328 AN EMBRYOLOGIST'S VIEW 



mechanisms, to evolve in allopatric populations. Furthermore, 

 I feel that it is unnecessary to assume a culminating sympatric 

 phase involving selection against hybridization as has been done 

 by Dobzhansky. In developing my reasons questioning these 

 views, I shall first cite a sequence of cases that can serve as a 

 model for the origin of fully effective isolating mechanisms in 

 allopatric populations. 



Stages in the Evolution of Frogs 



The processes involved in the evolution of one species into one 

 or more different species requires a very long time. It has been 

 suggested that one million years might be taken as a rough aver- 

 age. A period of this magnitude is equivalent to at least 20,000 

 lifetimes of work for a student of evolution. Clearly one individual 

 could not expect to reach any significant conclusions during 

 his own lifetime by measuring the changes in a single species. 

 Instead he must study a variety of organisms in different stages 

 of speciation and then reconstruct the probable course of 

 events. It must always be remembered that this procedure is 

 indirect, and that the answer depends to a considerable extent on 

 the choice of examples. 



We will now list some data obtained from several frogs in a 

 manner suggesting a pattern of evolution. 



There are five common species of the genus Rana in the New 

 York area. All are "good" species and no taxonomist questions 

 their distinctness. Each one has its own characteristic habitat, 

 morphological features, behavior pattern, and geographic dis- 

 tribution. Some years ago, I presented data showing the close 

 relation between the breeding behavior, geographic distribu- 

 tion, and temperature adaptations of the embryos of these species 

 (Moore, 1949b). There is no question of the importance of these 

 adaptations. Rana sylvatica, which is an early spring breeder, 

 lias embryos with a cold-adapted physiology that could not allow 

 survival during the 1 summer when R. catesheiana breeds. Neither 

 could the heat-adapted R. catesheiana embryos survive in the 

 cold water of ponds in early spring. 



In all but one species the temperature adaptations correlated 



