16 CONCEPTS AND DEFINITIONS 



Yet, as a basic, nonarbitrary yardstick, this is the species concept 

 on which we have to fall back whenever we encounter a border- 

 line situation. 



The Third Species Concept. This is a concept of an entirely 

 different kind, it is the concept of the polytypic or multidimen- 

 sional species. In contradistinction to the other two concepts, of 

 which one is based on a degree of difference, the second one on 

 the completeness of a discontinuity, this concept is a collective 

 one. It considers species as groups of populations, namely such 

 groups as interbreed with each other, actually or potentially. 

 Thus this species concept is a concept of the same sort as the 

 higher categories, genus, family, or order. Like all collective cate- 

 gories it faces the difficulty, if not impossibility, of clear demarca- 

 tion against other similar groupings. What this species gains in 

 actuality by the extension of the nondimensional situations in 

 space and time, it loses in objectivity. As unfortunate as this is, 

 it is inevitable since the natural populations, encountered by the 

 biologist, are distributed in space and time and cannot be di- 

 vorced from these dimensions. Thus, this species concept likewise 

 has its good and its bad points. 



Species Definitions 



All our reasoning in discussions of "the species" can be traced 

 back to the stated three primary concepts. As concepts, of course, 

 they cannot be observed directly, and we refer to certain ob- 

 served phenomena in nature as "species," because they conform 

 in their attributes to one of these concepts or to a mixture of 

 several concepts. From these primary concepts, just discussed, 

 we come thus to secondary concepts, based on particular aspects 

 of species. We have already mentioned the so-called morpho- 

 logical species concept, which, in most cases, is merely an ap- 

 plied typological concept, using morphological criteria. The case 

 of the so-called genetic species concept shows that all three of 

 the basic concepts can be expressed, on this level, in genetic 

 terms. Some geneticists, for instance, subscribed to the typolog- 

 ical concept and defined species by the degree of genetic differ- 

 ence as did Lotsy or DcVrics; others stressed the genetic basis 



