next discussed. This state of
things Darwin regards as derived from an original hermaphroditism. In
regard to the mammary glands of the male he does not accept the theory
that they are vestigial, but considers them rather as not fully
developed.

The last chapter of Part I deals with the question whether the
different races of man are to be regarded as different species, or as
sub-species of a race of monophyletic origin. The striking differences
between the races are first emphasised, and the question of the
fertility or infertility of hybrids is discussed. That fertility is
the more usual is shown by the excessive fertility of the hybrid
population of Brazil. This, and the great variability of the
distinguishing characters of the different races, as well as the fact
that all grades of transition stages are found between these, while
considerable general agreement exists, tell in favour of the unity of
the races and lead to the conclusion that they all had a common
primitive ancestor.

Darwin therefore classifies all the different races as sub-species of
_one and the same species_. Then follows an interesting inquiry into
the reasons for the extinction of human races. He recognises as the
ultimate reason the injurious effects of a change of the conditions of
life, which may bring about an increase in infantile mortality, and a
diminished fertility. It is precisely the reproductive system, among
animals also, which is most susceptible to changes in the environment.

The final section of this chapter deals with the formation of the
races of mankind. Darwin discusses the question how far the direct
effect of different conditions of life, or the inherited effects of
increased use or disuse may have brought about the characteristic
differences between the different races. Even in regard to the origin
of the colour of the skin he rejects the transmitted effects of an
original difference of climate as an explanation. In so doing he is
following his tendency to exclude Lamarckian explanations as far as
possible. But here he makes gratuitous difficulties from which, since
natural selection fails, there is no escape except by bringing in the
principle of sexual selection, to which, he regarded it as possible,
skin-colouring, arrangement of hair, and form of features might be
traced. But with his characteristic conscientiousness he guards
himself thus: "I do not intend to assert that sexual selection will
account for all the