self from the destructive methods of natural selection, has accumulated a new series of dangers and difficulties with which he must incessantly contend.
14. Man and Disease.
In the extra-human system of Nature there is no disease and there is no conjunction of incompatible forms of life, such as Man has brought about on the surface of the globe. In extra-human Nature the selection of the fittest necessarily eliminates those diseased or liable to disease. Disease both of parasitic and congenital origin occurs as a minor phenomenon. The congenitally diseased are destroyed before they can reproduce: the attacks of parasites great and small either serve only to carry off the congenitally weak, and thus strengthen the race, or become harmless by the survival of those individuals which, owing to peculiar qualities in their tissues, can tolerate such attacks without 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 presence of Man as an observer itself implies human intervention. 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
