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, fi-om 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 agi'eemcnt 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. 



