E. MAYR 377 



nations occur also in the animal kingdom as, for instance, in the 

 snail genus Cerion (Mayr and Rosen, 1956). When the morpho- 

 logical differences between such populations far exceed those 

 normally found between good species in related genera, the 

 taxonomist is reluctant to unite them into a single species. These 

 cases are in a way the exact opposite to sibling species. The ac- 

 quisition of reproductive isolating mechanisms is lagging far 

 behind the general genetic divergence of these populations as 

 indicated by their morphological divergence. 



Deficiencies in the Isolating Mechanisms. One, if not the most 

 important, attribute of a species is its possession of isolating 

 mechanisms. These isolating mechanisms are usually composite 

 and mutually reinforcing, but often they maintain only partial 

 isolation between populations. This becomes evident when a tem- 

 porary isolation between geographical isolates breaks down, re- 

 sulting in allopatric, or if the isolation was primarily ecological, 

 in sympatric hybridization. Numerous cases are known where 

 natural populations acted toward each other like good species in 

 areas of contact as long as their habitats were undisturbed. Yet 

 when the characteristics of these habitats were suddenly changed 

 in a drastic manner, usually by the interference of man, the re- 

 productive isolation broke down. Before this breakdown every- 

 one would have agreed that these populations were species, but 

 after the breakdown and after the loss of reproductive isolation, 

 they agreed better with the specifications of conspecific popula- 

 tions. 



The frequency of hybridization, that is, the susceptibility to a 

 secondary breakdown of partial reproductive isolation, is very 

 different in different groups of animals and plants. In most 

 higher animals hybridization is sufficiently rare (or else the hy- 

 brids sterile) not to cause any serious difficulties in species 

 delimitation. In cases like the snail genus Cerion, discussed 

 above, it is a major source of difficulty. Grant has discussed in 

 detail ( in this symposium ) the effect of this phenomenon on the 

 species problem in plants and has supplied quantitative data 

 indicating its relative importance in different genera. It is obvious 

 that the delimitation of species becomes a serious problem in 



