The Descent of Man

a during 1837, Dr. Blair found that the deathrate
of the immigrants was proportional to the
latitude of the country whence they had come.
With the negro the immunity, as far as it is the
result of acclimatisation, implies exposure during
a prodigious length of time; for the aborigines
of tropical America who have resided
there from time immemorial, are not exempt
from yellow fever; and the Rev. H.B. Tristram
states, that there are districts in Northern Africa
which the native inhabitants are compelled
annually to leave, though the negroes can remain
with safety.
That the immunity of the negro is in any degree
correlated with the colour of his skin is a mere
conjecture: it may be correlated with some difference
in his blood, nervous system, or other
tissues. Nevertheless, from the facts above alluded
to, and from some connection apparently
existing between complexion and a tendency to
consumption, the conjecture seemed to me not