346 THE ORIGIN OF SPECIES 



to Europe, but have not been found in the intermediate tropical 

 seas. 



From the foregoing facts, namely, the presence of temperate 

 forms on the highlands across the whole of equatorial Africa, and 

 along the peninsula of India, to Ceylon and the Malay Archipel- 

 ago, 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 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 

 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 min- 

 gled 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 per- 

 haps a still greater preponderance of temperate forms. So again in 

 the mountainous island of Fernando Po, in the Gulf of Quinea, 

 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. CroU'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 dis- 

 tribution of various organisms in the temperate parts of both hemi- 

 spheres, and on the mountains of the tropics. The Glacial period, 

 as m.easured by years, must have been very long; and when we 

 remember over what vast spaces some naturalized plants and ani- 

 mals 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 temper- 

 ate 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 subtropical regions of the south, for 

 the southern hemisphere was at this period warmer. On the de- 

 cline of the Glacial period, as both hemispheres gradually recov- 



