There is no doubt that the course and character of the feared 'European war'... will become the first world war in the full sense of the word.

The cell never acts; it reacts.

Civilization and the life of nations are governed by the same laws as prevail throughout nature and organic life.

Man is not distinguished from [the animals] by a special kind of soul, or by any peculiar and exclusive psychic function, but only by a higher degree of psychic activity, a superior stage of development.

Ontogeny re-capitulates phylogeny.

The real cause of personal existence is not the favor of the Almighty, but the sexual love of one's earthly parents.

In the course of individual development, inherited characters appear, in general, earlier than adaptive ones, and the earlier a certain character appears in ontogeny, the further back must lie in time when it was acquired by its ancestors.