vii HEREDITY, VARIATION 105 



the privilege and delight of the naturalist to seek out, to 

 study, and to marvel 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, commonly 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, which is yet so frequently 

 misapprehended, 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 variation 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 distinctness of species, 

 which they considered to be a fundamental and certain fact 

 of nature. Hence, perhaps, it was that Darwin 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 compared. 

 He therefore always guarded himself against assuming its 

 presence whenever required by using such expressions in 

 regard to the power of natural selection as, " If they vary, for 

 unless they do so, natural selection can effect nothing." 



This was the more strange because wherever we look 

 around us we find, in our own species, in our own race, 

 in our own special section of that race an amount of 



