HEEEDITY, VAEIATION 113 



great world of life ; and has been thus enabled nut only to keep 

 all in complete adaptation to an ever-varying environment, but 

 to fill up, as it were, every element, every different station, 

 every crack and crevice in the earth's surface with wonderful 

 and beautiful creatures which it is the privilege and delight 

 of the naturalist to seek out, to study, and to mar\^el at. 



The Variation of Species, its Frequency and its Amount 



Having now shown something of the nature of heredity, its 

 universality and its limitations, we pass on to a rather fuller 

 discussion of the nature and amount of those limitations, com- 

 monly known as the variability of species. It is this variability 

 that constitutes the most important of the factors which bring 

 about adaptation, and that peculiar change or modification of 

 living things which we term distinct species. This change is 

 often very small in amount, but it always extends to various 

 parts or organs, and so pervades the whole structure as to 

 modify to a perceptible extent the habits and mode of life, the 

 actions and motions, so that we come to recognise each species 

 as a complete entity distinct from all others. 



There is no subject of such vital importance to an adequate 

 conception of evolution, w^hich is yet so frequently misappre- 

 hended, as variability. Perhaps owing to the long-continued 

 and inveterate belief in the immutability of species, the earlier 

 naturalists came to look upon those conspicuous cases of varia- 

 tion which forced themselves upon their attention as something 

 altogether abnormal and of no importance in the scheme of 

 nature. Some of them went so far as to reject them altogether 

 from their collections as interfering with the well-marked dis- 

 tinctness of species, which they considered to be a fundamental 

 and certain fact of nature. Hence, perhaps, it was that Dar- 

 win himself, finding so little reference to variation among wild 

 animals or plants in the works of the writers of his time, had 

 no adequate conception of its universality or of its large general 

 amount whenever extensive series of individuals were com- 

 pared. He therefore always guarded himself against assuming 



