20 Outline of Genetics 



pyortional to the size of the organ involved, and that any retrogres- 

 sive evolution of this sort would be slower than progressive evolu- 

 tion. 



3. Diseases. — Roughly speaking, diseases are either 

 the results of infection by bacteria or fungi or some inher- 

 ent organic weakness. Since the latter condition is 

 chiefly serious only in inviting attacks by bacteria and 

 fungi, we are concerned chiefly with diseases caused by 

 these pathogenic forms. Realizing this, true inheritance 

 of disease seems to be an impossibility, for if the parasite 

 enters the germ cell it is practically sure to destroy it, 

 and there will be no progeny. It is true that in many 

 cases progeny are born diseased, but this is due to rein- 

 fection of the young embryo from the body of the mother. 

 Many examples of this phenomenon are available in man 

 and other mammals. In plants, also, diseases (e.g., 

 smut) are sometimes passed on by means of spores car- 

 ried upon or even within the seeds. Such a thing, how- 

 ever, can in no sense be spoken of as inheritance, since 

 it always involves a reinfection. 



In one respect, however, one may speak of disease 

 inheritance. Breeding experiments have shown that 

 predisposition to disease and disease resistance, com- 

 monly called susceptibility and immunity, are inherited. 

 In practically all cases, these characteristics are evidently 

 of germinal origin, having been hereditary in the begin- 

 ning rather than acquired. Such cases, of course, have 

 no bearing on the present problem. There remain a 

 few instances, however, that rather suggest the inherit- 

 ance of acquired characters. 



GuYER and Smith (10), by inoculating female rabbits 

 either with typhoid vaccine or with the hving bacilli. 



