374 DIFFICULTIES AND IMPORTANCE OF THE CONCEPT 



sider them as variants within a single interbreeding population 

 or rather as several similar species. There is no automatic solu- 

 tion. The splitting of every bimodal curve into two morphological 

 species would lead to the separation of males and females or of 

 age classes into separate species and to other equally unbiological 

 conclusions. Sylvester-Bradley (1956) and Imbrie (this sympo- 

 sium) have pointed to the fallacy of this approach. "The paleon- 

 tologist who classes members of a single interbreeding commu- 

 nity in more than one species is liable to confuse even himself 

 when he refers to hybridism. The distinction, in fact, between 

 morphological species and bio-species can only be overlooked at 

 the peril of utter confusion." Paleontologists cannot afford to for- 

 get that fossils are nothing but the remains of formerly living 

 organisms and that these organisms when still alive occurred in 

 the form of genetically defined populations exactly as the species 

 still living today. It is these populations which the paleontologist 

 attempts to classify with the help of fossil remains, and morpho- 

 logical criteria are used merely as a means to an end. Paleontolo- 

 gists classify their material on the basis of inferences. The better 

 they understand the nature of biological populations, the rela- 

 tions of genotype and phenotype, and the results of developmen- 

 tal physiology, the more skillfully they can interpret the available 

 morphological evidence. There is no justification for abandoning 

 the biological approach merely because it is sometimes difficult 

 to decide whether or not several morphological types in a popu- 

 lation are eonspecifie. 



A second type of difficulty is introduced when there is insuffi- 

 cient information on the reproductive isolation of populations 

 that are not in contact with each other. Some paleontologists 

 have insisted that the classification of samples of discontinuous 

 vertical sequences introduces a new element in the evaluation of 

 populations, not appreciated by the neontologist. This is not alto- 

 gether true. The taxonomic, genetic, and ecological problems of 

 l lie multidimensional species arc quite the same whether one 

 deals with a series of populations in a chronological series, as 

 docs the paleontologist, or with geographically isolated popula- 

 tions, as does the neontologist. This has been recognized cor- 



