The Descent of Man

posed to be more influential in modifying the
colour of the skin than mere heat; but as D'Orbigny
in South America, and Livingstone in
Africa, arrived at diametrically opposite conclusions
with respect to dampness and dryness,
any conclusion on this head must be considered
as very doubtful. (55. Livingstone's 'Travels
and Researches in S. Africa,' 1857, pp. 338, 339.
D'Orbigny, as quoted by Godron, 'De l'Espece,'
vol. ii. p. 266.)
Various facts, which I have given elsewhere,
prove that the colour of the skin and hair is
sometimes correlated in a surprising manner
with a complete immunity from the action of
certain vegetable poisons, and from the attacks
of certain parasites. Hence it occurred to me,
that negroes and other dark races might have
acquired their dark tints by the darker individuals
escaping from the deadly influence of the
miasma of their native countries, during a long
series of generations.