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Show ISLAND LIFE. . (rART T. north temperate zone, which bad been buried in snow or i?e, would become again clothed with vegetation and stocked w1th animal life, both of which, as the cold again came on, would be ddven southward, or perhaps partially exterminated. Forms usually separated would thus be crowded together, and a struggle for existence would follow, which must have led to the modification or the extinction of many species. When the survivors in the struggle had reached a state of equilibrium, a fresh field would be opened to them by the later ameliorations of climate; the more successful of the survivors would spread and multiply; and after this had gone on for thousands of generations, another change of climate, another southward migration, another struggle of northern and southern forms would take place. But if tho last glacial epoch has coincided with, and has bee~ to a considerable extent caused by, a high excentricity of the earth's orbit, we are naturally led to expect that earlier glacial epochs would have occurred whenever the excentricity .. was unusually large. Dr. Croll has published tables showing tlo varying amounts of excentricity for three million years back ; and from these it appears that there have been many periocls of high exceutricity, which has often been far greater than a~ the time of the last glacial epoch.1 The accompanying d1agram has been drawn from these tables, and it will be seen that the highest excentricity occurred 850,000 years ago, at which time the difference between the sun's distance 3 t aphelion and perihelion was thirteGn and a half millions of mile~, whereas during the last glacial period the maximum difference was ten and a half million miles. Now, judging by the amount of organic and physical chanrrc that occurred during and since the glacial epoch, and tb:t which has occurred since the Miocene period, it is consiclercd probable that this ma.ximum of excentricity coincided with some part of the latter period; and Dr. Croll maintains that a glacial epo.ch must then have occurred surpassing in severity that of wh10h we have such convincing proofs, and consisting like it of 1 London, Edinburgh and Dublin Philosophical Magazine Vol. XXXVI. pp. 144-150 (1868). ' ' CilAP. IX.] ANUIENT GLACIAL EPOCHS. alternations of co ld and warm phases elvery 10,500 years. The diao-ram ~ a so shows h b " . us anot er long-continued r-- o-+-.;..--___:o:~_ penod of high excentricity from 1,750,000 to 1 ' 950 ' 000 years arro and !et another almost equal to tll~ maximum 2,500,000 years back. The.s e may perha ps h ave occurred durmg the Eocene and Cr e t aceous epoch~ respectively, or all may have been m~luded within the limits of t~e Tertiary period. A.s two of these lngh excentricities greatly exceed th~t which caused our glacial epoch while the third is almost equal to i~ a~d of longer duration, they seem to affor~ us the means of testing rival theones of the causes of glaciation If: ~s J?r. Croll argues, high excen~ tnCity IS the great and dominatjnO' agency in bringing on glacial epoch: g~ographical changes being subor~ dmate, then there must have b 1 . 1 een g aCia epochs of great severity at all these three periods ; w bile if he is also correct in supposing that the ~lte~nate phases of precession would :ev:tably produce glaciation in one e.misphere, and a proportionately mild .and equable climate in the opposite hemisphere, then we should hav~ to look for evidence of exceptwnal~ y warm and exceptionally cold P.enods, occurring alternately ~nd With several repetitions, with1m ·a space of time which , geo- . ogwally speaking, is very short mdeed. l_ __ _ 165 |