agree, though
it must be recollected that it is one thing to allow that a given
migration is possible, and another to admit there is good reason to
believe it has really taken place.

But I can find no sufficient ground for accepting the fourth
proposition; and I doubt if it would ever have obtained its general
currency except for the circumstance that fair Europeans are very
readily tanned and embrowned by the sun. But I am not aware that there
is a particle of proof that the cutaneous change thus effected can
become hereditary, any more than that the enlarged livers, which
plague our countrymen in India, can be transmitted;--while there is
very strong evidence to the contrary. Not only, in fact, are there
such cases as those of the English families in Barbadoes, who have
remained for six generations unaltered in complexion, but which are
open to the objection that they may have received infusions of
fresh European blood; but there is the broad fact, that not a single
indigenous Negro exists either in the great alluvial plains of
tropical South America, or in the exposed islands of the Polynesian
Archipelago, or among the populations of equatorial Borneo or Sumatra.
No satisfactory explanation of these obvious difficulties has been
offered by the advocates of the direct influence of conditions. And as
for the more important modifications observed in the structure of the
brain, and in the form of the skull, no one has ever pretended to show
in what way they can be effected directly by climate.

It is here, in fact, that the strength of the Polygenists, or those
who maintain that men primitively arose, not from one, but from many
stocks, lies. Show us, they say to the Monogenists, a single case in
which the characters of a human stock have been essentially modified
without its being demonstrable, or, at least, highly probable, that
there has been intermixture of blood with some foreign stock. Bring
forward any instance in which a part of the world, formerly inhabited
by one stock, is now the dwelling-place of another, and we will prove
the change to be the result of migration, or of intermixture, and not
of modification of character by climatic influences. Finally, prove
to us that the evidence in favour of the specific distinctness of many
animals, admitted to be distinct species by all zoologists, is a whit
better than that upon which we maintain the specific distinctness of
men.

If presenting unanswerable objecti