376 ALTERNATE GLACIAL PERIODS 



they would have long survived like the arctic forms on the 

 mountains of Europe. They might have survived, even it 

 the climate was not perfectly fitted for them, for the change 

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

 doubtedly possess a certain capacity for acclimatization, 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 

 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 spe- 

 cies 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 south- 

 ern temperate zones and on the mountains of the interme- 

 diate 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 sep- 

 arated 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 sus- 

 pect 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 



