E. MAYR 17 



of the isolating mechanisms between species thereby endorsing 

 the nondimensional species concept; still others finally empha- 

 sized the gene flow among interbreeding populations in a multi- 

 dimensional system, thus adopting the multidimensional collective 

 species concept. All three groups of geneticists thought they 

 were dealing with a uniquely "genetic species concept," yet they 

 were merely observing secondary manifestations of the primary 

 concepts. 



It is evident from the analysis of the morphological and ge- 

 netic species concepts, that such derived concepts are attempts 

 to deal directly with the discontinuities in nature. In the past, 

 almost every taxonomist worked with his own personal yardstick 

 based on a highly individual mixture of elements from the three 

 basic concepts. As a consequence one taxonomist might call 

 species every polymorph variant, a second one every morpholog- 

 ically different population, and a third one every geographically 

 isolated population. Such lack of standards, which is still largely 

 characteristic for the taxonomic literature, has been utterly con- 

 fusing to taxonomists and other biologists alike. It has therefore 

 been the endeavor of many specialists within recent decades to 

 find a standard yardstick, on which there could be general agree- 

 ment. A historical study of species definitions indicates clearly 

 a trend toward acceptance of a synthetic species definition, often 

 referred to as "biological species" definition. It is essentially 

 based on the nondimensional ("reproductive gap") and the 

 multidimensional ("gene flow") species concepts. Nearly all 

 species definitions proposed within the last fifty years incorporate 

 some elements of these two concepts. This is evident from the 

 species definitions of Jordan (Mayr, 1955), Stresemann (1919), 

 and Rensch (1929). Du Rietz (1930) called the species "a 

 syngameon . . . separated from all others by . . . sexual isola- 

 tion." Dobzhansky (1935) was apparently the first geneticist to 

 define species in the terms customary among naturalists and 

 taxonomists, namely interbreeding and reproductive isolation; 

 other recent definitions are variants of the same theme. Mayr 

 (1940) defined species as "groups of actually or potentially in- 

 terbreeding natural populations which are reproductively iso- 



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