m THE NORTE AND SO UTH. 401 



their transmitting to their ofEspriug diSerent constitutional 

 powers of resisting heat and cold. 



In the regular course of events the southern hemisphere 

 would in its turn be subjected to a severe Glacial period, 

 with the northern hemisphere rendered warmer; and then 

 the southern temperate forms would invade the equatorial 

 lowlands. The northern forms which had before been left 

 on the mountains would now descend and mingle with the 

 southern forms. These latter;, when the warmth returned, 

 would return to their former homes, leaving some few 

 species on the mountains, and carrying southward with 

 them some of the northern temperate forms which had 

 descended from their mountain fastnesses. Thus, we 

 should have some few species identically the same in the 

 northern and southern temperate zones and on the mount- 

 ains of the intermediate tropical regions. But the species 

 left during a long time on these mountains, or in opposite 

 hemispheres, would have to compete with many new forms 

 and would be exposed to somewhat different physical con- 

 ditions; hence, they would be eminently liable to modifica- 

 tion, and would generally now exist as varieties or as rep- 

 resentative 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 in- 

 habiting 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 sus- 

 pect 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 

 homes in greater numbers, and having consequently been 

 advanced through natural selection 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 alterna- 

 tions of the Glacial periods, the northern forms were the 



