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_