114 THE PRINCIPLES OF HEREDITY 



third, an unnamed doctrine, supposes that variations are due 

 to influences from the environment acting on the germ-plasm. 

 Whichever doctrine be true, it is plain that a race long 

 afflicted by any prevalent and lethal or serious disease must 

 undergo change. All the races of mankind sprang from a 

 common stock, as is proved by the fact that they are able to 

 interbreed. They were, therefore, once alike. By comparing 

 races which have been afflicted for hundreds or thousands of 

 generations by a given disease with races which have had 

 little or no experience of it, we should be able to ascertain 

 the kind of racial change caused by the disease, and so be 

 able to judge of the correctness of one or other theory. 

 Thus if long experience of tuberculosis has rendered a race 

 highly resistant, we must suppose that variations arise "spon- 

 taneously." But if it renders the race feeble and degenerate, 

 or less resistant, it is plain we must reject the Neo- Darwinian 

 theory and accept one of the other two. 



185. The doctrine of Natural Selection has rarely been 

 applied by medical men to the study of disease. The 

 Lamarckian doctrine has been, and is still, very widely held. 

 The doctrine that parental disease acting directly on the 

 germ-plasm tends to render offspring subsequently born 

 degenerate is practically universal, 1 not only among medical 

 men, but even amongst biologists, who in this particular have 

 naturally accepted medical opinion. Enormous confusion has 

 arisen in medical literature by the loose use of the terms 

 " acquired " and " congenital." Before attempting to analyze 

 and estimate the racial effects of disease, it is necessary, 

 therefore, to gather ideas as clear as possible respecting the 

 exact nature of certain effects of disease on the individual, 

 especially the nature of that most important effect of all 

 acquired immunity. 



1 Witness the prevalent notion of the causation of gout in offspring, 

 and the equally prevalent belief that races have grown resistant to 

 disease through the transmission of acquirements. 



