EVIDENCE FROM DISTRIBUTION 139 



^Egean Seas were land; the junction of Italy and 

 Spain with Africa cut the Mediterranean into two 

 land-locked basins; the North Sea was land and the 

 British Islands were joined to each other and to the 

 continent. This, taken in connection with the 

 barrier made by the Desert of Sahara, explains why 

 North Africa belongs now to the great Holarctic 

 region and not to the Ethiopian, which covers the 

 continent south of the desert. 



I do not wish to give the impression that all the 

 problems of distribution have been solved and all the 

 difficulties removed, for such an impression would 

 be far from the truth. What can fairly be said is 

 that the main outline has been satisfactorily ex- 

 plained on the evolutionary theory and no other 

 theory even pretends to account for the facts. Just 

 in proportion to the completeness with which the 

 past history of any mammalian group is known, is 

 it easy to account for its present distribution. In 

 the same way, the more fully the geographical and 

 biological history of any continent has been ascer- 

 tained, the more readily can the arrangement and 

 relationships of its animals and plants be under- 

 stood. It is largely because the history is incom- 

 plete, that difficulties still remain. 



It is often very puzzling to explain why groups 

 that range together in the region of their origin 

 should behave so differently when new land-con- 

 nections are established and new regions invaded; 

 of this there are very many instances. The mas- 



