"The highest possible stage in moral culture is when we recognize that we ought to control our thoughts."

"Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows."

"One day, on looking at an orchid, I was struck with the idea that the structure of the flower was adapted to the visits of insects."

"A scientific man ought to have no wishes, no affections – a mere heart of stone."

"The expression of the emotions in man and animals."