E. MAYR 9 



ical version meant merely "kind of." This, as far as inanimate 

 objects are concerned, is measured in terms of difference. But 

 one cannot apply this same standard to "kinds of" organisms, 

 because there are various biological "kinds." Males and females 

 may be two very different "kinds" of animals. Jack may be a 

 very different "kind" of a person from Bill, yet neither "kind" 

 is a species. Realization of the special aspects of biological varia- 

 tion has led to a restriction in the application of the term species 

 to a very particular "kind," namely the kind that would inter- 

 breed with each other. The first three authors found * by me who 

 state this clearly are Voigt (1817), "Man nennt Spezies . . . was 

 sich fruchtbar mit einander gattet, fortpflanzt"; Oken (1830), 

 "Was sich scharet und paaret, soil zu einer Art gerechnet wer- 

 den"; and Gloger ( 1833 ) , "What under natural conditions regu- 

 larly pairs, always belongs to one species." (He stated that by 

 stressing "regularly" he wanted to eliminate the complications 

 due to occasional hybridization.) Gloger later (1856) gave a 

 different, but similar definition: "A species is what belongs to- 

 gether either by descent or for the sake of reproduction." It is 

 interesting how completely all these definitions omit any refer- 

 ence to morphological criteria. They are obviously inapplicable 

 to asexually reproducing organisms. 



This is an exceedingly short outline of some of the trends in 

 the development of a modern species concept. More extensive 

 treatments can be found in the publications of Geoffrey St. Hi- 

 laire (1859), Besnard (1864), de Quatrefages (1892), Bachmann 

 (1905), Plate (1914), Uhlmann (1923), Du Rietz (1930), Kuhn 

 (1948), and other authors cited in the bibliography. Several 

 conclusions are self-evident. One is that biological or so-called 

 modern species criteria were already used by authors who pub- 

 lished more than one hundred years ago, long before Darwin. 

 Another is that a steady clarification is evident, yet that there is 

 still much uncertainty and widespread divergence of opinion 

 on many aspects of the species problem. It is rather surprising 

 that not more agreement has been reached during the past two 



* Still earlier statements can no doubt be found in the extensive litera- 

 ture, particularly on hybridization. 



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