Origin of Species, by Charles Darwin

Chapter V:
Laws of Variation

Effects of external conditions — Use and disuse, combined with natural selection; organs of flight and of vision — Acclimatisation — Correlation of growth — Compensation and economy of growth — False correlations — Multiple, rudimentary, and lowly organised structures variable — Parts developed in an unusual manner are highly variable: specific character more variable than generic: secondary sexual characters variable — Species of the same genus vary in an analogous manner — Reversions to long-lost characters — Summary