While the rocks themselves thus afford valuable testimony as to the climatic conditions which prevailed at the time and place of their formation, this testimony is fragmentary, missing for very long periods, and must be supplemented from the information presented by the fossils. As in all matters where fossils are involved, the evidence must be cautiously used, for hasty inferences have often led to contradictory and absurd conclusions. When properly employed, the fossils give a more continuous and complete history of climatic changes than can, in the present state of knowledge, be drawn from a study of the rocks alone. For this purpose plants are particularly useful, because the great groups of the vegetable kingdom are more definitely restricted in their range by the conditions of temperature and moisture than are most of the correspondingly large groups of animals. Not that fossil animals are of no service in this connection; quite the contrary is true, but the evidence from them must be treated more carefully and critically. To illustrate the use of fossils as recording climatic changes in the past, one or two examples may be given. In the Cretaceous period a mild and genial climate prevailed over all that portion of the earth whose history we know, and was, no doubt, equally the case in the areas whose geology remains to be determined. The same conditions extended far into the Arctic regions, and abundant remains of a warm-temperate vegetation have been found in Greenland, Alaska and other Arctic lands. Where now only scanty and minute dwarf willows and birches can exist, was then a luxuriant forest growth comprising almost all of the familiar trees of our own latitudes, a most decisive proof that in the Cretaceous the climate of the Arctic regions must have been much warmer than at present and that there can have been no great accumulation of ice in the Polar seas. Conditions of similar mildness obtained through the earlier part of the Tertiary. In the Eocene epoch large palm-trees were growing in Wyoming and Idaho, while great crocodiles and other warm-country reptiles abounded in the waters of the same region.