(2) As Evidence Of Geographical Changes

We have seen that from the composition and structure of the stratified rocks themselves much may be learned concerning the geographical conditions under which they were formed, and of the subsequent geographical changes of the region in which they occur. Fossils supplement this information regarding the body of water in which the rocks were laid down, whether fresh or salt, deep or shallow, near or far from land, in an open sea or a closed basin, and whether such a closed basin had occasional or constant communication with the ocean. Most of the stratified rocks which now form part of the land-surface give us information only concerning the former extension of bodies of water over what is now the land, but they can tell us nothing of the land areas which have disappeared beneath the sea. In this connection fossils are of great assistance, for, in certain instances, the distribution of marine fossils points to the presence of land barriers to migration which no longer exist, or to a continuity of coast-lines which are now broken up, while the fossils of land animals may demonstrate the former existence of land bridges between regions which have long been separated by water.

Thus it may be shown that North America was frequently and for long periods of time connected with Asia across Bering Sea, and that its union with South America is of geologically late date.

(3) As Evidence Of Climatic Changes

The remarkable climatic changes through which various parts of the earth have passed are indicated by fossils. Indeed, with the exception of glacial marks and ice-formed deposits, fossils offer almost the only trustworthy evidence available as to changes of temperature. Thus, when we find in the rocks of Greenland the remains of extensive forests of such trees as now grow in temperate latitudes, the only possible inference is that Greenland now has a far colder climate than when those forests existed. The same conclusion follows from the presence in the rocks of Wyoming and Idaho of great palm leaves and other subtropical plants associated with the bones of crocodiles and other reptiles, such as live only in warm regions. In deposits of a far later date occur bones of the reindeer in southern New England and in the south of France, walrus bones in the sands of New Jersey, and those of the musk-ox in Arkansas; all of which shows that at one time these regions had a much colder climate than at present.

The evidence as to climatic changes which is presented by fossils must, however, be treated with great caution, because even nearly allied species often have entirely different habits, and flourish in quite different climates. Most fossils belong to extinct 2 M species, as to whose climatic relations we have no knowledge. Before any conclusion concerning changes of climate can be regarded as established, we should have the testimony of species still living, or, if that is not possible, the evidence must be drawn from large assemblages of different kinds of animals and plants. Such an extreme case as the fossil plants of Greenland is sufficient evidence without further corroboration.