[All beings], remote in the scale of nature, are bound together by a web of complex relations. . . . In the case of every species, many different checks . . . come into play; . . . The dependency of one organic being on another, as of a parasite on its prey . . . the structure of every organic being is related, in the most essential yet often hidden manner, to that of all other organic beings, with which it comes into competition. . .

On the Origin of Species, 1859

There is in Nature a struggle for existence. . . Can it be doubted that any minute variation in structure, habits, or instincts, adapting that individual better to the new conditions, would tell upon its vigour and health?

Darwin's Manuscripts, 1839 to 1844

At some future period, not very distant as measured by centuries, the civilised races of man will almost certainly exterminate, and replace, the savage races throughout the world.

Descent of Man, 1898