The masterly generalisation of William Smith that strata can be identified by their fossil contents established by close study of the rocks and fossils of the British Oolites has been confirmed generally by subsequent work. The comparative study of the fossil contents of rocks in widely separated areas has proved to be the most valuable means by which the correlation of the rocks can be effected and their identity of age established. In some cases the recognition of a single fossil species in two areas separated, perhaps, by thousands of miles may suffice to demonstrate that the rocks are of the same age. For example, a graptolite such as _Phyllograptus typus_ is found in many parts of the world, but has only a very restricted range in time. It has been found only in rocks of Lower Ordovician age. Its occurrence in Wales and in the rocks of Bendigo practically suffices to establish the identity in age of the rocks in these widely separated areas. Generally, however, much closer study and a more detailed examination of a large number of the fossils of a rock series are required before the age of the rocks can be surely established and a safe correlation made with distant localities. The stratigraphical generalisations to be made from the study of fossils however must be qualified by certain considerations. Among these are the fact that our knowledge of the life forms of a given geological period is necessarily incomplete, that the differences in the fossil contents of rocks may depend not only on differences of age but also in the conditions under which the organisms lived and the rocks were accumulated, and that forms of life originating in one area do not spread themselves immediately over the earth but migrate at velocities depending on their mode of life and the presence or absence of barriers to their progress. Our incomplete knowledge of the forms living in remote geological periods arises partly from the fact that some forms had no permanent skeleton and were therefore incapable of preservation, partly to the obliteration of the skeletons of organisms through subsequent earth movements in the rocks or through the solvent action of water. Many land forms, too, probably disintegrated on the surface before deposits were formed over the area. Apart from these causes which determine that a full knowledge of the fossils from ancient rocks in particular, will never be acquired, our knowledge is incomplete by reason either of difficulty of access to certain areas or incomplete search. As a result of later discoveries earlier conclusions based on incomplete evidence as to the age of a rock series, have not infrequently been modified.