RODENTS OF LIBYA 53 



of the Nile Valley, and the river would thus have entered the Gulf of 

 Suez, rather than emptying into the Mediterranean Sea as it does 

 today. 



Other less tenable hypotheses include the diversion of the Nile 

 along the great depressions of northern Egypt and entering the Medi- 

 terranean near the present city of Benghazi, Libya, on the Gulf of 

 Sirte. It has also been suggested that the freshwater source for the 

 chain of oases stretching irregularly across the central Sahara from 

 Egypt to Morocco represents an "underground Nile" and thus a 

 remnant of a previous course followed by the Nile River. The similarity 

 of the fish fauna of these oases, many of which are today separated by 

 vast tracts of desert, tends to support this hypothesis. 



These latter changes in the course of the Nile are strictly conjectural, 

 but some similar phenomenon is required to explain the present dis- 

 tribution of Saharan and southwest Asian rodents. In the absence 

 of the Nile River as a barrier, the Arabian Peninsula, the Middle 

 Eastern countries, and the Isthmus of Sinai would have provided a 

 dispersal corridor allowing free interchange of Afro-Eurasian popula- 

 tions of rodents. 



To establish breeding populations of desert rodents on the west side 

 of the modern Nile in northwestern Egypt and northern Libya, a 

 faunal immigration of relatively short duration would have been 

 required. Following the reestablishment of the Nile along its present 

 course, these "founder populations" of desert rodents in Egypt and 

 Libya would have been isolated from the ancestral Eurasian popula- 

 tions and could have provided the "genetic pool" for the ensuing 

 dispersal of these rodents throughout large portions of North Africa 

 and the Sahara. 



Zoogeographical Relationships of the Rodents of the Sahara and 

 Southwest Asia 



During this study of the rodents of Libya it became necessary to 

 examine specimens from adjacent parts of the Sahara and from the 

 desert areas of Southwest Asia. As the study progressed, rather 

 striking similarities were noted in the composition of the fauna of 

 such widely scattered areas as the Saharan portions of North Africa, 

 the deserts of Saudi Arabia, and the dry portions of the Middle 

 East and Southwest Asia. Several genera of rodents occur exclusively 

 in these arid portions of North Africa and Southwest Asia, and many 

 others are widespread and occur only marginally in adjacent parts of 

 Eurasia and Africa. Most of these genera have little taxonomic 

 relationship with the rodents of marginal areas. 



In view of the high degree of endemism shown by the rodent fauna 

 of these desert regions of North Africa, Saudi Arabia, and Southwest 



