NATURE'S INSURGENT SON 33 



out injury, resulting in the establishment of immune 

 races. It is a remarkable thing which possibly may 

 be less generally true than our present knowledge seems 

 to suggest that the adjustment of organisms to their 

 surroundings is so severely complete in Nature apart 

 from Man, that diseases are unknown as constant and 

 normal phenomena under those conditions. It is no 

 doubt difficult to investigate this matter, since the pre- 

 sence of Man as an observer itself implies human inter- 

 vention. But it seems to be a legitimate view that 

 every disease to which animals (and probably plants 

 also) are liable, excepting as a transient and very 

 exceptional occurrence, is due to Man's interference. 

 The diseases of cattle, sheep, pigs, and horses, are not 

 known except in domesticated herds and those wild 

 creatures to which Man's domesticated productions 

 have communicated them. The trypanosome lives in 

 the blood of wild game and of rats without producing 

 mischief. The hosts have become tolerant of the 

 parasite. It is only when man brings his unselected, 

 humanly-nurtured races of cattle and horses into con- 

 tact with the parasite, that it is found to have deadly 

 properties. 1 The various cattle-diseases which in Africa 

 have done so much harm to native cattle, and have in 



1 This has been established in the case of the Trypanosoma Brucei 

 a minute parasite living in the blood of big game in south-east Africa, 

 amongst which it is disseminated by a blood-sucking fly, the Glossina 

 morsitans or Tsetze fly. The parasite appears to do little or no harm 

 to the native big game, but causes a deadly disease both in the horses 

 and cattle introduced by Europeans and in the more anciently intro- 

 duced native cattle (of Indian origin). Similar cases are found where a 

 disease germ (such as that of measles) produces but a small degree of 

 sickness and mortality in a population long associated with it, but is 

 deadly to a human community to which it is a new-comer. Thus 

 Europeans have introduced measles with deadly results in the South 

 Sea Islands. A similar kind of difficulty, of which many might be 



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