2 THE THEORY OF EVOLUTION 



writes: "The more deeply I pursued the alleged evi- 

 dence for it [the theory] and sought to gain, through 

 special investigations, some essential proof of the 

 genetic relationships of animals, the more clearly I 

 recognized that the theory is a seductive romance, 

 which deceptively pretends to give results and ex- 

 planations, rather than a doctrine built upon positive 

 foundations." 1 In another passage he speaks of the 

 "collapse of the theory." This opinion of Professor 

 Fleischmann's stands almost entirely alone in modern 

 biological literature, yet is not altogether surprising, 

 for, from the very nature of the case, such complete 

 and indubitable proof as Professor Fleischmann de- 

 mands is unattainable. 



What, then, has led to the belief, mentioned at 

 the outset, that naturalists have so largely grown 

 sceptical and are inclined to abandon the theory? 

 This belief is principally due to the fact that there 

 is no agreement among men of science as to the 

 manner in which evolution operates (modes) or as 

 to its efficient causes (factors) and, further, to the 

 often fierce controversies which long have been and 

 still are held concerning these problems of modes 

 and factors. Some biologists maintain that these 

 questions are unanswerable in the present state of 

 knowledge and have taken up other lines of investi- 

 gation, but that is very far from being an abandon- 

 ment of the theory of evolution itself. It is one 

 thing to accept a fact as substantially proved and 



1 Albert Fleischmann: Die Descendenztheorie, Leipzig, 1901, p. iii. 



