E. MAYR 373 



crete situations in nature faces, as all the speakers have empha- 

 sized, numerous difficulties. At first sight there appears to be a 

 bewildering diversity of perplexities, but these can be classified 

 into some major groups as was shown by Grant (this sym- 

 posium), and a study of the various classes of difficulties helps 

 considerably in an understanding of the species problem. Basi- 

 cally all obstacles in the application of a biological species con- 

 cept are due either to a lack of pertinent information on some 

 essential property of the investigated material or to its evolu- 

 tionary intermediacy. 



Lack of Information 



Different kinds of information are needed to permit the cor- 

 rect assignment of individuals to species. Most commonly the 

 question arises whether certain morphologically rather distinct 

 individuals belong to the same species or not. The long list of 

 synonyms, characteristic for some groups of animals and plants, 

 are a concrete expression of this difficulty (Mayr, Linsley, and 

 Usinger, 1953). This difficulty is particularly acute in three 

 branches of animal taxonomy. In many families of insects, par- 

 ticularly hymenoptera, males and females are so different that 

 separate classifications for males and females have to be adopted 

 until the proper associations have been made. Even more dif- 

 ferent are the larval stages in many groups of insects and aquatic 

 organisms. The same is true for parasites for which it is likewise 

 sometimes necessary to have two sets of names, one for larval 

 and one for adult stages, until association can be demonstrated 

 through elucidation of the life cycle. The difficulties caused by 

 sexual dimorphism, age differences, or nongenetic habitat differ- 

 ences which the neontologist faces in his work must be empha- 

 sized because some taxonomists seem to believe that paleontolo- 

 gists are the only ones who have to cope with the difficulty of 

 having to draw inferences from morphological types. That this 

 difficulty is particularly acute in paleontology no one will deny. 

 The worker who finds two or more essentially similar, yet some- 

 what different, morphological types in a single sample of fossils 

 is forced to make a somewhat arbitrary decision whether to con- 



