166 ALTERNATE GLACLAX PERIODS [Chap. XU. 



where, if sufficiently lofty, they would have long sur- 

 vived 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 acclimatisation, as shown 

 by their transmitting to their offspring different con- 

 stitutional 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 moun- 

 tains, and carrying southward with them some of the 

 northern temperate forms which had descended from 

 tin ir 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 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 inhabiting the same 



