IN THE NORTH AND SOUTH. 375 



continents were everywhere tenanted under the equator by a 

 considerable number of temperate forms. At this period the 

 equatorial climate at the level of the sea was probably about 

 the same with that now experienced at the hei'ght of from 

 five to six thousand feet under the same latitude, or perhaps 

 even rather cooler. During this, the coldest period, the low- 

 lands under the equator must have been clothed with a 

 mingled tropical and temperate vegetation, like that de- 

 scribed 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 tem- 

 perate 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 actu- 

 ally warmer, throws any clear light on the present appar- 

 ently 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 naturalized 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 equato- 

 rial lowlands. The inhabitants of these hot lowlands would 

 at the same time have migrated to the tropical and subtropi- 

 cal 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 tem- 

 perature, the northern temperate forms living on the low- 

 lands 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, 



