Seite - 422 - in The Origin of Species

422 ORIGIN OF SPECIES like that described by Hooker as growing luxuriantly at the height of from four to five thousand feet on the lower slopes of the Himalayas, but with perhaps a still greater prepon- derance of temperate forms. So again in the mountainous islands of Fernando Po, in the Gulf of Guinea, Mr. Mann found temperate European forms beginning to appear at the height of about five thousand feet. On the mountains of Panama, at the height of only two thousand feet, Dr. See- mann found the vegetation like that of Mexico, "with forms of the torrid zone harmoniously blended with those of the temperate." Now let us see whether Mr. Croll's conclusion that when the northern hemisphere suffered from the extreme cold of the great Glacial period, the southern hemisphere was actu- ally warmer, throws any clear light on the present apparently inexplicable distribution of various organisms in the tem- perate parts of both hemispheres, and on the mountains of the tropics. The Glacial period, as measured by years, must have been very long; and when we remember over what vast spaces some naturalised plants and animals have spread within a few centuries, this period will have been ample for any amount of migration. As the cold became more and more intense, we know that Arctic forms invaded the tem- perate regions; and, from the facts just given, there can hardly be a doubt that some of the more vigorous, dominant and widest-spreading temperate forms invaded the equa- torial lowlands. The inhabitants of these hot lowlands would at the same time have migrated to the tropical and sub- tropical regions of the south, for the southern hemisphere was at this period warmer. On the decline of the Glacial period, as both hemispheres gradually recovered their former temperatures, the northern temperate forms living on the lowlands under the equator, would have been driven to their former homes or have been destroyed, being replaced by the equatorial forms returning from the south. Some, however, of the northern temperate forms 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