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 a case of response in an adap- 

 tive direction that could not have been the outcome of experi- 

 ence in the past. It is 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, i.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 



