Darwinism stated by Darwin himself · Charles Darwin
THE RACES GRADUATE INTO EACH OTHER.
Every naturalist who has had the misfortune to undertake the description of a group of highly-varying organisms, has encountered cases (I speak after experience) precisely like that of man; and, if of a cautious disposition, he will end by uniting all the forms which graduate into each other under a single species; for he will say to himself that he has no right to give names to objects which he can not define. Cases of this kind occur in the order which includes man, namely, in certain genera of monkeys; while in other genera, as in _Cercopithecus_, most of the species can be determined with certainty. In the American genus _Cebus_, the various forms are ranked by some naturalists as species, by others as mere geographical races. Now, if numerous specimens of _Cebus_ were collected from all parts of South America, and those forms which at present appear to be specifically distinct were found to graduate into each other by close steps, they would usually be ranked as mere varieties or races; and this course has been followed by most naturalists with respect to the races of man.
Text from Project Gutenberg, public domain in the United States.