Chap. XII.] IN THE NORTH Aj\D SOUTH. 339 ; 



any clear light on the present apparently inexplicable distribution of 

 various organisms in the temperate parts of both hemispheres, and on 

 the mountains of the tropics. The Glacial period, as measured by 

 years, must have been very long ; and when we remember over what 

 vast spaces some naturalised plants and animals have spread within 

 a few centuries, this period will have been ample for any amount of 

 migration. As the cold became more and more intense, we know 

 that Arctic forms invaded the temperate regions ; and, from the 

 facts just given, there can hardly be a doubt that some of the more 

 vigorous, dominant and widest-spreading temperate forms invaded 

 the equatorial lowlands. Tho inhabitants of these hot lowlands 

 would at the same time have migrated to the tropical and sub- 

 tropical regions of the south, for the southern hemisphere was at this 

 period warmer. On the decline of the Glacial period, as both hemi- 

 spheres gradually recovered 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 returning from the south. 

 Some, however, of the northern temperate forms would almost 

 certainly have ascended any adjoining high land, where, if suffi- 

 ciently 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 tem- 

 perature must have been very slow, and plants undoubtedly possess a 

 certain capacity for acclimatisation, 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 northern 

 hemisphere rendered wanner ; 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 mountains 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 conditions ; hence 

 they would be eminently liable to modification, and would generally • 

 now exist as varieties or as representative species; and this ia the' 



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