E. MAYR 11 



in nature. Yet they are secondary, derived concepts, based on 

 underlying philosophical concepts, which might also be called 

 primary or theoretical concepts. I believe that the analysis of 

 the species problem would be considerably advanced, if we could 

 penetrate through such empirical terms as phenotypic, morpho- 

 logical, genetic, phylogenetic, or biological, to the underlying 

 philosophical concepts. A deep, and perhaps widening gulf has 

 existed in recent decades between philosophy and empirical 

 biology. It seems that the species problem is a topic where pro- 

 ductive collaboration between the two fields is possible. 



An analysis of published species concepts and species defini- 

 tions indicates that all of them are based on three theoretical 

 concepts, neither more nor less. An understanding of these three 

 philosophical concepts is a prerequisite for all attempts at a 

 practical species definition. And all species criteria or species 

 definitions used by the taxonomist in his practical work trace 

 back ultimately to these basic concepts. 



The Typological Species Concept. This is the simplest and 

 most widely held species concept. Here it merely means "kind 

 of." There are languages, as for instance German, where the term 

 for "kind" (Art) is also used for "species." A species in this con- 

 cept is "a different thing." This concept is very useful in many 

 branches of science and it is still used by the mineralogist who 

 speaks of "species of minerals" (Niggli, 1949) or the physicist 

 who speaks of "nuclear species." This simple concept of every- 

 day life was incorporated in a more sophisticated manner in the 

 philosophy of Plato. Here, however, the word eidos (species, in 

 its Latin translation) acquired a double meaning that survives 

 in the two modern words "species" and "idea" both of which are 

 derived from it. According to Plato's thinking objects are merely 

 manifestations, "shadows," of the eidos. By transfer, the indi- 

 viduals of a species, being merely shadows of the same type, do 

 not stand in any special relation to each other, as far as a typol- 

 ogist is concerned. Naturalists of the "idealistic" school endeavor 

 to penetrate through all the modifications and variations of a 

 species in order to find the "typical" or "essential" attributes. 

 Typological thinking finds it easy to reconcile the observed 



