CHAP. XII.] IN THE NORTH AND SOUTH. 319 



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 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 

 northera 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 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 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 Candolle in regard to Australia, that 

 many more identical or slightly modified species 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 perfection, or dominating power, than the 

 southern forms. And thus, when the two sets became com- 

 mingled in the equatorial regions, during the alternations of 

 the Glacial periods, the northern forms were the more powerful 

 and were able to hold their places on the mountains, and after- 



