Geological Department,
The University, Melbourne.

William Smith, the Father of English Geology, used to apologize for the study of palaeontology by claiming that “the search for a fossil is at least as rational a proceeding as the pursuit of a hare.” Those of us who are accustomed to take the field, armed with a hammer, in the search for “medals of creation” and from time to time have experienced the sporting enjoyment of bringing to light a rare or perfect specimen are quite prepared to support his claim. But the student of fossils needs the help of a text book to guide him to the literature on the subject, to help him with his identifications or to indicate that some of his finds are new and hitherto undescribed. European and American workers have long been provided with excellent books treating generally of fossils, but the illustrations have been quite naturally taken mainly from forms occurring in the Northern Hemisphere. Our own fossil forms both plants and animals are numerous, interesting and in many cases peculiar, but the literature concerning them is so widely scattered in various scientific publications that a warm welcome should be given to this book of Mr. Chapman’s, in which the Australian evidence is brought together and summarised by one, whose training, long experience, and personal research qualify him to undertake the task. Especially will teachers and students of Geology and Palaeontology value such an undertaking. Workers in other countries who have only partial access to the Australian literature on the subject should also find this a valuable book of reference.

In the study of fossils we are concerned with the nature, evolution and distribution of the former inhabitants of the earth. The study of Palaeontology may be justified as a means of scientific discipline, for the contributions the subject makes to the increase of natural knowledge and the unfolding of panoramas of ancient life. It also provides perhaps the most positive evidence in the story of evolution. So, too, the student of the present day distribution of animals and plants finds the key to many a problem in zoo-geography in the records of past migrations yielded by the study of fossils in different lands. The stratigraphical geologist is of course principally concerned with two important aspects of the study of fossils.

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.

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.

Attention should also be paid to the conditions under which the deposits accumulated, since it is clear that rocks may be formed at the same time in different areas and yet contain many distinct fossils by reason of climatic or bathymetrical differences. Among living marine organisms we find certain forms restricted to sandy or muddy sea-bottoms and others to clear water, and these changes in the conditions of deposition of sediment have played their part in past geological periods in determining differences in the fossil faunas of rocks which were laid down simultaneously. We not infrequently find mudstones passing laterally into limestones, and this lithological change is always accompanied by a more or less notable change in the fossil contents of the two rock types. Such facts emphasize the close connection between stratigraphy and palaeontology, and indicate that the successful tracing out of the geological history of any area is only possible when the evidence of the stratigrapher is reinforced by that provided by the palaeontologist. The fact that species of animals and plants which have been developed in a particular area do not spread all over the world at once but migrate very slowly led Huxley many years ago to put forward his hypothesis of “homotaxis.” He agreed that when the order of succession of rocks and fossils has been made out in one area, this order and succession will be found to be generally similar in other areas. The deposits in two such contrasted areas are homotaxial, that is, show a similarity of order, but, he claimed, are not necessarily synchronous in their formation. In whatever parts of the world Carboniferous, Devonian and Silurian fossils may be found, the rocks with Carboniferous fossils will be found to overlie those with Devonian, and these in their turn rest upon those containing Silurian fossils. And yet Huxley maintained that if, say, Africa was the area in which faunas and floras originated, the migration of a Silurian fauna and flora might take place so slowly that by the time it reached Britain the succeeding Devonian forms had developed in Africa, and when it reached North America, Devonian forms had reached Britain and Carboniferous forms had developed in Africa. If this were so a Devonian fauna and flora in Britain may have been contemporaneous with Silurian life in North America and with a Carboniferous fauna and flora in Africa.

This could only be true if the time taken for the migration of faunas and floras was so great as to transcend the boundaries between great geological periods. This does not appear to be the case, and Huxley’s idea in its extreme form has been generally abandoned. At the same time certain anomalies in the range in time of individual genera have been noted, and may possibly be explained on such lines. For instance, among the group of the graptolites, in Britain the genus Bryograptus occurs only in the Upper Cambrian and the genus Leptograptus only in the Upper Ordovician rocks. In Victoria these two genera, together with typical Lower Ordovician forms, may be found near Lancefield preserved on a single slab of shale. In the same way, in a single quarry in Triassic rocks in New South Wales, a number of fossil fish have been found and described, some of which have been compared to Jurassic, others to Permian, and others to Carboniferous forms in the Northern Hemisphere.

Another point which the palaeontologist may occasionally find evidence for is the existence of “biological asylums,” areas which by means of land or other barriers may be for a long period separated from the main stream of evolution. We know that the present fauna and flora of Australia is largely of archaic aspect, as it includes a number of types which elsewhere have long ago become extinct or were never developed. This appears to be due to the long isolation of Australia and, as Professor Gregory happily puts it—its “development in a biological backwater.” We have some evidence that similar asylums have existed in past geological periods, with the result that in certain areas where uniform conditions prevailed for a long time or where isolation from competition prevented rapid evolution, some organisms which became extinct in other areas, persisted unchanged in the “asylum” into a younger geological period.

The broad generalizations that rocks may be identified by their fossil contents and that the testimony of the rocks demonstrates the general order of evolution from simple to complex forms, have only been placed on a surer footing by long continued investigations. The modifications produced by conditions of deposit, of climate and of natural barriers to migration, while introducing complexities into the problems of Palaeontology, are every year becoming better known; and when considered in connection with the variations in the characters of the rocks, provide valuable and interesting evidence towards the solution of the ultimate problems of geology and palaeontology, which include the tracing out of the evolution of the history of the earth from the most remote geological period to that point at which the geologist hands over his story to the archaeologist, the historian, and the geographer.

ERNEST W. SKEATS.