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

"A man who dares to waste one hour of time has not discovered the value of life."

"There is no fundamental difference between man and the higher mammals in their mental faculties."

"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."

"No one can feel more strongly than I do the extreme difficulty of accounting for the origin of species."