496 HUMAN BIOLOGY 



years or even through a long lifetime. There is obviously 

 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 abso- 

 lutely decide the matter. They do give evidence, however, 

 that familial differences in resistance exist. 



Some light has been thrown on this case by animal 

 experimentation. Guinea pigs are very susceptible to 

 inoculation tuberculosis. These animals have also been 

 favored as subjects for genetic experiments. There exist a 

 few families of the species which have been propagated for 

 years by the closest possible inbreeding. With regard to cer- 

 tain characters, color, growth rates, fertility, etc., the family 

 characters are distinctive beyond question. It has also 

 been possible to show that the families differ in their suscepti- 

 bility to inoculation tuberculosis. The differences are of 

 degree only. That is, all are susceptible, but the disease 

 advances much more rapidly in some families than in others. 

 Animal experiments cannot in general be transferred to the 

 interpretation of human phenomena without scrupulous 

 consideration. But the laws of inheritance have been proved 

 in other cases to be among the most fundamental of biological 

 phenomena. Wherever there is sexual reproduction the laws 

 of Mendel have been found to govern the inheritance. 

 And wherever a certain quality has been found to be definitely 

 inherited in any species it is found to be inherited in other 

 species possessing the quality. The details governing the 

 inheritance of the quality may differ from species to species, 

 but this only means that the relative importance of certain 

 qualities may be found to vary in relation to other qualities 

 which may or may not be definitely heritable. The quality, 

 to repeat, if subject to inheritance in one species will be 

 similarly controlled in any other in which it may occur, 

 although it may be a much more important and significant 

 quality, in the one species than in the other. Also it may be 

 considered certain, that, in its fundamentals, inoculation 

 tuberculosis in the guinea pig reproduces the condition of 

 spontaneous tuberculosis in man. There are doubtless 

 important departures in the intimate nature of the disease 



