( xlii ) 



differencps found within a sjieoies between the various component, varieties and 

 individuals. Tlieroiore it is necessary, in order to nnderstand tlie origin of tlie 

 specific barrier, to stndy the varietal differences, and find out among which 

 varieties there is a rndimeutary specific barrier, and hence which varieties are 

 rndimentary (= incipient) species. It has been shown by one of ns * (and 

 therefore we do not again fnlly enter into the same question) that the development 

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

 efiective extraneous barrier between the varieties, which barrier prevents the 

 fusion of the varieties, as docs the specific barrier the fusion of the species, 

 and, further, that tliis extraneous barrier is provided by geographical separation. 

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

 species subjected to them becoming diiferent from the other components which 

 stand under other influences. All our researches confirm this 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. Geographical variation leads to a mu'tiplication of the 

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

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

 therefore geographical varieties have a ditferent standing in the evohrtion of the 

 organic world from the individual and generatory varieties. 



Geograjihieal varieties as incipient species are the ne.xt 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, tlierefore, 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 geographical separation. Synoecic varieties — i.e. varieties 

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

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

 varieties. There are comjjaratively very few species which do not vary 

 geograi)liically. 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 such general basis for evolution. Systematists have proved by their 

 minute research that geographical variation is the rule and not the exce])tion, 

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

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

 to fully comprehend its bearing on the theory of descent. 



A species which has not developed into subsjiecies (= geographical varieties 

 = geograjihical 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 



• See " Mechanical Selection " in .Vut'. Zoul. iii. p. 42G (18%); " Reproductive Divergence, etc., in 

 XaiuTal Scienct' xii. p. 45 (1898). 



