Comparative Evolution of Biomes 



301 



particular biomes it is essential to know which continents now dis- 

 connected were once connected and vice versa, as well as when 

 the connections existed. Definite information is difficult to obtain 

 because of current disagreements concerning the history of the 

 continental masses, disagreements which are not only highly con- 

 troversial but extreme in divergence of view. The different hy- 

 potheses are treated in more detail in Chapter 1; only information 

 pertinent to this chapter is summarized here. 



One hypothesis maintains that the continents have occupied the 

 same relative positions since the origin of life, and that since the 

 course of organic evolution began, connecting corridors between 

 various continents such as the Bering bridge, the Isthmus of Pan- 

 ama, and the Asiatic-Australian bridge have arisen or sunk many 

 times (Fig. 130). These connections would have acted like gates 

 opening and closing at different times. 



BANDA 

 6E0SYNCLINE 



WESTRALIAN— /'.:-./f! 

 GEOSYNCLINE ^:v ' 





S/. 



'<^^ 



\ 



\ 



<^;^ 



FIJI IS 





-SAMOA 





^ I I — PACIFIC 



^ I I BORDER 



Fig. 130. Geomorphic features of the connecting link between Asia and Aus- 

 tralia. The Banda geosyncHne and the Melanesian arcs are thought to have 

 been unstable several times. (From Ross, compiled from various sources.) 



The opposing hypothesis holds that the continents have drifted 

 over the face of the globe and have had entirely different spatial 

 relations with each other during the evolution of current taxonomic 



