J. A. MOORE 331 



This brief synopsis of the situation in R. pipiens will serve as 

 the first stage in our model of geographic speciation. It is a wide- 

 spread species occupying a diversity of habitats. So far as the data 

 go, it appears that it is a continuous group of interbreeding popu- 

 lations. Some of the widely separated populations, however, have 

 diverged to such an extent that they behave as different genetic 

 species. This divergence could not have been promoted by selec- 

 tion against hybrids, according to Dobzhansky's hypothesis, since 

 there is no possibility of hybrids being formed. 



Our question now is this: Where does the species go from 

 here? Some have regarded divergence in a continuous popula- 

 tion, such as R. pipiens, as a blind alley in evolution and, unless 

 some extrinsic factors can divide the species, it can never be 

 more than a group of interbreeding allopatric populations. I favor 

 this view, but the evidence for it is so incomplete that there is 

 little point in one being counted in adherence or in opposition. 



There is no difficulty in imagining the formation of two species 

 in R. pipiens if we invoke some extrinsic factors which would 

 obliterate the populations in the middle portions of the species' 

 range or if there arose several well-placed barriers to gene flow. 

 A variety of physical and biological factors could serve this pur- 

 pose. 



The next stage in our model of speciation will be a species 

 that at one time undoubtedly had a continuous distribution but 

 it is now found in two widely separated regions. 



Crinia signifera occurs in eastern and in western Australia but 

 not in a wide band of unsuitable country that extends from the 

 Great Australian Bight to the northwest coast. There are no 

 detectable morphological differences between the eastern and 

 western individuals and no one thought they were other than 

 the same species until cross fertilization experiments were per- 

 formed. When one crosses eastern and western individuals, how- 

 ever, profound hybrid defects and generally so little survival are 

 observed that it is necessary to consider them separate species. 



The fact that the eastern and western forms arc morphologi- 

 cally identical is best interpreted in terms of their descent from 

 a common ancestor. During the Pleistocene the climate was 



