Natural selection acts solely by accumulating slight successive favorable variations, it can produce no great or sudden modification; it can act only by very short steps.

Disbelief crept over me at a very slow rate, but at last it was complete.

A mathematician is a blind man in a dark room looking for a black cat which isn't there.

It is really laughable to see what different ideas are prominent in various naturalists' minds, when they speak of 'species'; in some, resemblance is everything and descent of little weight-in some, resemblance seems to go for nothing, and Creation the reigning idea-in some, descent is the key,-in some, sterility an unfailing test, with others it is not worth a farthing. It all comes, I believe, from trying to define the undefinable.

Intelligence is based on how efficient a species became at doing the things they need to survive.

Often a cold shudder has run through me, and I have asked myself whether I may have not devoted myself to a fantasy.

In the distant future I see open fields for far more important researches. Psychology will be based on a new foundation, that of the necessary acquirement of each mental power and capacity by gradation. Light will be thrown on the origin of man and his history.