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 



