The study of the present distribution of animals and plants over the earth is a help in the attempt to decide how far the fossil differences in the sets of rocks are due to differences in the ages of the rocks or to differences in the conditions under which the organisms lived. The present, in this, as in many other geological problems, is the key to the past. We know, for instance, that differences of climate largely control the geographical distribution of land animals and especially of land plants, and for that reason among others, fossil plants are generally less trustworthy guides to geological age than fossil animals. In the distribution of marine animals at the present day we find that organisms of simple structure are generally more wide-spread and less susceptible to changes in their environment than are the more complex organisms with specialised structures. Hence we find, for instance, a fossil species of the Foraminifera may persist unchanged through several geological periods, while a species of fossil fish has in general not only a short range in time but often a restricted geographical extent. If we consider the marine organisms found at the present day we find a number of free-swimming forms very widely distributed, while a large number are restricted either by reason of climate or of depth. Certain organisms are only to be found between high and low tide levels, others between low tide level and a depth of thirty fathoms, while many quite different forms live in deeper waters. If we confine our attention to shallow-water marine forms we note that certain forms are at the present day restricted to waters of a certain temperature. We find, therefore, a contrast between arctic and tropical faunas, while other types characterize temperate latitudes. Climatic and bathymetrical differences at the present day therefore lead to distinct differences in the distribution of certain organisms, while other forms, less sensitive to these factors, range widely and may be almost universally distributed. Similar conditions obtained in past geological times, and therefore in attempting to correlate the rocks of one area with those of another those fossils which are most wide-spread are often found to be the most valuable.