The geographical distribution of animals, with a study of the relations ...

152 DISTRIBUTION OF EXTINCT ANIMALS. [part ii.in mountains of ice around the two poles, would lower the generallevel of the ocean about 2,000 feet. This would be equivalentto a general elevation of the land to the same amount, and wouldthus tend to intensify the cold ; and this elevation may enableus to understand the recent discoveries of signs of glacialaction at moderate elevations in Central America and Brazil, farwithin the tropics. At the same time, the weight of ice piled upin the north would cause the land surface to sink there, perhapsunequally, according to the varying nature of the interior crustof the earth ;and since the weight has been removed land wouldand thus the phenomenarise again, still somewhat irregularly ;of raised beds of arctic shells in temperate latitudes, are explained.Now, it is evident, that the phenomena we have been considering—ofthe recent changes of the mammalian fauna inEurope, North America, South Temperate America, and thehighlands of Brazil—are such as might be explained by the mostextreme views as to the extent and vastness of the ice-sheet,and especially as toand southern hemispheres ;and where two such completely independentsets of facts areits simultaneous occurrence in the northernfound to combine harmoniously, andsupplement each other on a particular hypothesis, the evidencein favour of that hypothesis is greatly strengthened. An objectionthat will occur to zoologists, may here be noticed. Ifthe Glacial epoch extended over so much ofthe temperate andeven parts of the tropical zone, and led to the extinction of somany forms of life even within the tropics, how is it that somuch of the purely tropical fauna of South America has maintaineditself, and that there are still such a vast number offorms, both of mammalia, birds, reptiles, and insects, that seemorganized for an exclusive existence in tropical forests ? NowMr. Belt's theory, of the subsidence of the ocean to the extent ofabout 2,000 feet, supplies an answer to this objection ; for weshould thus have a tract of lowland of an average width ofsome hundreds of miles, added to the whole east coast of Centraland South America.This tract would, no doubt, become coveredwith forests as it was slowly formed, would enjoy a perfectly