CiiAr. XI. OF THE NOKTII AND SOUTH. 347 



splicre was at this period warmer. On the decline of the Ghi- 

 cial ]>eriod, as both liemis])heres gradually recovered their 

 former temperatures, the northern temperate forms, livins; on 

 the lowlands under tlie equator, would be driven to their former 

 homes or be destroyed, being replaced by the equatorial forms 

 returning from the south. Some, howeveK, of the northern 

 temperate forms would almost certainly ascend any adjoining 

 highland, where, if sufficiently lofty, they would long survive, 

 like the Arctic forms on the mountains of Europe. They might 

 survive, even if the climate was not perfectly fitted for them, 

 for the change of temperature must have been very slow, and 

 plants undoubtedly possess a certain capacity for acclimatiza- 

 tion, as slio\\Ti by their transmitting to their offspring different 

 constitutional powers of resisting heat and cold. 



In the regular course of events the southern hemisphere 

 would be subjected to a severe Glacial period, with the northern 

 hemisphere rendered warmer ; and then the southern temperate 

 forms would in their turn invade the equatorial lowlands. The 

 northern forms which had before been left on the mountains 

 would now descend and mingle Avith the southern forms. 

 These latter, Avhen the warmth returned, Avould return to their 

 former homes, leaving some few species on the mountains, and 

 carrying southward with them some of the northern temper- 

 ate 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 moun- 

 tains of the intermediate tropical regions. But the species left 

 during a long time on these mountains, or in opposite hemi- 

 spheres, would have to compete Avith many new forms, and 

 would be exposed to somewhat clifTerent physical conditions ; 

 hence tlicy would be eminently lialjle to modification, and 

 would generally now exist as varieties or as representative s]>c- 

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

 occurrence in both hemispheres of former Glacial jieriods ; 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 remarkaljlc fact, strongly insisted on by Hooker in 

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

 tralia, that inany more identical or now slightly-modified spe- 

 cies have migrated from the north to the south, than in a re- 

 versed direction. We see, however, a few southern forms on 



