372 DIFFICULTIES AND IMPORTANCE OF THE CONCEPT 



ently, rediscovered much that has been considered axiomatic in 

 population systematics for seventy to eighty years. 



Our thinking on these questions is one-sided not only because 

 it is so strongly affected by our working material, but also be- 

 cause it is based on a limited number of selected examples. An 

 author who works with a hybrid complex will be impressed by 

 the importance of hybridization, another one who works with 

 insular forms by the importance of geographic isolation. We now 

 have reached the point where we are badly in need of compara- 

 tive systematics and of a strictly quantitative approach. Analyses 

 such as were done by Verne Grant (this symposium) should be 

 clone for as many groups of animals and plants as possible. Ob- 

 viously, this method can be applied only where the group has 

 reached a considerable degree of taxonomic maturity, but there 

 are now many such groups in the vertebrates, insects, and plants. 

 The need for such comparative systematics emphasizes the im- 

 portance of sound orthodox taxonomic monographs. 



Difficulties in the Application of Species Concepts 



In the introduction I attempted to describe the three basic 

 philosophical species concepts which play a role in systematics. 

 Much of the discussion of this symposium dealt with the problem 

 of the utilization of such concepts in the taxonomic practice. It 

 may be useful at this point to say a word or two on the basic 

 method of the application of concepts. In particular it must be 

 emphasized that a concept is not necessarily invalidated if it 

 cannot be applied in an individual case. The concept tree is un- 

 questionably valid, yet one may have doubts whether or not to 

 include in this concept such plants as a spreading juniper, a 

 dwarf willow, a giant cactus, and a strangler fig. Some of our 

 most universally accepted concepts encounter the same diffi- 

 culties as the species concept, namely, borderline cases or insuf- 

 ficient information. Child and adult are two concepts which are 

 not invalidated by the fact that the adolescent is a borderline 

 stage. Father is a completely valid concept, but its application 

 sometimes encounters difficulties as is evident in paternity suits. 

 The student who attempts to apply the species concept to con- 



