14 Evolution and Adaptation 
that if he is suddenly brought back to the normal condition 
of the race he will die. 
Immunity to the poison of venomous snakes can also be 
acquired by slowly increasing the amount given to an animal. 
It is possible to make a person so immune to the poison of 
venomous snakes that he would become, in a sense, adapted 
to live amongst them without danger to himself. It is to be 
noted, moreover, that this result could be reached only by 
quite artificial means, for, under natural conditions it is incon- 
ceivable that the nicely graded series of doses of increasing 
strength necessary to bring about the immunity could ever be 
acquired. Hence we find here acase of response in an adap. 
tive direction that could not have been the outcome of experi- 
ence in the past. Itis important to emphasize this capacity of 
organisms to adapt themselves to certain conditions entirely 
new to them. 
These cases lead at once to cases of immunity to certain 
bacterial diseases. An animal may become immune to a par- 
ticular disease in several ways. First, by having the disease 
itself, which renders it immune for a longer or a shorter 
period afterwards; or, second, by having a mild form of the 
disease as in the case of smallpox, where immunity is brought 
about by vaccination, z.e. by giving the individual a mild 
form of smallpox; or, third, by introducing into the blood 
an antidote, in the form, for example, of antitoxin, which has 
been made by another animal itself immune to the disease. 
The first two classes of immunity may be looked upon as 
adaptations which are of the highest importance to the or- 
ganism ; the last case can scarcely be looked upon as an 
adaptive process, since the injurious effect of the poison may 
as well be neutralized outside of the body by mixing it with the 
antitoxin. We may suppose, then, that in the body a similar 
process goes on, so that the animal itself takes no active part 
in the result. 
When we consider that there are a number of bacterial 
