the explanation of this fact on Darwinian principles? If there were not
barbarous tribes like the Fuegians, one might imagine America to have
been peopled when mankind was somewhat more advanced and more capable of
diffusing itself over an entire continent. But I cannot well understand
why isolation such as accompanies a very low state of social progress
did not cause the Neo-tropical and Neo-arctic regions to produce by
varieties and Natural Selection two very different human races. May it
be owing to the smaller lapse of time, which time, nevertheless, was
sufficient to allow of the spread of the representatives of one and the
same type from Canada to Cape Horn? Have you ever touched on this
subject, or can you refer me to anyone who has?--Believe me ever most
truly yours,

CHA. LYELL.

* * * * *

TO SIR C. LYELL


1867.

Dear Sir Charles,--Why the colour of man is sometimes constant over
large areas while in other cases it varies, we cannot certainly tell;
but we may well suppose it to be due to its being more or less
correlated with constitutional characters favourable to life. By far the
most common colour of man is a warm brown, not very different from that
of the American Indian. White and black are alike deviations from this,
and are probably correlated with mental and physical peculiarities which
have been favourable to the increase and maintenance of the particular