HEREDITY AND DISEASE 1 1 1 



a baby. Similarly the microbes of certain diseases 

 demand for their soil the tissues of man or of a hi^rher 

 ape, the lower ape, the lemur, and all other animals 

 being insusceptible or immune. Again, there is one 

 kind of sheep Avhich is immune to a disease that 

 readily attacks all other sheep ; and so on. without 

 limit. Plainly there is a question of heredity here. 



When we attempt to name the factors of suscepti- 

 bility or of imnumity we find ourselves in ditliculties. 

 For immunity or heightened susceptibility may be 

 acquired by the individual as a result of actual 

 experience of the disease. Immunity or suscepti- 

 bility may vary in its degree from day to day, or 

 from decade to decade. It may be relative or 

 absolute, temporary or permanent, natural or ac- 

 quired. But in every case true immunity or sus- 

 ceptibility must ultimately depend upon the facts 

 of cell-chemistry, and in many cases these facts 

 must be of very ancient origin. The natural sus- 

 ceptibility of the chimpanzee and of man to a 

 certain disease plainly depends upon inborn charac- 

 ters which have been transmitted through thousands 

 of generations. Their immunity to the tsetse-fly 

 disease of the horse is similarly a fact of heredity. 



But we have already seen that imnumity or 

 susceptibility may be confined to certain varieties 

 within a species, as in the case of the sheep. Simi- 

 larly it appears that different races of men, and even 

 different families of the same race, varv widelv in 

 these respects. And the most important fact is that 

 these characters of natural imnumity or susceptibility 

 to one disease or anoth<'r are transmissible ])y heredity. 

 This fact serves to solve our practical problems. 



