CLASSIFICATION. 59 



and relations to surrounding circumstances and external 

 objects." 



On a closer examination, however, it will be found that 

 these two leading ideas in the definition of species — external 

 resemblance and community of descent — are both defective, 

 and liable to break down if rigidly applied. Thus, there 

 are in nature no assemblages of plants or animals, usually 

 grouped together into a single species, the individuals of 

 which exactly resemble one another in every point. Every 

 naturalist is compelled to admit that the individuals which 

 compose any so-called species, whethei of plants or animals, 

 differ from one another to a greater or less extent, and in 

 respects which may be regarded as more or less important. 

 The existence of such individual differences is attested by 

 the universal employment of the terms ''varieties" and 

 "races." Thus, a "variety" comprises all those individuals 

 which possess some distinctive peculiarity in common, but 

 do not differ in other respects from another set of indi- 

 viduals sufficiently to entitle them to take rank as a separate 

 species. A "race," again, is simply a permanent or "per- 

 petuated " variety. The question, however, is this — How 

 far may these differences amongst individuals obtain with- 

 out necessitating their being placed in a separate species ? 

 In other words : How great is the amount of individual 

 difference which is to be considered as merely " z^^r/r/^z/," 

 and at what exact point do these differences become of 

 ''specific'' value? To this question no answer can be given, 

 since it depends entirely upon the weight which different 

 naturalists would attach to any given individual difference.* 

 Distinctions which appear to one observer as sufficiently 

 great to entitle the individuals possessing them to be grouped 

 as a distinct species, by another are looked upon as simply 



• As an example of this, it is sufficient to allude to the fact that hardly 

 any two botanists agree as to the number of species of Willows and 

 Brambles in the British Isles. What one observer classes as mere 

 varieties, another regards as good and distinct species. 



