GEOGRAPHICAL DISTRIBUTION 347 



ered their former temperature, the northen temperate forms liv- 

 ing on the lowlands under the equator, would have been driven to 

 their former homes or have been destroyed, being replaced by the 

 equatorial forms returning from the south. Some, however, of the 

 northern temperate forms would almost certainly have ascended 

 any adjoining high land, where, if sufficiently lofty, they would 

 have long survived like the arctic forms on the mountains of 

 Europe. They might have survived, 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 acclimatization, as shown by their transmitting to 

 their offspring different 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 north- 

 ern hemisphere rendered warmer; and then the southern temper- 

 ate form? 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 temper- 

 ate zones and on the mountains of the intermediate tropical re- 

 gions. 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 

 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 oc- 

 currence 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 CandoUe in regard to Aus- 

 tralia, that many more identical or slightly modified species have 

 migrated from the north to the south, than in a reversed direc- 

 tion. 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 



