NATURAL SELECTION AMONG CIVILIZED MEN 113 



different qualities. Tuberculosis should cause an evolution 

 of what we know as inborn immunity, while measles should 

 cause an evolution of something quite distinct the inborn 

 power of acquiring immunity. Moreover, since immunity 

 against any one disease does not imply immunity against any 

 other, we should find that evolution against any one malady 

 does not imply evolution against any other. 



182. On the other hand, if acquired characters are trans- 

 missible, we should have different and exceedingly curious 

 results. Tuberculosis tends always to injure and weaken the 

 individual afflicted by it. Seemingly it never confers any 

 sort of benefit. Consequently, if acquirements are inherited, 

 a race afflicted by tuberculosis should, through the accumu- 

 lation of injuries during generations, become more and more 

 injured, feeble, and degenerate, more and more unfit for 

 existence, till at last it becomes extinct. But measles 

 generally confers a distinct benefit on the individual ; he 

 acquires immunity. If this were transmitted it would, of 

 course, appear as inborn immunity in the next generation. 

 If, then, acquirements are transmissible, no matter how 

 "faintly and fitfully," a race afflicted by measles should 

 become in time quite immune to it, quite incapable of being 

 infected by it. 



183. Lastly, if acquired characters are not transmissible, 

 but if influences from the environment tend, with more or 

 less readiness, to affect the germ-plasm, we should again get 

 definite results in the case of each disease, though in each 

 instance it would not be possible to forecast, on logical 

 grounds, the nature of the changes in the race. Presum- 

 ably they would always be in the direction of degeneracy. 

 Certainly they would be in this direction in the case of such 

 diseases as tuberculosis, in which the influences are always 

 harmful. Even in the case of measles and kindred disorders 

 the changes should also probably be in the direction of 

 degeneracy, since, for a period at least, the germ-plasm is 

 subjected to acute poisoning. 



184. It is evident, therefore, that disease affords ideal 

 material for testing the truth of the three principal doctrines 

 of heredity. All these doctrines suppose that offspring tend, 

 with variations, to reproduce the inborn characters of their 

 parents. But they differ in the way in which they account 

 for the variations. The first doctrine, the Neo-Darwinian, 

 supposes that practically all variations arise "spontaneously"; 

 the second, the Lamarckian, supposes that some variations 

 at least are due to the transmission of acquirements ; the 



