Chap. XII.] IN THE NORTH AND SOUTH. 165 



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 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 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 equatorial lowlands. The inha- 

 bitants 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 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 re- 

 placed by the equatorial forms returning from the south. 

 Some, however, of the northern temperate forms would 

 almost certainly have ascended any adjoining high land, 



