264 SIGNIFICANCE OF SEXUAL REPRODUCTION [V. 



The supposition of such a phyletic force of transformation 

 (see Appendix I, p. 306) possesses, in my opinion, the greatest 

 defect that any theory can have, — it does not explain the pheno- 

 mena. I do not mean to imply that it is incapable of rendering 

 certain subordinate phenomena intelligible, but that it leaves a 

 larger number of facts entirely unexplained. It does not afford 

 any explanation of the purposefulness seen in organisms : and 

 this is just the main problem which the organic world offers for 

 our solution. That species are, from time to time, transformed 

 into new ones might perhaps be understood by means of an 

 internal transforming force, but that they are so changed as to 

 become better adapted to the new conditions under which they 

 have to live, is left entirely unintelligible by this theory. For 

 we certainly cannot accept as an explanation Nageli's statement 

 that organisms possess the power of being transformed in an 

 adaptive manner simply by the action of an external stimulus 

 (see Appendix II, p. 308). 



In addition to this fundamental defect, we must also note 

 that there are absolutely no proofs in support of the foundation 

 of this theory, viz. of the existence of an internal transforming 

 force. 



Nageli has very ingeniously worked out his conception of 

 idioplasm, and this conception is certainly an important ac- 

 quisition and one that will last, although without the special 

 meaning given to it by its author. But is this special meaning 

 anything more than pure hypothesis ? Can we say more than 

 this of the ingenious description of the minute molecular struc- 

 ture of the hypothetical basis of life ? Could not idioplasm be 

 built up in a manner entirely different from that which Nageli 

 supposes? And can conclusions drawn from its supposed 

 structure be brought forward to prove anything? The only 

 proof that idioplasm must necessarily change, in the course of 

 time, as the result of its own structure, is to be found in the 

 fact that Nageli has so constructed it ; and no one will doubt 

 that the structure of idioplasm might have been so conceived 

 as to render any transformation from within itself entirely 

 impossible. 



But even if it is theoretically possible to imagine that idio- 

 plasm possesses such a structure that it changes in a certain 

 manner, as the result of mere growth, we should not be 



