THE NATURAL HISTORY OF MAN 31 



weak or strong against any given disease, tend to 

 transmit their peculiarities to children. For this 

 reason consumption, for example, is said to run in 

 families. Three facts, therefore, are apparent : 

 ( I ) that men differ in their powers of resisting any 

 given disease ; (2) that offspring tend to inherit 

 their parents' powers of resistance ; (3) that disease 

 is highly selective in its action. It follows that 

 every deadly and prevalent zymotic disease plays 

 the part of a breeder. It eliminates the unfittest, 

 leaving the fittest to continue the race. Thus, in 

 the case of man, the only animal with whose 

 conditions we are thoroughly familiar, we find the 

 thing which has been denied so often. Natural 

 Selection in full swing — Natural Selection of the 

 most stringent kind, for, as I say, many death- 

 dealing diseases are so prevalent within their areas 

 of distribution that no man escapes infection unless 

 he be immune, nor death, unless he be resistant. 

 Here then is a test case. If there be truth in the 

 Darwinian doctrine, disease should be a great 

 cause of evolution. 



It is most significant that every race is resistant 

 to every deadly disease strictly in proportion to its 

 past experience of it. West African negroes are 

 much more resistant to malaria than Englishmen, 

 who, on the other hand, are as highly resistant to 

 consumption when compared to Australian blacks. 



