6 CONCEPTS AND DEFINITIONS 



today even though most geneticists, under the leadership of 

 Dobzhansky, Huxley, Ford, and others, have swung into Jordan's 

 camp. The principal opponents of the concept of objectively 

 delimitable species are today found among philosophers and 

 paleontologists. Publications maintaining this viewpoint are those 

 of Gregg (1950), Burma (1949, 1954), Yapp (1951), and Arkell 

 ( 1956 ) . These are only the most recent titles in a vast literature, 

 some of which is cited in the bibliography. 



The point which is perhaps most impressive when one studies 

 these voluminous publications is the amount of disagreement that 

 has existed and still exists. The number of possible antitheses 

 that have been established in this field may be characterized by 

 such alternate views, to mention only a few, as follows: 



Subjective versus objective; 



Scientific versus purely practical; 



Degree of difference versus degree of distinctness; 



Consisting of individuals versus consisting of populations; 



Only one kind of species versus many kinds of species; 



To be defined morphologically versus to be defined biologically. 



To give a well-documented history of the stated controversies 

 would fill a book. As interesting as this chapter in the history 

 of human thought is, the detailed presentation of the gropings 

 and errors of former generations would add little to the task 

 before us. Let us concentrate therefore on the gradual emergence 

 of the ideas which we, today, consider as central and essential. 

 Three aspects are stressed in most modern discussions of species, 

 that (1) they are based on distinctness rather than on difference 

 and are therefore to be defined biologically rather than morpho- 

 logically, (2) they consist of populations, rather than of uncon- 

 nected individuals, a point particularly important for the solution 

 <>l the problem of speciation, (3) they are more succinctly de- 

 fined by isolation from non-conspecific populations than by the 

 relation of conspecific individuals to each other. The crucial 

 species criterion is thus not the fertility of individuals, but rather 



