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

civilized man has proceeded so far in his interference with
extra-human nature, has produced for himself and the living
organisms associated with him such a special state of things
by his rebellion against natural selection and his defiance
of Nature's pre-human dispositions, that he must either go