The Descent of Man

the intercrossed races of man, if established,
would not absolutely preclude us from ranking
them as distinct species.
Independently of fertility, the characters presented
by the offspring from a cross have been
thought to indicate whether or not the parentforms
ought to be ranked as species or varieties;
but after carefully studying the evidence, I
have come to the conclusion that no general
rules of this kind can be trusted. The ordinary
result of a cross is the production of a blended
or intermediate form; but in certain cases some
of the offspring take closely after one parentform,
and some after the other. This is especially
apt to occur when the parents differ in characters
which first appeared as sudden variations
or monstrosities. (15. 'The Variation of
Animals,' etc., vol. ii. p. 92.) I refer to this point,
because Dr. Rohlfs informs me that he has frequently
seen in Africa the offspring of negroes
crossed with members of other races, either