No. 496] ASPECTS OF THE SPECIES QUESTION 



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as merely "varietal/' and at what exact point do these 

 differences become of "specific" value? To this ques- 

 tion no answer can be given, since it depends entirely 

 upon the weight which different naturalists would attach 

 to any given individual difference. 2 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 of varietal 

 value; and, in the nature of the case, it seems impossible 

 to lay down any definite rules. To such an extent do 

 individual differences sometimes exist in particular 

 genera— termed "protean" or "polymorphic" genera— 

 that the determination of the different species and vari- 

 eties becomes an almost hopeless task. 



The second point in the definition of species— namely, 

 community of descent— is hardly in a more satisfactory 

 condition, since the descent of any given series of indi- 

 viduals from a single pair, or from pairs exactly similar 

 to one another, is at best but a probability, and is in no 

 case capable of proof. 



Upon the whole, then, it seems in the meanwhile safest 

 to adopt a definition of species which implies no theory, 

 and does not include the belief that the term necessarily 

 expresses a fixed and permanent quantity. Species, 

 therefore, may be defined as an assemblage of individuals 

 ivhich resemble each other in their essential characters, 

 are able, directly or indirectly, to produce fertile indi- 

 viduals, and which do not (as far as human observation 

 goes) give rise to individuals which vary from the gen- 

 eral type through more than certain definite limits. The 

 production of occasional monstrosities does not, of 

 course, invalidate this definition. 



