40 A STUDY IN HEREDITY 



ing the worst of these areas can have undergone 

 the whole of their evolution within them ; the 

 death-rate of a non-evolved race being so high as 

 to cause extinction, not evolution. They probably- 

 dwelt first within less malarious regions, and under- 

 went part of their evolution there. But now, after 

 the sufferings of uncounted generations, after pay- 

 ing toll in millions of lives, they dwell secure in 

 their fever-haunted fastnesses. They cannot be 

 displaced by incoming hordes. They may be con- 

 quered by the superior weapons of civilisation, but 

 even then, with the advance of their own culture, 

 they must in time regain their freedom. Not for 

 them is the fate of so many perished and forgotten 

 races, whose poor relics, rude implements and 

 mouldering fragments of bone, alone attest their 

 former existence, and tell of their extinction by 

 stronger and fiercer invaders. They are safe for all 

 time, unless, indeed, the march of sanitary science 

 destroys malaria, always their scourge, but now their 

 principal rock of defence as well. 



If malaria and its congeners have played a great 

 part, consumption and other wholly parasitic dis- 

 eases have played a greater. Being parasitic and 

 earth- and air-borne, they are, necessarily, diseases 

 of crowded populations. Among scattered and 

 nomadic peoples they tend to die out, unless 

 renewed from denser communities. During the 



