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Show 224 ISLAND LIFE. [rART I. . t . 't cor it is impossible to doubt that a varying distance excen nci y, 1 . ' from 86 to 99 mi. lh•o ns of m1·1 es (w h I' c h of the sun m summer . . is what occurred during-as supposed-the Mwc~ne peno~, 850,000 years ago) would prod~ce an i~p?rt~nt difference m the summer temperature and m the actmiC mtlu,ence of sunshine on vegetation. For the intensity of the suns rays would vary as the square of the distance, o.r .nearly as 7 4 to 98, so that the earth would be actually receivmg one-fourth less sunheat during summer at one time than at the other. An equally high excentricity occurred 2,500,000 years back: and no doubt was of t en r eached durin()o' still earlier epochs, while .a lower bu. t t'll very hi<Yh excentricity has frequently prevailed, and IS s I b bl nea; its avera<Ye value. Changes of climate, therefore, pro a y o . d' d d f every 10 , 500 years ' of the character above m wate an . o varying intensity, have been the rule rather than the exc~ptwn in past time; and these cha~ges mus.t. have been vanously modified by changing geographical conditiOns so ~s .to produce climatic alterations in different directions, and givmg to the ancient lands either dry or wet seasons, storms or calms, equable or excessive temperatures, in a variety of combinations of which the earth perhaps affords no example ~nder. the p~esent low phase of excentricity and consequent shght mequahty of sun heat. . . Present Condition of the Earth one of exceptional Stabtltty as rega1·ds Climate.-It will be seen, by a re~er~nce to the diagram at page 165, that during the last three nnlhon ye~rs the excentricity has been less than it is now on eight occasiOns, for sh~rt periods only, making up a total of about 280,000 !ears ; while it has been more than it is now for many long penods, of from 300,000 to 700,000 years each, making a total of 2,720,?00 years, or nearly as 10 to 1. For nearly half the entire penod, or 1,400,000 years, the excentricity has been nearly double what it is now and this is not far from its mean condition. We have no reaso~ for supposing that this long period of three m~llion years, for which we have tables, was in any way exceptional as regards the degree or variation of excentricity; ?ut, on ~he contrary, we may pretty safely assume that its variatwns dunng this time fairly represent its average state of increase and CJIAP. X.] TilE HATE OF OHGANIC CHANGE. 225 decrease during all known geological time. But when the glacial epoch ended, 72,000 years ago, the excentricity was about double its present amount; it then rapidly decreased till, at 60,000 years back, it was very little greater than it is now, and since then it has been uniformly small. It follows that, for about 60,000 years before our time, the mutations of climate every 10,500 years have been comparatively unimportant, and that the temperate zones have enjoyed an exceptional stability of climate. During this time those powerful causes of organic change which depend on considerable changes of climate and the consequent modifications, migrations, and extinctions of species, will not have been at work; the slight changes that did occur would probably be so slow and so little marked that the various species would be able to adapt themselves to them without much disturbance; and the result would be an epoch of exceptional stabiUty of species. But it is from this very period of exceptional stability that we obtain our only scale for measuring the rate of organic change. It includes not only the historical period, but that of the Swiss Lake dwellings, the Danish shell-mounds, our peat-bogs, our sunken forests, and many of our superficial alluvial depositsthe whole in fact, of the iron, bronze, and neolithic ages. Even some portion of the palreolithic age, and of the more recent gravels and cave-earths may come into the same general period if they were formed when the glacial epoch was passing away. Now throughout all these ages we find no indication of change of species, and but little, comparatively, of migration. We thus get an erroneous idea of the permanence and stability of specific forms, due to the period immediately antecedent to our own being a period of exceptional pern~anence and stability as regards climatic and geographical conditions.1 1 This view was, I believe, first put forth by myself in a paper read before the Geological Section of the British Association in 1869, and subsequently in an article in Natu?·e, Vol. I. p. 454. It was also stated by Mr. S. B. K. Skertchley in his Phy.sical System of the Universe, p. 363 (1878); but we both founded it on what I now consider the erroneous doctrine that actual glacial epochs recurred each 10,500 years during periods of high excentricity. Q |
