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 larve 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 larve, 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. 
(¢) Under the subject of the inherited effects of use 
and disuse the following cases may be 
mentioned as showing how inconclusive 
much of the evidence is: : 
(x) In the first place, this whole line of argument 
starts with the asfumption 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 
Use and disuse. 
