"Why acts nature thus? and why does she thus crowd her creatures one upon another? Because she would produce the greatest number and variety of living beings in the least space, so that one crushes another, and an equilibrium of powers can alone produce peace in the creation. Every species cares for itself, as if it were the only one in existence: but by its side stands another, which confines it within due bounds: and in this adjustment of opposing species creative nature found the only mean of maintaining the whole. She weighed the powers, she numbered the limbs, she determined the instincts of the species toward each other; and left the earth to produce what it was capable of producing."