"The best thermometer we can use to test the character of a climate is the flora and fauna which lived while it prevailed. This is not only the best, but is virtually the only thermometer available when we inquire into the climate of past geological ages. Other evidence is always sophisticated by the fact that we may be attributing to climate what is due to other causes; boulders can be rolled by the sea as well as by sub-glacial streams, and conglomerates can be formed by other agencies than ice. But the biological evidence is unmistakable; cold-blooded reptiles cannot live in icy water; semi-tropical plants, or plants whose habitat is in the temperate zone, cannot ripen their seeds and sow themselves under arctic conditions.... We may examine the whole series of geological horizons, from the earliest Palaeozoic beds down to the so-called Glacial beds, and find, so far as I know, no adequate evidence of discontinuous and alternating climates, no evidence whatever of the existence of periods of intense cold intervening between warm periods, but just the contrary. Not only so, but we shall find that the differentiation of the earth's climate into tropical and arctic zones is comparatively modern, and that in past ages not only were the climates more uniform, but more evenly distributed over the whole world." Without attempting to follow through the whole series of formations we may note a few characteristic statements of the text-books. Thus Dana says of the Cambrian: "There was no frigid zone, and there may have been no excessively torrid zone." While of the Silurian coral limestones of the Arctic regions he says: "The formation of thick strata of limestone shows that life like that of the lower latitudes not only existed there, but flourished in profusion."[69] Howorth thus quotes Colonel Fielden, the Arctic explorer, regarding the fossil Sclerodermic corals of the Silurian, widely distributed in the Arctic regions: "These undoubted reef-forming corals of the Silurian epoch were just as much inhabitants of warm water in northern latitudes at that period as are the Sclerodermata of to-day in the Indo-Pacific and Atlantic oceans.... These corals were forms of life which must have been tropical in habits and requirement." In fact coral limestones of the Carboniferous system are the nearest known fossiliferous rocks to the North Pole, and from the strike of the beds must underlie the Polar Sea. In the words of Howorth, "Coal strata with similar fossils have occurred all round the Polar basin ... and may be said, therefore, to have occupied a continuous cap around the North Pole."[70] Again I quote from Howorth regarding the Mesozoic rocks: