2l8 READINGS IN BIOLOGICAL SCIENCE 



certain diseases which are peculiar to it, and neither naturally nor arti- 

 ficially transmissible to any other species. Influenza and malaria are fair 

 examples of such diseases of human beings. Asiatic cholera is another. 

 Many cases may be cited in which species lines are not rigidly respected 

 and are yet very influential. Smallpox is such a human disease. It may 

 spread to milch cattle under suitable conditions, but in them produces a 

 modified type of disease similar to the naturally occurring cowpox. Rabies 

 is widely disseminated among the domestic animals, is very frequently 

 transmitted to man but is not known as a disease of birds. 



The questions at issue really become debatable when we consider the re- 

 lation of the racial, familial or individual inheritances within the species. 

 It is now clear that here the lines are much less rigid. There are very cer- 

 tain instances, particularly among plants, where families or strains within 

 the race are quite immune to a particular disease from which the race as a 

 whole suffers most severely. The rust-resistant varieties of wheat and 

 asparagus are familiar cases. Similar cases can be made out among animals. 

 There is no certain instance of an infectious disease affecting one or more 

 race of the human species and leaving another untouched. There are a 

 number of instances when it seems that certain races are less susceptible 

 than others to particular diseases but even here it is impossible in the 

 present state of knowledge to be sure of the significance of the cases. Racial 

 habits as to diet, for example, and the continued state of contact with the 

 disease are apparently influential factors about which there is as yet in- 

 sufficient information. 



When we turn from the race to the individual, vision apparently be- 

 comes clearer, for there can be no doubt that with reference to most in- 

 fectious diseases there are wide individual variations in resistance. 



But if we go back for a moment to an earlier period we find a fixed and 

 universal opinion that certain infectious diseases follow family lines to 

 a considerable extent. This is not true of measles or smallpox. It seems con- 

 spicuously true of tuberculosis. Most of us can doubtless call to mind 

 families in w^iich severe illnesses and deaths from tuberculosis have been 

 common, and other families in which they have been rare. Large groups of 

 family histories have been collected and submitted to the best available 

 mathematical analysis and these have also given evidence of some differ- 

 ence in the inheritance. But it is also known that under conditions of uni- 

 versal exposure as in crowded cities, practically all individuals have some 

 tuberculosis at some time or other. The disease is one which often lasts in 

 individual cases for years or even through a long lifetime. There is ob- 

 viously unusual opportunity for infection to follow a family in which it 

 is established. In the face of such considerations on the contrary side, it 

 cannot be maintained that such studies of human family histories as have 

 been made absolutely decide the matter. They do give evidence, however, 

 that familial differences in resistance exist. 



