Origin of Different Kinds of Adaptations 377 
of its use may be found to be a very simple phenomenon that 
requires no special explanation at all to account for its exist- 
ence in the individual, further than that the muscles are of 
such a kind that this is a necessary physical result of their 
action. But until we know more of the physiology in- 
volved in the process, it is idle to speculate about the ori- 
gin of the phenomenon. 
REACTIONS OF THE ORGANISM TO POISONS, ETC. 
In this case also we meet with a number of responses for 
whose origin we can give not the shadow of an explanation. 
On the other hand, the cases are significant in so far as a 
number of them show quite clearly that the response cannot 
have been acquired through the experience of the organism, 
or the selection of those individuals that have best resisted 
the particular poison. This is true, because in a number of 
cases the poison is a substance that the animal cannot possi- 
bly have met with during the ordinary course of its life, or 
of that of its ancestors. It may be argued, it is true, that 
in the case of the poisons produced by certain bacteria the 
power of resistance has been acquired through the survival 
of the less susceptible, or more resistant, individuals. Im- 
probable as this may be in some cases, it does not, even if 
it were true, alter the real issue, for it can be shown, as has 
just been said, that the same power of responding adaptively 
is sometimes shown in cases of poisons that are new to the 
animal. 
There is no question that different individuals respond in 
very different degrees to these poisonous substances, and it 
is easy to imagine in the case of contagious diseases that 
a sort of selective process might go on that would bring 
the race up to the highest point to which fluctuating varia- 
tions could be carried, even to complete immunity ; but even 
if this were the case, it seems to be true that the moment 
