Nothing is constant but change! All existence is a perpetual flux of being and becoming! That is the broad lesson of the evolution of the world.

Man is not above nature, but in nature.

By ecology we understand the total science of the connections of the organism to the surrounding external world.

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.

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

Politics is applied biology.

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

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.

It is, however, a most astonishing but incontestable fact, that the history of the evolution of man as yet constitutes no part of general education. Indeed, our so-called educated classes are to this day in total ignorance of the most important circumstances and the most remarkable phenomena which Anthropogeny has brought to light.

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.

Phylogeny and ontogeny are, therefore, the two coordinated branches of morphology. Phylogeny is the developmental history [Entwickelungsgeschichte] of the abstract, genealogical individual ontogeny, on the other hand, is the developmental history of the concrete, morphological individual.

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 succession of individuals, connected by reproduction and belonging to a species, makes it possible for the specific form itself to last for ages. In the end, however, the species is temporary it has no eternal life. After existing for a certain period, it either dies or is converted by modification into other forms.

We are compelled by reflection to recognize that God is not to be placed against the material world [as in Christianity], but must be placed as a 'divine power' or 'moving spirit' within the cosmos itself ... All the wonderful phenomena of nature around us, organic as well as inorganic, are only various products of one and the same original force.