“There can be no doubt that the existing Fauna and Flora is but the last term of a long series of equally numerous contemporary species, which have succeeded one another, by the slow and gradual substitution of species for species, in the vast interval of time which has elapsed between the deposition of the earliest fossiliferous strata and the present day.”

“History warns us ... that it is the customary fate of new truths to begin as heresies and to end as superstitions.”

“The man of science has learned to believe in justification, not by faith, but by verification.”

“Perhaps the most valuable result of all education is the ability to make yourself do the thing you have to do, when it ought to be done, whether you like it or not. It is the first lesson that ought to be learned and however early a man's training begins, it is probably the last lesson that he learns thoroughly.”

“The scientific spirit is of more value than its products, and irrationally held truths may be more harmful than reasoned errors.”

“I am too much of a skeptic to deny the possibility of anything.”

“The most considerable difference I note among men is not in their readiness to fall into error but in their readiness to acknowledge these inevitable lapses.”

“The great end of life is not knowledge but action.”

“Act decidedly and take the consequences. No good is ever done by hesitation.”

“The most considerable difference I note among men is not in their readiness to fall into error but in their readiness to acknowledge these inevitable lapses.”

“Of moral purpose I see no trace in Nature. That is an article of exclusively human manufacture and very much to our credit.”