SIS ALTERNATE GLACIAL PERIODS [CEAV. XIL 



forms on the highlands across the whole of equatorial Africa, 

 and along the Peninsula of India, to Coy Ion and the Malay 

 Archipelago, and in a less well-marked manner across the wide 

 expanse of tropical South America, it appears almost certain that 

 at some former period, no doubt during the most severe part of a 

 Glacial period, the lowlands of these great continents were every- 

 where 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 height of from five to six thousand feet under 

 the same latitude, or perhaps even rather cooler. During this, 

 the coldest period, the lowlands under the equator must have 

 been 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 temparate 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 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 



