V. GRANT 41 



omy, and the clone, defined by descent from a single zygote. 

 Similarly, various authors have been forced to recognize different 

 kinds of species. 



The task of finding objective criteria for the definition of the 

 biological units, that is, criteria for marking off their boundaries 

 which can be applied by different observers with equivalent 

 results, has in general proved easier in the case of the microscopic 

 units than with the macroscopic ones. Thus the chromosome and 

 the cell are clearly definable units; there is every reason to expect 

 that the recombination gene and the functional gene can be 

 satisfactorily defined; but an inordinate amount of controversy 

 has been devoted to the species and the biotic community. As 

 between the latter two entities, moreover, it is apparent that 

 biologists are much closer at the present time to an understand- 

 ing of the species question than they are to finding a basis for 

 the definition of the community. This largest and most complex 

 of the biological units presents the greatest difficulties of defini- 

 tion and delimitation of them all. 



Evolution of the Species Concept 



The original species concept was that of primitive man. His 

 idea of species was based on the observation of discrete units in 

 the local fauna and flora. He had a specific name for each kind 

 of animal and plant to which he paid any attention; these names 

 corresponded rather well to objectively existing populations in 

 nature. Essentially the same species concept was adopted by the 

 local naturalist. Thus John Ray in 1686 defined the species as the 

 unit which breeds true within its own limits (cf. Darlington, 

 1940). 



The primitive species concept was not equipped for coping 

 with the phenomenon of geographical variation, which only be- 

 came apparent when the stage of man's activities expanded from 

 the restricted home territory to an area equal in extent to the 

 distribution range of many species. A species which was clear-cut 

 in the local community might become the center of a "species 

 problem" when a series of collections from different localities 

 was assembled and compared. To accommodate the expanded 



