[39] Nordenskjöld, A. E., The Voyage of the Vega, Vol. I. Chap. IV.
[40] Ibid. Chap. VII.
Amongst the marine invertebrates reef-building corals and mollusca perhaps furnish the best evidence of climatic conditions. The coral-reefs of the Jurassic rocks with large gastropods and lamellibranchs clustered around them have been appealed to in proof of the existence of sub-tropical conditions during their formation; further back in time we find evidence of climate furnished by the fossils of the Silurian rocks of the Isle of Gothland in the Baltic Sea. Of these, Lindström writes "The fauna had a tropical character. In consideration of the great numbers of Pleurotomariae, Trochi, Turbinidae and the large Pteropods the assumption of a tropical character of the fauna may seem justifiable[41]."
[41] Lindström, G., On the Silurian Gastropoda and Pteropoda of Gotland, Stockholm, 1884, p. 33.
Structure may give some indication of climate even though the organism is not allied to living species. The bark of trees in arctic regions is often thicker than in more temperate regions, and the leaves of arctic plants often have special characters to enable them to resist the long periods during which they are deprived of water, though the fact that desert-plants frequently shew similar modifications deprives this test of any particular value except as a means of corroborating conclusions reached from other evidence[42]. The shells of arctic mollusca may become stunted, but this is not by any means universal, and the same result may be brought about by other abnormal conditions, as for instance the increase of salt in a water area by evaporation.
[42] For an account of the modifications of the leaves of arctic plants, see Warming, Eug., Om Grønlands Vegetation, Meddelelser om Grønland, 12th part, p. 105.
On the whole, an examination of the evidence available for ascertaining the character of climate by reference to included organisms, shews that inferences may be drawn within certain limits, but that the task is a difficult one not unaccompanied by danger, and every kind of available evidence derived from a study of physical phenomena and the included organisms should be utilised before any conclusion is drawn.
The likelihood of accurate inference is increased by comparing the faunas of various areas; should they seem to indicate a progressive lowering of climate when passing from lower to higher latitudes, it is probable that the indication is correct. The student is referred to a paper by the late Professor Neumayr for an account of the existence of climatic zones during the Mesozoic Period[43].
[43] Neumayr, M., "Ueber klimatische Zonen während der Jura- und Kreidezeit," Denkschrift. der Math.-Naturwissensch. Classe der k. Akad. der Wissenschaften, Bd. XLVII. Vienna, 1883.