42 A STUDY IN HEREDITY 



now are — as little capable of dwelling in dense 

 communities, and consequently of achieving civilisa- 

 tion.^ Our civilisation, therefore, is conditioned by 

 our power of resisting certain infectious diseases, 

 a power which arose, during the advance of 

 civilisation, by a long and painful process of 

 evolution. 



Malaria infests a large portion of the earth's 

 surface, but, except in the south of Asia, its habitat, 

 mainly the great tropical swamps and forests, is 

 thinly populated by human beings. Consumption, 

 the type and the worst of purely parasitic diseases, 

 afflicts a larger and more densely peopled area. 

 It is not unknown in African forests and Indian 

 jungles, and therefore Africans and Hindustanis 

 have undergone some evolution against it ; but it 

 has longest, or at least principally, afflicted parts 

 of Western Europe and Eastern Asia, where, for 

 thousands of years, teeming populations have lived 

 in houses so designed for warmth as to be more 

 or less exclusive of light and air. Like all, or most 



1 " They (the Baggara) hve always out in some desert place where 

 no trees are, or water, and the houses they inhabit in these ' deyms ' 

 are always isolated and irregularly dotted over a wide space in such a 

 manner as to avoid anything like street or enclosed communication. 

 These desert tribes, by the way, are terribly susceptible to all kinds 

 of infectious diseases, which invariably attack them with almost 

 incredible violence, and it is quite possible that they cling to their 

 mode of living, which is undoubtedly highly sanitary, as being in a 

 measure a precaution against epidemics." — London Correspondent of 

 the Daily News, 23rd Oct., 1 896. 



