Comparative Evolution of Biomes 309 



from the eastern segment by the flat central area of the continent 

 as it is from the segment in northeastern Asia by the Bering Sea. 

 In terrestrial and freshwater biomes, therefore, inter-regional mix- 

 ing on the same continent may be governed by a set of factors 

 comparable to those involved in intercontinental mixing. 



Dispersal Availability 



When a bridge connects two segments of a biome, one would ex- 

 pect that all the species on each side would disperse across it, 

 resulting in a thorough mixing of the components of the two pre- 

 viously separated regional units. There is some evidence that this 

 may occasionally happen. Known fossils of the holarctic, middle 

 Cenozoic, temperate deciduous forest recorded by Chancy ( 1940 ) 

 contained sufficient species common to both North America and 

 Eurasia to indicate a fairly complete panmixia. However, for every 

 case like this there are many that indicate a much lesser degree of 

 mixing. Two factors cause a reduction from complete mixing: the 

 ecological nature of the bridging area and the geographic dis- 

 tribution of species in the biome. 



The connecting area may be within the limits of ecological 

 tolerance of only a few of the many species living at either end 

 of it. To cite an inter-regional example, in North America the 

 Rocky Mountain and the Appalachian mountain systems were con- 

 nected during the Pleistocene and are connected at present by a 

 non-mountainous area having cold lakes and streams. During this 

 time a few species of montane caddisflies such as Rhyacophila acro- 

 pedes and angelita spread from the Rocky Mountains to the Ap- 

 palachian Mountains. Only those which were able to live in the 

 slower cold streams of the flatter country dispersed in this fashion. 

 Many other eastern and western species which appear to be com- 

 pletely restricted to faster mountain streams either did not spread 

 over the routes or failed to persist in their new homes (Ross, 1956a). 

 An example from the monotremes explained by de Beaufort ( 1951 ) 

 also fits this concept. In early Cenozoic times populations of the 

 form ancestral to Echidna and Zaglossus spread over New Guinea 

 and Australia; during isolation in later Cenozoic time, Echidna 

 evolved in Australia and Zaglossus in New Guinea. During the 

 Pleistocene connection between these areas Echidna reached south- 

 ern New Guinea and Tasmania, but Zaglossus, which lives only 

 in the mountains, could not reach Australia because the connection 

 was entirely lowland in nature. 



