Origin of Biomes and Succession 



245 



so many elements in common that the two areas certainly represent 

 a community which was split by Pleistocene events (Fig. 105) 



RANGE OF INOIGO SNAKE 

 RANGE OF GOPHER TORTOISE 



Fig. 105. Range of the indigo snake Dnjmarchon corais and of the gopher 

 tortoises Gopherus pohjphemiis in the East, G. berkindieri in Texas and eastern 

 Mexico, and G. agassizi in the West. (After Conant and Blair.) 



(Neill, 1957). A division producing such community fragments 

 would have followed the southern movement of a community for- 

 merly continuous to the north of the present fragmented ranges. 

 A community movement of this type has been demonstrated for 

 five lizards which now occur together some distance south of the 

 locality in which all five lived together during the warmer Sangamon 

 interglacial period of the Pleistocene (Fig. 106) (Etheridge, 1958). 

 The occurrence of closely related species in physically similar 

 communities on either side of Central America attests to the former 

 continuity of these marine areas before the elevation during Pliocene 

 time of the Central American land barrier which now separates 

 them. To cite only a few examples, Walton (1950) recorded the 

 marine crayfish Parapijlocheles glasselli from the Pacific waters of 

 Mexico and its close relative P. scorpio from the Caribbean Sea; 

 and another marine crayfish Xijlopagiinis cancellariiis from the Pa- 

 cific waters of Colombia and its close relative X. rectus only from 



