214 
THE DESCENT OF MAN. 
Part I. 
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. 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 conjec- 
ture : 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 connec- 
tion apparently existing between complexion and a 
tendency to consumption, the conjecture seemed to me 
not improbable. Consequently I endeavoured, with but 
little success , 48 to ascertain how far it held good. The 
48 In the spring of 1862 I obtained permission from the Director- 
General of the Medical department of the Army, to transmit to the 
surgeons of the various regiments on foreign service a blank table, 
with the following appended remarks, but I have received no returns. 
“ As several well-marked cases have been recorded with our domestic 
“ animals of a relation between the colour of the dermal appendages 
“ and the constitution ; and it being notorious that there is some limited 
“ degree of relation between the colour of the races of man and the 
“ climate inhabited by them ; the following investigation seems worth 
u consideration. Namely, whether there is any relation in Europeans 
“ between the colour of their hair, and their liability to the diseases of 
u tropical countries. If the surgeons of the several regiments, when 
“ stationed in unhealthy tropical districts, would be so good as first to 
“ count, as a standard of comparison, how many men, in the force 
“ whence the sick are drawn, have dark and light-coloured hair, and 
u hair of intermediate or doubtful tints ; and if a similar account were 
“ kept by the same medical gentlemen, of all the men who suffered 
“ from malarious and yellow fevers, or from dysentery, it would soon 
“ be apparent, after some thousand cases had been tabulated, whether 
“ there exists any relation between the colour of the hair and consti- 
“ tutional liability to tropical diseases. Perhaps no such relation would 
