Though nature grants vast periods of time for the work of natural selection, she does not grant an indefinite period; for as all organic beings are striving, it may be said, to seize on each place in the economy of nature, if any one species does not become modified and improved in a corresponding degree with its competitors, it will soon be exterminated.

He who is not content to look, like a savage, at the phenomena of nature as disconnected, cannot any longer believe that man is the work of a separate act of creation ... Man is the co-descendant with other mammals of a common progenitor.

Nature appears to have been the cruel stepmother rather than the mother of many animals.

Men and nature must work hand in hand. The throwing out of balance of the resources of nature throws out of balance also the lives of men.

There is no result in nature without a cause; understand the cause and you will have no need of the experiment.

I do not see why man should not be just as cruel as nature.