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



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 

 n-hen the northern hemisphere sufEered from the ex- 

 treme 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 hemi- 

 spheres, 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 naturahsed 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 vigor- 

 ous, dominant, and widest-spreading temperate forms in- 

 vaded the equatorial 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 of the Glacial period, as both hemi- 

 spheres 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, 



