The conditions that direct the order of . . . the living world . . . are marked by their persistence in improving the birthright of successive generations.

All male animals, including men, when they are in love, are apt to behave in ways that seem ludicrous to bystanders.

The publication in 1859 of the Origin of Species by Charles Darwin made a marked epoch in my own mental development, as it did in that of human thought generally. Its effect was to demolish a multitude of dogmatic barriers by a single stroke, and to arouse a spirit of rebellion against all ancient authorities whose positive and unauthenticated statements were contradicted by modern science.

One of the effects of civilization is to diminish the rigour of the application of the law of natural selection. It preserves weakly lives that would have perished in barbarous lands.

What nature does blindly, slowly and ruthlessly, man may do providently, quickly, and kindly. As it lies within his power, so it becomes his duty to work in that direction.

Well-washed and well-combed domestic pets grow dull; they miss the stimulus of fleas.

Eugenics is the study of the agencies under social control that may improve or impair the racial qualities of future generations either physically or mentally.