great areas. 

 If they push down along the sides of a great river 

 or a mountain range over which they cannot pass, 

 they are as effectively divided in one direction as 

 if the geographical feature had sprung up in the 



