From my early youth I have had the strongest desire to understand or explain whatever I observed. ... To group all facts under some general laws.

I suppose you are two fathoms deep in mathematics, and if you are, then God help you. For so am I, only with this difference: I stick fast in the mud at the bottom, and there I shall remain.

Nothing can be more improving to a young naturalist, than a journey in a distant country.

A grand and almost untrodden field of inquiry will be opened, on the causes and laws of variation, on correlation of growth, on the effects of use and disuse, on the direct actions of external conditions, and so forth.

About weak points [of the Origin] I agree. The eye to this day gives me a cold shudder, but when I think of the fine known gradations, my reason tells me I ought to conquer the cold shudder.

Light may be shed on man and his origins.

[Alexander von Humboldt was the] greatest scientific traveller who ever lived.