376 DIFFICULTIES AND IMPORTANCE OF THE CONCEPT 



discussed by Imbrie ( this symposium ) . On the other hand, there 

 are conspicuous gaps in most of the celebrated cases of so-called 

 unbroken lineages such as for instance Brinkmann's Kosmoceras. 

 In some cases of real continuity between morphological species 

 the differences are so slight that most contemporary neontologists 

 would not hesitate to consider these forms merely subspecies of 

 a single polytypic species. Even though the number of cases that 

 cause real difficulties is very small, the fact remains that an ob- 

 jective delimitation of species in a multidimensional system is an 

 impossibility. 



Acquisition of Reproductive Isolation without Equivalent Mor- 

 phological Change. The reconstruction of the genotype which 

 is responsible for the reproductive isolation between two species 

 takes place sometimes without visible effect on the phenotype. 

 The resulting "sibling species" qualify as biological species, but 

 not as morphological species. Sonneborn has shown that the 

 varieties of Paramecium belong in this class. In plants most sib- 

 ling species appear to be allopolyploids. Where such sibling 

 species clearly differ in their ecology, nothing is gained by ignor- 

 ing them merely because the morphological difference is slight. 

 On the other hand, where morphological and ecological differ- 

 ences are not discernible, as seems to be the case in some in- 

 stances of allopolyploidy in plants or where the differences can 

 be established only by breeding in the laboratory such as the 

 varieties of Paramecium, it would seem impractical to separate 

 these forms as species in routine taxonomic work. 



Strong Morphological Differences without Reproductive Isola- 

 tion. A number of genera of animals and plants are known 

 where even strikingly different populations interbreed freely 

 wherever they come in contact. Grant (this symposium) has 

 discussed this situation in detail and has given numerous illus- 

 trations. The attitude of calling every morphologically distinct 

 population a species, which was widespread among classical 

 taxonomists, is definitely losing ground. Yet to combine all mor- 

 phological species that freely hybridize in zones of contact also 

 leads to absurdity. Full agreement as to where to compromise 

 between these two extremes has not yet been reached. Such sit- 



