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Show 196 ISLAND LT.F' E. (PAin' I. persistent cold epochs when the excentricity was as low as it is now or lower, for that would imply that the duration of cold conditions was greater than that of warm. Why then should the fauna and flora of the cold epochs ne'ver be preserved ? Mollusca and many other forms of life are abundant in the Arctic seas, and there is often a luxuriant dwarf woody vegetation on the land, yet in no one case has a single example of such a fauna or flora been dis-covered of a date anterior to the last glacial epoch. And this argument is very much strengthened when we remember that an exactly analogous series of facts is found over all the temperate zones. Everywhere we have abundant floras and faunas indicating warmer conditions than such as now prevail, but never in a single instance one which as clearly indicates colder conditions. The fact that drift with Arctic shells was deposited during the last glacial epoch, as well as gravels and crag with the remains of arctic anima]s and p1ants, shows us that there is nothing to prevent such deposits being formed in cold as well as in warm periods ; and it is quite impossible to believe that in every place and at all epochs all records of the former have been destroyed, while in a considerable number of instances those of the latter have been preserved. When to this uniform testimony of the palreon~ ol~gic~l evidence w_e add the equally uniform absence of any mdwatwn of those we-borne rocks, boulders, and drift, which are the. c~nstant and. necessary accompaniment of every period of glacmtwn, and whwh must inevitably pervade all the marine d.eposits ~ormed over a wide area so long as the state of glaciatwn contmues, we are driven to the conclusion that the last glacial epoch of the northern hemisphere was exceptional, and was not preceded by numerous similar o'lacial epochs throughout Tertiary and Secondary time. 0 . But although glacial epochs (with the one or two excephons already referred to) were certainly absent considerable changes of cl~mate may have frequently occurr~d, and these would lead to Important changes in the organic world. We can hardly doubt that some such change occurred between the Lower and Upper Cretaceous periods, the floras of which exhibit such an extraordinary contrast in general character. We have also CHAP. IX.] GEOLOGICAL CLIMATES. 197 the testimony of Mr. J. S. Gardner, who has long worked at the fossil floras of the Tertiary deposits, and who states, that there is strong negative and some positive evidence of alternating warmer and colder conditions, not glacial, contained not only in English Eocene, but all Tertiary beds throughout the world. 1 In the case of marine faunas it is more difficult to judge, but the numerous changes in the fossil remains from bed to bed only a few feet and sometimes a few inches apart, may be sometimes due to change of climate; and when it is recognised that such changes have probably occurred at all geological epochs and their effects are systematically searched for, many peculiarities in the distribution of organisms through the different members of one deposit may be traced to this cause. General view of Geological Climates as dependent on the Physical Peat'ltTes of the Earth's Surface.-In the preceding chapters I have earnestly endeavoured to arrive at an explanation of geological climates in the temperate and Arctic zones, which should be in harmony with the great body of geological facts now available for their elucidation. If my conclusions as here sot forth diverge considerably from those of Dr. Croll, it is not from any want of appreciation of his facts and arguments, since for many years I have upheld and enforced his views to the best of my ability. But a careful re-examination of the whole question has now convinced me that an error has been made in estimating the comparative effect of geographical and astronomical causes on changes of climate, and that, while the ln.tter have undoubtedly played an important part in bringing about the glacial epoch, it is to the former that the mild climates of the Arctic regions are almost entirely duo. If I have now succeeded in approaching to a true solution of this difficult problem, I owe it mainly to the study of Dr. Croll's writings, since my theory is entirely based on the facts and principles so clearly set forth in his admirable papers on "Ocean Currents in relation to the Distribution of Heat over the Globe." The main features of this theory as distinct from that of Dr. Croll I will now endeavour to summarise. Looking at the subject broadly, we see that the climatic 1 Geological ~fagcudne, 1877, p. 137. |
