[35] For an account of the deep-sea fauna, see Hickson, S. J., The Fauna of the Deep Sea, 1894.
[36] Suess, E., Das Antlitz der Erde, 2er. Bd., p. 266.
As one would naturally expect, the actual depth at which deposits were formed can generally be calculated with a greater degree of certainty amongst the newer rocks than amongst the older ones. In the case of the Pliocene Crags, the depth in fathoms may be confidently given. In the Cretaceous rocks attempts have been made to give numerical estimates of the depths at which different accumulations were formed, but some differences of opinion have arisen in the case of these rocks. In the Palæozoic rocks, only a rough idea of the general depth can usually be obtained, and no attempt to calculate the depth in fathoms is likely to be even approximately correct in the present state of our knowledge.
The comminution of fossils has sometimes been taken as an indication of shallower water origin of the deposits which contain them, but although the hard parts of organisms in a broken condition have frequently been shattered by the action of the waves, they may also be broken at great depths by predaceous creatures, and in many instances the fracture is the result of earth-movements occurring subsequently to the formation of the deposits.
Turning now to the difference in organisms which results from difference of station, it will be sufficient to give a quotation from Woodward's Manual of the Mollusca as an illustration:—"In Europe the characteristic genera of rocky shores are Littorina, Patella, and Purpura; of sandy beaches, Cardium, Tellina, Solen; gravelly shores, Mytilus; and on muddy shores, Lutraria and Pullastra. On rocky coasts are also found many species of Haliotis, Siphonaria, Fissurella, and Trochus; they occur at various levels, some only at the high-water line, others in a middle zone, or at the verge of low-water. Cypræa and Conus shelter under coral-blocks, and Cerithium, Terebra, Natica and Pyramidella bury in sand at low-water, but may be found by tracing the marks of their long burrows (Macgillivray)[37]."
[37] Woodward, S. P., A Manual of the Mollusca, p. 151.
The geologist will naturally select sporadic forms rather than endemic ones in comparing the strata of different areas, but how far differences in faunas are the result of existence at different times, and how far they are due to difference of conditions affecting contemporaneous organisms can only be discovered as the result of accurate observation. The main points to be regarded when comparing the successive faunas of different regions have been noticed in this and the preceding chapters, and it has been shown that as the evidence is cumulative, it requires the collection of a large number of facts obtained by observation of the strata before accurate inferences can be drawn.
The indications of climatic conditions furnished by organisms require some consideration. In the comparatively recent deposits it is not difficult to get some notion of the prevalent climatic conditions when the fossils belong to forms closely related to modern genera. The existence of the arctic birch and arctic willow, and of shells belonging to species now living north of the British Isles, in deposits of comparatively recent date in Britain would afford convincing evidence of the occurrence of colder climatic conditions than those which are now prevalent in the area, even if the evidence were not confirmed as it is, by physical proof of glaciation in deposits of the same age. Nevertheless, even in these recent beds, we have a useful warning, by finding species of elephant and rhinoceros associated with northern forms like the lemming, glutton, and musk-ox. We know that the species of elephant and rhinoceros (the mammoth and woolly rhinoceros) were provided with thick coverings which would enable them to resist the severity of an arctic climate, but had not these coverings been found, we might have been puzzled by the association of forms whose nearest allies are sub-tropical with others of arctic character. As we go back in time and deal with earlier deposits, the ascertainment of the climatic conditions becomes more difficult, as the fossils mostly belong to extinct species, genera or even families.
In these circumstances, it is very dangerous to draw conclusions as to climatic conditions from examination of a few forms, but when we find that plants and animals, terrestrial and marine forms, vertebrates and invertebrates alike point to the same conclusion, as in the London Clay, where all the fossils belong to forms allied to those now living under sub-tropical conditions, the state of the climate may be inferred with considerable certainty[38]. The character of the fossils must be taken into account rather than their size. There was a tendency amongst geologists to believe that large organisms probably indicate warm conditions. Recent researches in arctic seas have dispelled this belief. Marine algæ of enormous size are found in the cold seas, and the size of creatures, abundance of individuals and variety of forms in the arctic faunas of some regions is very noteworthy. In the Kara Sea, for instance, a variety of creatures were dredged up during the voyage of the Vega, and Baron Nordenskjöld makes the following pertinent remarks about them: "For the science of our time, which so often places the origin of a northern form in the south, and vice versa, as the foundation of very wide theoretical conclusions, a knowledge of the types which can live by turns in nearly fresh water of a temperature of +10°, and in water cooled down to -2·7° and of nearly the same salinity as that of the Mediterranean, must have a certain interest. The most remarkable were, according to Dr Stuxberg, the following: a species of Mysis, Diastylis Rathkei Kr., Idothea entomon Lin., Idothea Sabinei Kr., two species of Lysianassida, Pontoporeia setosa Stbrg., Halimedon brevicalcar Goës, an Annelid, a Molgula, Yoldia intermedia M. Sars, Yoldia (?) arctica Gray, and a Solecurtus[39]. "The temperatures were taken by a centigrade thermometer. Again we read of the results of dredging off Cape Chelyuskin. "The yield of the trawling was extraordinarily abundant; large asterids, crinoids, sponges, holothuria, a gigantic sea-spider (Pycnogonid), masses of worms, crustacea, etc. It was the most abundant yield that the trawl-net at any one time brought up during the whole of our voyage round the coast of Asia, and this from the sea off the northern extremity of that continent[40]."
[38] For a discussion as to the value of plants as indices of climate see Seward, A. C., Sedgwick Essay for 1892.