GEOGRAPHICAL DISTRIBUTION 175 



compete with many new forms and would be exposed 

 to somewhat different physical conditions; hence they 

 would be eminently liable to modification, and would 

 generally now exist as varieties or as representative 

 species; and this is the case. We must, also, bear in 

 mind the occurrence in both hemispheres of former 

 Glacial periods; for these will account, in accordance 

 with the same principles, for the many quite distinct 

 species inhabiting the same widely separated areas, and 

 belonging to genera not now found in the intermediate 

 torrid zones. 



It is a remarkable fact strongly insisted on by Hooker 

 in regard to America, and by Alph. de Candolle in 

 regard to Australia, that many more identical or slightly 

 modified species have migrated from the north to the 

 south than in a reversed direction. We see, however, 

 a few southern forms on the mountains of Borneo and 

 Abyssinia. I suspect that this preponderant migration 

 from the north to the south is due to the greater extent 

 of land in the north, and to the northern forms having 

 existed in their own bomes in greater numbers, and 

 having consequently been advanced through natural selec- 

 tion and competition to a higher stage of perfection, or 

 dominating power, than the southern forms. And thus, 

 when the two sets became commingled in the equatorial 

 regions, during the alternations of the Glacial periods, 

 the northern forms were the more powerful and were 

 able to hold their places on the mountains, and afterward 

 to migrate southward with the southern forms; but not 

 so the southern in regard to the northern forms. In the 

 same manner at the present day we see that very many 

 European productions cover the ground in La Plata, New 



