“It is not enough to discover and prove a useful truth previously unknown, but that it is necessary also to be able to propagate it and get it recognized.”

The form follows the function.

It is not always the magnitude of the differences observed between species that must determine specific distinctions, but the constant preservation of those differences in reproduction.

All knowledge that is not the real product of observation, or of consequences deduced from observation, is entirely groundless and illusory.

Habits form a second nature.

What nature does in the course of long periods we do every day when we suddenly change the environment in which some species of living plant is situated.

All known living bodies are sharply divided into two special kingdoms, based upon the essential differences which distinguish animals from plants, and in spite of what has been said, I am convinced that these two kingdoms do not really merge into one another at any point.

It is not the organs-that is, the character and form of the animal’s bodily parts-that have given rise to its habits and particular structures. It is the habits and manner of life and the conditions in which its ancestors lived that have in the course of time fashioned its bodily form, its organs and qualities.