258 THE SIGNIFICANCE OP SEXUAL REPRODUCTION 



they are so changed as to become better adapted to the new con- 

 ditions 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. 300). 



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 idio- 

 plasm, and this conception is certainly an important acquisition 

 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 de- 

 scription of the minute molecular structure 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 con- 

 ceived as to render any transformation from within itself entirely 

 impossible. 



But even if it is theoretically possible to imagine that idioplasm 

 possesses such a structure that it changes in a certain manner, as 

 the result of mere growth, we should not be justified in thus 

 assuming the existence of a new and totally unknown principle 

 until it had been proved that known forces are insufficient for the 

 explanation of the observed phenomena. 



Can any one assert that this proof has been forthcoming ? It 

 has been again and again pointed out that the phyletic development 

 of the vegetable kingdom proceeds with regularity and according to 

 law, as we see in the preponderance and constancy of so-called 

 purely ' morphological ' characters in plants. The formation of 

 natural groups in the animal and vegetable kingdoms compels us 

 to admit that organic evolution has frequently proceeded for longer 

 or shorter periods along certain developmental lines. But we aiv 

 not on this account compelled to adopt the supposition of un- 



