82 A NEW THEORY OF EVOLUTION. 



successor that come into action at that stage 

 and differentiate the types ? And the ques- 

 tion is, How or whence come those differen- 

 tiating forces? 



Darwin says the differentiation is caused 

 by changes in external conditions of life, or 

 by the accumulation of beneficial differences 

 that " spontaneously " arise. 



But we cannot see how external in- 

 fluences, sometimes destructive, can become 

 constructive and evolve higher specialisa- 

 tion, and before any argument can be based 

 on " spontaneous " variation there must be 

 evidence of such variation. But Darwin 

 offers no evidence of persistent specific 

 variation. 



Specific variation between a successor and 

 its immediate known antecessor is sometimes 

 great, and the similarity between phases of 

 their foetus is only observable in the early 

 stages of growth. In such cases the mature 

 animals are very dissimilar : the foundations, 

 so to speak, are alike, but the superstructures 

 are very different. The difficulties of evolu- 

 tion, by secondary causes, are more obvious in 

 such cases, but they are not really increased 

 by the width of the gap between antecessor 

 and successor. The difference is in kind, 

 not in degree. 



