400 ALTERNATE GLACIAL PERIODS 



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 begiuing to appear ab 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 temper- 

 ate.'' 



Now let us see whether Mr. CrolFs 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 ap- 

 parently 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 remem- 

 ber 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 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 subtropical regions of the south, for the 

 southern hemisphere was at this period warmer. On the 

 decline of the Glacial period, as both hemispheres gradu- 

 ally recovered their former temperature, the northern tem- 

 perate 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 return- 

 ing from the south. Some, however, of the northern 

 temperate forms would almost certainly have ascended any 

 adjoining high land, where, if sufficiently lofty, they would 

 have long survived like the Arctic forms on the mountains 

 of Europe. They might have survived, even if the climate 

 was not perfectly fitted for them, for the change of tem- 

 perature must have been very slow, and plants undoubtedly 

 possess a cerfeain capacity for acclimatization, as shown by 



