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.
Pages:
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633