124 “The Descent of Man” 
hair-covering in man as having some relation to a warm climate, 
while elsewhere he is inclined to make sexual selection responsible 
for it. Darwin recognises the great gap between man and his nearest 
relatives, but similar gaps exist at other parts of the mammalian 
genealogical tree: the allied forms have become extinct. After the 
extermination of the lower races of mankind, on the one hand, and of 
the anthropoid apes on the other, which will undoubtedly take place, 
the gulf will be greater than ever, since the baboons will then bound 
it on the one side, and the white races on the other. Little weight need 
be attached to the lack of fossil remains to fill up this gap, since the 
discovery of these depends upon chance. The last part of the chapter 
is devoted to a discussion of the earlier stages in the genealogy of 
man. Here Darwin accepts in the main the genealogical tree, which 
had meantime been published by Haeckel, who traces the pedigree 
back through Monotremes, Reptiles, Amphibians, and Fishes, to 
Amphioxus. 
Then follows an attempt to reconstruct, from the atavistic 
characters, a picture of our primitive ancestor who was undoubtedly 
an arboreal animal. The occurrence of rudiments of parts in one 
sex which only come to full development in the other is 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 popula- 
tion 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 environ- 
ment. 
