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.

As our mother-earth is a mere speck in the sunbeam in the illimitable universe, so man himself is but a tiny grain of protoplasm in the perishable framework of organic nature.

I established the opposite view, that this history of the embryo (ontogeny) must be completed by a second, equally valuable, and closely connected branch of thought - the history of race (phylogeny). Both of these branches of evolutionary science, are, in my opinion, in the closest causal connection; this arises from the reciprocal action of the laws of heredity and adaptation... 'ontogenesis is a brief and rapid recapitulation of phylogenesis, determined by the physiological functions of heredity (generation) and adaptation (maintenance).

Among the Spartans all newly born children were subject to a careful examination or selection. All those that were weak, sickly, or affected with any bodily infirmity, were killed. Only the perfectly healthy and strong children were allowed to live, and they alone afterwards propagated the race.

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.

The whole of organic nature on our planet exists only by a relentless war of all against all. ... The raging war of interests in human society is only a feeble picture of an unceasing and terrible war of existence which reigns throughout the whole of the living world.

Mental differences between the lowest men and the animals are less than those between the lowest and the highest man.