296 Origin of Biomes and Succession 



biomes, as many still are today. Thus climatic changes would result 

 in what might be termed either a new biome by separation or a 

 new biome from old ingredients. This latter is reminiscent of the 

 subclimax sand communities in the neighboring temperate decid- 

 uous forest. 



CONGREGATION 



If biomes and associations can arise by separation, there is a theo- 

 retical possibility that the separated units might reunite. If, for 

 example, in the future the climate of southwestern North America 

 reverted to the more uniform one of Oligocene time, would the 

 five existing associations intermingle to form somewhat the same 

 single biome represented by the Madro-Tertiary flora of earUer 

 Cenozoic time? 



The question of a reunion of biomes may be of special interest 

 in questions arising from the geographic displacement of biomes 

 during maximum ice advances of the Pleistocene. Did the various 

 biomes maintain their taxonomic characteristics as we now know 

 them and simply condense in distinctive narrow bands, or did they 

 reunite to a considerable extent to form biomes unlike any now in 

 existence? Two circumstances favor the latter view. In the first 

 place during the ice maxima spruce pollen was deposited in several 

 localities near the Gulf Coast, many hundreds of miles south of 

 existing spruce stands. In the second place, the eastern states south 

 of the ice unquestionably had a climate different from any present 

 eastern climate, and therefore more than likely they had forests 

 with a combination of species different from any now extant. Per- 

 haps the result was somewhat like the Cretaceous forests suggested 

 by Peirce, probably with many more species than those earlier 

 forests but having a mixture of conifers and hardwoods which would 

 seem unusual to a student of existing biomes. 



SUMMARY 



Geomorphic changes may cause a simultaneous range fission of all 

 the species in a community and in this manner bring into existence 

 two daughter communities composed of diflFerent if closely related 

 species. If later geomorphic events reunite these communities, com- 

 petition between dominant species results in the establishment of 

 different communities existing side by side. 



The continued operation of this mechanism of alternating com- 

 munity fission and .reunion has led to the formation of the sue- 



