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The Crinoidea mentioned in connexion with the Coral-reefs may perhaps be quoted independently as favouring the opinion of a prevalent high temperature in these northern regions in the Paleozoic periods, for the living Pentacrinite, very similar to the fossil groups in the London Clay and Mesozoic Strata, has been found only in the West Indian Seas. If we look at a map of isothermal lines, it will appear that a mean temperature like that to be inferred for the land of the Coal-measures from the evidence of Plants, ought to be assigned to the sea of the same period from the evidence of the Marine Radiata.

This inference, according to discoveries in the northern parts of America, would seem to carry the warmth of ancient times very much beyond the experience of geologists in Europe-even to the shores of the Arctic Sea-where mean temperatures below that of congelation now prevail.

The Mollusca of Tertiary Strata in Europe may sometimes be appealed to for evidence of the former connexion of the basins in which they were collected with the ocean, in such a manner as to allow of at least occasional communication; so that Voluta, Conus, Cypræa, Nautilus, &c. might be introduced among the fossils of the basins of London and Paris. But in earlier geological periods, evidence of this

kind is very faint; and a very large proportion of the fossil shells of all orders must be passed over as yielding no sufficient data for a sound conclusion. If we may trust to the few species of the recent genus Nautilus, as indicative of a warm climate, and include all the fossil groups in the same inference, the conclusion already obtained as to the Paleozoic and Mesozoic seas of the north temperate zone would be confirmed.

With much confidence we may appeal to the class of Reptiles for proof of the warm climates of the land, sea, and fresh water of the whole Mesozoic period, for this whole class, as represented in modern times, has such a dependence on temperature as to diminish rapidly in number and dwindle to small size beyond the tropics, and to require special provisions for enduring the winter cold. Not that it seems necessary to suppose, for their comfortable life, a climate heated above ordinary temperatures, in the same proportion that their magnitude exceeds that of recent species. The living Crocodilia are confined to rivers which open into warm seas, and their own geographical range does not pass the isothermal of 64° mean annual temperature. Within this range are most of the races of Serpents and Batrachians on land, and of Turtles in the sea; except that now and then some species wander beyond their usual

bounds; as we may perhaps suppose the marine Turtles and Saurians of the Mesozoic and Cainozoic periods occasionally to have done. In migrations of this kind it is difficult to assign the limits; individuals may arrive and live, where the race would soon perish in the struggle with unfavourable natural conditions.

In the earlier Tertiary periods, the excessive predominance of Pachydermatous Genera, allied to Tapir, Rhinoceros, Hippopotamus, Elephas, and the occurrence of their remains in old lakes and marshes, &c. seem to require the hypothesis of their having lived through long ages in the latitudes of Paris, the Rhine-land, and England. Granting that they indicate a warmer climate then prevailing, we may confirm it by the remains of Serpents and Monkeys found in the London clay at Kyson near Ipswich1. But in the later Tertiary epochs, the frequent mixture of Rhinoceros and Hippopotamus and Elephant with Horse, Ox, Deer, Wolf, Bear and other quadrupeds, whose relatives are found in various climates, and are known by experience to prosper in the climates now prevalent where their remains occur, renders inferences as to climate from them too vague and indecisive to be trusted. With these mixed land quadrupeds lie in several situations, in old Pleistocene lakes, shells 1 Owen in British Fossil Mammalia.

of the land and fresh waters, which are identical with those now living in the same neighbourhood, unmixed with any from latitudes further south, or further north; so that here we have a decisive test, and must admit in these cases that the climate was nearly the same as now. This applies to the Postglacial period, in some part of which we place the commencement of the History of Man.

The Glacial period itself-marked over a great part of the north temperate zone in America and Europe, by marine deposits heaped over what had been dry land-covering sometimes the lacustrine and peaty layers of that earlier (Preglacial) land,—this period was one of considerable refrigeration, within the large areas mentioned, for the shells found in the deposits are of the colder arctic types; and the deposits are found to be consistent with the idea of icebergs floating in deep water over all but the mountainous tracts, (these being covered with ice and snow, the source of glaciers and icebergs,) and not consistent with any other probable condition of things.

Thus we have one well-marked period, at least, of considerably greater cold in the northern temperate regions, succeeding many periods of greater warmth in these same regions. It has been supposed that toward the close of the Paleozoic period, a very early glacial sea was spread round several of our

British and Scottish mountains, and that the remarkable conglomerate of the Malvern and Abberley hills, now supposed to be of Permian age, is due to the floating and stranding of icebergs along the edge of the sea which washed the Longmynd, Abberley, and Malvern hills'; and a more comprehensive conjecture was once proposed by Agassiz, that the ancient climate was subject to several sudden depressions, coincident with great destructions of life, followed by some rise of temperature, and a renewal of life.

Admitting that such great changes have happened in the climate of our north temperate zones, how are they to be accounted for?

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If in agreement with the conclusions of Herschel 2 we admit the earth's orbit to be only in a small degree variable, and so the sun's influence nearly constant; and decline to accept the hypothesis of Poisson that the solar system in its wandering through space has encountered various temperatures ; we shall find our power of explaining the great differences of climate, on the same area in ancient and modern times, reduced to estimating the effect of variations proper to the planet. It has been sup

1 See Memoirs of the Geological Survey, II. 1 for my description of this conglomerate. Professor Ramsay is the author of the hypothesis referred to.

2 Geological Transactions, Ser. 2, Vol. II.

3 Whewell in Reports of British Association, 1835.

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