vol. ii. pp. 227, 335.) It has long been known that negroes, and even
mulattoes, are almost completely exempt from the yellow-fever, so
destructive in tropical America. (57. See, for instance, Nott and
Gliddon, Types of Mankind, p. 68.) They likewise escape to a large
extent the fatal intermittent fevers, that prevail along at least 2600
miles of the shores of Africa, and which annually cause one-fifth of
the white settlers to die, and another fifth to return home invalided.
(58. Major Tulloch, in a paper read before the Statistical Society,
April 20, 1840, and given in the Athenaeum, 1840, p. 353.) This
immunity in the negro seems to be partly inherent, depending on some
unknown peculiarity of constitution, and partly the result of
acclimatisation. Pouchet (59. The Plurality of the Human Race
(translat.), 1864, p. 60.) states that the negro regiments recruited
near the Soudan, and borrowed from the Viceroy of Egypt for the Mexican
war, escaped the yellow-fever almost equally with the negroes
originally brought from various parts of Africa and accustomed to the
climate of the West Indies. That acclimatisation plays a part, is shewn
by the many cases in which negroes have become somewhat liable to
tropical fevers, after having resided for some time in a colder
climate. (60. Quatrefages, Unite de l Espece Humaine, 1861, p. 205.
Waitz, Introduction to Anthropology, translat., vol. i. 1863, p. 124.
Livingstone gives analogous cases in his Travels. ) The nature of the
climate under which the white races have long resided, likewise has
some influence on them; for during the fearful epidemic of yellow fever
in Demerara during 1837, Dr. Blair found that the death-rate 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 improbable. Consequently I endeavoured,
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