274 THE ORIGIN OF SPECIES 



of the Glacial period, as both hemispheres gradually re- 

 covered their former temperatures, the northern temperate 

 forms living on the lowlands under the equator would 

 have been driven to their former homes or have been 

 destroyed, being replaced by the equatorial forms return- 

 ing from the south. Some, however, of the northern 

 temperate forms would almost certainly have ascended 

 any adjoining high land, where, if sufficiently loft}', 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 ac- 

 climatization, as shown by their transmitting to their 

 offspring different constitutional powers of resisting heat 

 and cold. 



In the regular course of events the southern hemi- 

 sphere 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 fast- 

 nesses. Thus, we should have some few species identi- 

 cally the same in the northern and southern temperate 

 zones and on the mountains of the intermediate tropical 

 regions. But the species left during a long time on these 

 mountains, or in opposite hemispheres, would have to 



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