EVIDENCE FROM CLASSIFICATION 35 



the others to require separation as a distinct species. 

 Yet if the natural origin of these minor subdivisions 

 be admitted, there can be no valid reason for denying 

 the natural origin of species, genera and the higher 

 groups of classification, for the difference is one of 

 degree, not of kind, and the intergradation is often so 

 perfect, that it is a matter of personal judgment and 

 experience, whether the systematist shall regard a 

 given group of individuals as a variety, a subspecies, 

 or a species, whence the many differences and con- 

 flicts in practice. 



If a species, say of birds, having a very wide 

 geographical range, be carefully examined and com- 

 pared, it very frequently happens that specimens 

 taken from the extreme points of that range differ 

 so notably from one another, that the systematist 

 would not hesitate to separate them as distinct 

 species, were it not that in the intervening area all 

 the intergradations between the extreme forms can 

 be found. The recognition of species is thus often a 

 purely arbitrary procedure, as to which different 

 writers diverge widely in their judgment. All this 

 flux and inconstancy and intergradation between 

 very distinct extremes are hard to comprehend, if 

 we adopt the hypothesis of special creation, while, 

 on the other hand, they are precisely what would 

 be anticipated under the hypothesis of evolution. 



A study of the races of domesticated animals and 

 plants, which Darwin l investigated so thoroughly, 



1 The Variation of Animab and Plants under Domestication, 2 vols., 1868. 



