What men need is, as much knowledge as they can assimilate and organize into a basis for action; give them more and it may become injurious. One knows people who are as heavy and stupid from undigested learning .

No one can help another very much in these crises of life; but love and sympathy count for something.

The struggle for existence holds as much in the intellectual as in the physical world. A theory is a species of thinking, and its right to exist is coextensive with its power of resisting extinction by its rivals.

Living things have no inertia, and tend to no equilibrium.

If a man cannot see a church, it is preposterous to take his opinion about its altar-piece or painted window.

As a natural process, of the same character as the development of a tree from its seed, or of a fowl from its egg, evolution excludes creation and all other kinds of supernatural intervention.

Veracity is the heart of morality.