112 THE PRINCIPLES OF HEREDITY 



qualities which enable the survivors to escape. It follows, in 

 the case of disease, that selection should cause an evolution 

 of an inborn power of resisting infection, or of an inborn power 

 of recovering from infection. The two qualities 'are quite 

 distinct, and are generally useful against quite distinct 

 diseases. Thus only inborn immunity, inborn power of 

 resisting infection, is of avail against tuberculosis. As is well 

 known, experience of and recovery from tuberculosis does not 

 confer increased resisting power. In other words, when a 

 person, who has suffered and recovered from tuberculosis, has 

 increased resisting power, we have every reason to believe 

 that his added security is due solely to improved conditions 

 of general health and surroundings, not to any benefit con- 

 ferred by the disease. The case is quite different as regards 

 some other maladies. Thus almost every one is capable of 

 being infected by measles that is, scarcely any one has inborn 

 immunity against it. But most people are capable of recover- 

 ing from it after a short illness of pretty definite length. 

 They acquire an immunity, which is generally persistent, 

 even under the worst conditions, throughout life. Indeed, 

 were the acquired immunity, which arises during recovery 

 from measles, whooping-cough, and the like, not more or less 

 permanent, life would be difficult or impossible in all countries 

 where these diseases are prevalent. In the case of some 

 diseases, then, illness and recovery confer a distinct benefit ; 

 that is to say, an immunity which is acquired through the 

 existence of an inborn power of making this acquirement, 

 which may be enduring, as against measles or chicken-pox, 

 or more or less transient, as against diphtheria or common 

 cold, and whujh, as we know, is protective only against the 

 particular disease which causes its development in the 

 individual. 1 



181. Now, since tuberculosis, as a rule, weeds out those 

 who cannot resist infection by it, whereas measles, as a rule, 

 weeds out those who cannot recover from infection, it is evident, 

 if Natural Selection acts at all, that these diseases, which are 

 types of many others, should cause the evolution of entirely 



1 It must not be supposed, however, that there is a hard-and-fast line 

 of demarcation between the diseases against which immunity may be 

 acquired, and those against which the only kind of immunity possible 

 is the inborn. Some people seem congenitally immune to measles. 

 Many people appear congenitally immune to scarlatina ; but many 

 others, though susceptible to infection, are capable of acquiring 

 immunity against it. The fact remains, however, that the two kinds 

 of immunity are quite distinct, and that, as a general rule, they afford 

 protection against distinct diseases. 



