832 AN EMBRYOLOGIST'S VIEW 



such that there could have been a continuous population of 

 these frogs across Australia. There is good evidence for periods 

 during which most of the continent was moist. These humid 

 times were followed by dry intervals, some of which were drier 

 than the present-day climate. 



We can imagine, therefore, that at one time in the past the 

 frogs ancestral to C. signifera occupied a continuous zone across 

 Australia. Somewhat later the continuous distribution was broken 

 by the development of arid central regions. The east and west 

 populations are now different species. There is no way of telling 

 whether they became different after being isolated or whether, 

 as in R. pipiens, the differences developed while the range was 

 continuous. 



With either interpretation, or some combination of them, it is 

 clear that there is no need to invoke the concept of a culminating 

 sympatric stage in speciation. The differences that now serve 

 as isolating mechanisms arose in allopatric populations. 



Other Arguments against the Hypothesis 



We should not conclude from these two examples in frogs that 

 the hypothesis of the development of isolating mechanisms by 

 selection, in zones of overlap, against hybridization is incorrect. 

 For most cases, however, I do think it is unnecessary. The follow- 

 ing are some of the reasons. 



Undoubtedly there have been many instances where a species 

 population lias become divided and has remained so for long 

 periods of time. When the land connection between North 

 America and South America was reestablished in recent geo- 

 logical times, the continuous ranges of many marine organisms 

 was broken. At the present we find many pairs of related species, 

 the two members of each pair being separated by the Central 

 American isthmus. The most probable interpretation of this situa- 

 tion is that divergence was a consequence of evolution in isola- 

 tion. Since the two diverging groups never came together there 

 would have been no possibility of their developing or reinforcing 

 isolating mechanisms through the selection against hybridization. 



There must be numerous similar instances both for terrestrial 



