Comparative Evolution of Biomes 325 



order Cycadales, the individual genera occur in only one or the 

 other. In some plant groups entire families are restricted to one 

 region, as the Pandanaceae in the Old World and the Cyclanthaceae 

 in the New World. The large plant family Bromeliaceae is re- 

 stricted to the New World except for the single species Pitcairnia 

 Feliciana which occurs also on the west coast of Africa ( Lawrence, 

 1951). This lone colonist would seem to indicate the extreme in- 

 frequency of over-water dispersal of terrestrial organisms from 

 South America to Africa and the relatively absolute isolation of 

 these intercontinental segregates of the present tropical rain forest. 



Biome Extinction and Resurrection 



When tropical or subtropical climates presumably prevailed as far 

 north as the Bering Sea area in the Cretaceous and Eocene, moun- 

 tainous areas may not have been as high as at present. If boreal 

 forests or tundra communities had evolved previously, they might 

 have become extinct because of the disappearance of sufficiently 

 cold areas in which to live. In this event, the cold region biomes 

 of today would be the result of another colonization of cold areas 

 by species living in adjacent biomes. No direct evidence has yet 

 been presented on this possibility, but it is nevertheless a very real 

 one. It fits Axelrod's contention (1959) that many upland plant 

 species adapted to rigorous conditions evolved much earlier than 

 the record indicates, yet because of poor conditions for their fos- 

 silization may never be known. 



Indirect evidence for the extinction of past biomes adapted to 

 cool climates lies in the greater number of species which occur in 

 tropical regions as compared with the number found in temperate 

 and arctic regions. This latitudinal differential, well known to Wal- 

 lace (1878) and more recently treated by Darlington (1957) and 

 Fischer ( 1960 ) , can be explained best on the assumption that areas 

 for the survival of tropical biomes have always been available, 

 whereas temperate and arctic areas have periodically become so 

 reduced that many older temperate and arctic biomes became ex- 

 tinct. Although tropical areas have been relatively stable climat- 

 ically, most of them have been unstable geologically. As a result, 

 connected and disconnected island arcs, elevated then peneplaned 

 mountains, and other geomorphic events have produced in the 

 tropics a continuous succession of isolating and congregating condi- 

 tions leading to an equally continuous evolution of more and 

 more species and ecological communities. 



