How complete this conquest is, can best be realized perhaps by considering man's relation to the lower animals. When history opens, the animals are in their element; it is man who is the interloper. Two thousand years ago it was not the Society of Friends but wolves and wild boars who felt themselves at home on the site of Bournville Garden Village. To-day we are surprised when we read that in remote East Africa lions and giraffes venture occasionally to interfere in the murderous warfare between man and man. Man has imposed himself on the animals, by dint of his gradual accumulation of knowledge and his consequent power of organization and government. He has destroyed the conditions under which the animals prospered. He has, as we might say, destroyed their home life, exposing them to dangers of his own making against which they are now as powerless as he was once against them. 'It is a remarkable thing,' writes Sir E. Ray Lankester, 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 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 contact with the parasite, that it is found to have deadly properties. The various cattle-diseases which in Africa have done so much harm to native cattle, and have in some regions exterminated big game, have _per contra_ been introduced by man through his importation of diseased animals of his own breeding from Europe. Most, if not all, animals in extra-human conditions, including the minuter things such as insects, shellfish, and invisible aquatic organisms, have been brought into a condition of 'adjustment' to their parasites as well as to the other conditions in which they live: it is this most difficult and efficient balance of Nature which Man everywhere upsets.[54] And Sir E. Ray Lankester goes on to point out the moral to be drawn from this development. He points out that