330 AN EMBRYOLOGIST'S VIEW 



point to be emphasized, however, is that the northern and south- 

 ern forms have developed isolating mechanisms, in reference 

 to each other, to such a degree that they could coexist and re- 

 main distinct. Since these differences that serve as isolating 

 mechanisms have developed in widely separated places, it is 

 not necessary to invoke Dobzhansky's hypothesis of a culminating 

 sympatric phase of speciation. 



What is the basis of the differences between the southern and 

 northern frogs? Are they the incidental and fortuitous concom- 

 itants of divergence? I prefer to think not for the following 

 reasons. The northern and southern embryos differ greatly in 

 their embryonic temperature adaptations. They differ in their 

 range of temperature tolerance, temperature coefficient for 

 development, and rate of development. Clearly they are quite 

 dissimilar in their physiologies, which presumably are genetically 

 determined. When we cross individuals with these dissimilar 

 physiologies, the offspring are highly abnormal. It is my belief 

 that the developmental abnormalities in the "hybrids" are due to 

 the inharmonious association of two genetic systems that differ 

 in their- modes of action. If this is the correct interpretation, the 

 genetic differences that are the basis of the embryonic adapta- 

 tions are likewise the basis of an isolating mechanism, namely, 

 hybrid inviability. Two sets of data indirectly support this possi- 

 bility: 



1. The magnitude of the differences in embryonic temperature 

 adaptations between any two populations parallels the degree of 

 abnormality shown by their "hybrids." This suggests that the 

 same genes might be the basis for both phenomena. 



2. The genes that are responsible for the embryonic tempera- 

 ture adaptations and the genes that interact to produce the 

 hybrid malformations have their effects at precisely the same 

 time in the life cycle. This also suggests that the genetic basis 

 for the embryonic temperature adaptations and hybrid inviabil- 

 ity is the same. 



If this interpretation is correct, the very process of adaptation 

 to diverse climatic conditions has resulted in genetic differences 

 that would also be completely effective isolating mechanisms. 



