Reading material: The Descent of Man by Charles Darwin

http://www.sacred-texts.com/aor/darwin/descent/index.htm

In full.

From Part 1, Chapter 7;

ON THE RACES OF MAN

… Geographical distribution is often brought into play unconsciously and sometimes consciously; so that forms living in two widely separated areas, in which most of the other inhabitants are specifically distinct, are themselves usually looked at as distinct; but in truth this affords no aid in distinguishing geographical races from so-called good or true species….

He covers his wider ideas on the evolution of Man in this one (not, as is commonly held, Origin).

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