Origin of Biomes and Succession 257 



Comparable community aggregations undoubtedly exist in the wa- 

 ter layers, but perhaps because of a lack of data no consensus of 

 opinion has been reached as to how these communities should be 

 delineated or what basis should be used in their delineation. 



This situation is not surprising. Any system of communities is 

 the result of historic evolutionary processes superimposed on en- 

 vironmental gradients. Because the ecological gradients are dif- 

 ferent on land, on the ocean bottom, and in the sea, it is entirely 

 conceivable that in each of these realms quite different criteria 

 will eventually be used for community segregation and delineation. 



EVOLUTION WITHIN THE BIOME 



Considering biomes in the northern temperate regions, especially 

 the grassland, deciduous forest, and evergreen coniferous forest 

 biomes, one might be led to believe that the evolution of biomes 

 was associated with the evolution of taxonomic units of plants such 

 as the grasses and conifers. This is far from the case. The dominant 

 vegetation growing under the same ecological conditions may pro- 

 duce essentially the same tempering of the basic ecology for a very 

 long span of geologic time, but the taxonomic composition of the 

 vegetation may change radically and frequently. Thus vegetation 

 that can be described as a luxuriant evergreen tropical rain forest 

 has been in existence since Devonian time, but the kinds of trees 

 comprising it have changed many times. Comparing fossil floras 

 far-distant in time which we believe occupied essentially the same 

 ecological situation, we see that the community dominants and 

 therefore the communities themselves were quite different taxonom- 

 ically at these various times. 



In Devonian and Mississippian times, the equivalent of the 

 present-day evergreen tropical rain forest included as its dominants 

 chiefly tree ferns, scale trees, and pteridosperms. In the Mesozoic 

 there was another combination of dominants in the tropical rain 

 forests: tree ferns, cycads, and conifers. The present-day tropical 

 rain forests lack scale trees and pteridosperms and have only rare 

 cycads; the tree ferns are abundant only locally; their dominant 

 species are chiefly angiosperms. 



The changes occurred gradually through time, for some cycads 

 occurred in Paleozoic forests, and remnants of these and the tree 

 ferns still persist as minor elements of present-day forests. If we 

 had a continuous record of one of these "community columns" 

 moving through time, we would undoubtedly find it impossible to 



