494 HUMAN BIOLOGY 



INFECTIOUS DISEASES 



As previously pointed out there is in the rigid sense no 

 such thing as the positive inheritance of an infectious 

 disease. This hes in the nature of the case since the impelling 

 incident in such a disease is the entry of an agency: germ, 

 bacterium or protozoan, from the environment. None the less, 

 the inheritance is of very vital significance and within 

 certain hmits absolutely controls the prevalence of these 

 diseases. This is true when we approach the question from 

 a wide biological viewpoint, regarding species Hues. It then 

 becomes in truth a matter of common knowledge. It is 

 probably quite correct to state that each distinct species 

 of animal or plant has certain diseases which are peculiar 

 to it, and neither naturally nor artificially 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 relation 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 certain 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 

 races 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 knowl- 

 edge to be sure of the significance of the cases. Racial habits 

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



