ALTERNATE GLACIAL PERIODS 423 



of temperature must have been very slow, and plants un- 

 doubtedly 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 hemisphere 

 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 low- 

 lands. The northern forms which had before been left on 

 the mountains would now descend and mingle with the south- 

 ern 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 is the 

 case. We must, also, bear in mind the occurrence in both 

 hemispheres of former Glacial periods; for these will ac- 

 count, 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 inter- 

 mediate torrid zones. 



It is a remarkable fact strongly insisted on by Hooker in 

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

 Australia, that many more identical or slightly modified spe- 

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

 reversed direction. 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 in the north, and to the 

 northern forms having existed in their own homes in greater 

 numbers, and having consequently been advanced through 

 natural selection and competition to a higher stage of per- 



