e 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 differences between the
races." ("Descent of Man", page 308.)

I may be permitted a remark as to Darwin's attitude towards Lamarck.
While, at an earlier stage, when he was engaged in the