THE FACTORS OF ORGANIC EVOLUTION. 113 



or the other, is evidence that they are not represented in 

 the structure of the germinal plasm; and the fact that 

 definite extrinsic causes, such as salt or fresh water, act- 

 ing upon this plasm, produce results which are con- 

 stantly the same is the best evidence that the internal 

 mechanism — i. e., the structure of the germinal plasm — 

 is constantly the same. The same can be said of many 

 artificially produced modifications, such as the exogas- 

 trulas and potassium larvae of Herbst, all of which pro- 

 found changes are due entirely to extrinsic and not to 

 intrinsic causes, as is shown by the fact that they disap- 

 pear as soon as the immediate extrinsic cause is with- 

 drawn. The same thing is shown in Poulton's experi- 

 ments on the colours of lepidopterous larvae, and in this 

 case also it is known that the changes are not inherited, 

 at least during the limited period through which the 

 experiments were conducted ; and it should be observed 

 that to assume that this would take place at the end of 

 an indefinite number of generations is simply to beg the 

 question. 



Very many other cases of a similar character might 

 be instanced under this head, but I hasten on to another 

 class of evidence. 



(c) Under the subject of the inherited effects of use 



„ J J. and disuse the following cases mav be 



use and disuse. . •' 



mentioned as showing how inconclusive 

 much of the evidence is : 



(x) In the first place, this whole line of argument 

 starts with the assumption that the individual habits of 

 an animal are inherited, and that these habits ultimately 

 determine the structure, an assumption which really 

 begs the whole question ; for, after all, the substratum 

 of any habit must be some physical structure, and if 

 modified habits are inherited it must be because some 

 modified structure is inherited. I take an example 



