GEOGRAPHICAL DISTRIBUTION 173 



clothed with a mingled tropical and temperate vegetation, 

 like that described by Hooker as growing luxuriantly at 

 the height of from four to five thousand feet on the 

 lower slopes of the Himalaya, but with perhaps a still 

 greater preponderance of temperate forms. So again in 

 the mountainous island 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. Seemann 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 actually warmer, throws any clear light on the 

 present apparently inexplicable distribution of various 

 organisms in the temperate 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 natural- 

 ized plants and animals have spread within a few cen- 

 turies, this period will have been ample for any amount 

 of migration. As the cold became more and more in- 

 tense, we know that Arctic forms invaded the temperate 

 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 subtropical regions of the south, for the southern 

 hemisphere was at this period warmer. On the decline 



