( xlii ) 



clifTeroncps ionnd within a siiooios Ix'twcen tlic various com])niicat varii'ties and 

 iiulividiiiils. Tlicrdore it is nec.essar)', in order to nnderatand tlie origin of the 

 specilic liarrier, to study the varii'tal difTerences, and find out among which 

 varieties tiierc is a ruch'uieutary sjie('ific harrier, and lience wliich varieties are 

 rudimentary (= incipient) species. It has been shown by one of us* (and 

 therefore we do not again full}' enter into the same question) that the development 

 of gamogenetic species into two or several species is not possible without an 

 effective extraneous barrier between the varieties, whicii barrier prevents the 

 fusion (if tli(! varieties, as does tiie specific barrier the fusion of the species, 

 and, furtlier, tliat this extraneous barrier is provided by geographical separation. 

 Isolation of one or more mutating fiictors is the cause of the portion of a 

 s])ecies subjected to them becoming different from the other components which 

 stand under other influences. All our researches confirm tliis conclusion based 

 on the facts of variation, and all attempts to demonstrate the possibility of the 

 separation of a species into several without some kind of local isolation are 

 fallacious in reasoning. Geocjraphicdl variation leads to a muHi plication of the 

 species; non-geographical variation at the highest to polgmorpliism. Geographical 

 variation is, therefore, of another kind than non-geographical variation, and 

 therefore geographical varieties have a different standing in the evolution of the 

 organic world from the individual and generatory varieties. 



Geographical varieties as incijnent species are the next classificatory category 

 below species, just as subfamily is a degree lower than family, and no better 

 term could have been invented for them than subspecies. With subspecies we 

 designate, therefore, nothing else but the geographically separated different 

 components of one and the same type, which components represent together 

 a species. The criterion of a subspecies is not a certain amount of difference, 

 but bodily difference and geogra]ihical separation. Synoecic varieties — i.e. varieties 

 from the same locality— are never subspecies. We have to empliasise this 

 distinction, as many authors constantly confound subspecies with non-geographical 

 varieties. Tiiere are comparatively very few species which do not vary 

 geograi)liicaily. It was an ardent opponent of Darwin — Wiegand — who put forward 

 as an argument against the theory of evolution that geographical variation was a 

 conditio sine qua non for the correctness of the theory of descent, and that there 

 was no sucli general basis for evolution. Systematists have proved by their 

 minute research that geographical variation is the rule and not the exception, 

 and they may be justly prond of this result of their untiring labours. Curiously 

 enough, uon-systematists do not generally seem to be aware of this result, nor 

 to fully comprehend its bearing on the theory of descent. 



A sjiecies which has not developed into subspecies (= geograpliical varieties 

 = geographical races or forms) may be individually or seasonally di- or poly- 

 morphic, and similarly the individuals of a subspecies may all fall into seasonal 

 and these into individual varieties. As the species of a genus are co-ordinate 



* Boe " Meclianical Selection " in iVor. Zool. iii. p. 126 (1896); " ReprocUictive Divergence, etc., in 

 Natural Science xii. p. 45 (189i<). 



