LIMITATIONS 93 



1 have already referred to the fact that the operation 

 of what are called ''unknown factors in evolution" is 

 increasingly reaHzed by biological investigators. Thus 

 V. L. Kellogg, whose antipathy to any teleological 

 interpretation of evolution is openly expressed, says, 

 "Let no ambitious student hesitate to take up the search 

 for truth about evolution from the notion that biology 

 is a read book. The 'Origin of Species' was the first 

 opening of the book — that the world recognized at 

 least; poor Lamarck opened the book but could not 

 make the world read in it — and that time when it 

 shall be closed because read through is too far away 

 even to speculate about. With Osborn let us join the 

 believers in the 'unknown factors in evolution.'" ^ A 

 few pages further on in the same chapter, he states that 

 "by no means all biologists find in natural selection a 

 sufficient explanation of adaptation"; ^ and makes this 

 acknowledgment in spite of his own predilection for 

 that explanation. 



These unknown factors are apparently to be identi- 

 fied with directive forces working either within the 

 developing organisms, or in nature at large, or in both. 

 It is because they he beyond direct observation that 

 their nature escapes scientific description, and no par- 

 ticular attempt to describe them has gained general 

 acceptance. Such attempts are represented in several 

 orthogenetic theories, thus called because they assume 



^Darwinism To-day, p. 377 (cf. note 4, pp. 390, 391, where a 

 significant quotation from H. F. Osborn is given). 



2 Page 380. 



