Page:EB1911 - Volume 20.djvu/641

Rh ancient chordates in the Cambrian or even pre-Cambrian. Thus all recent discovery tends to carry the centres of origin and of dispersal of all animal types farther and farther back in geological time.

Geology and Palaeophysiography.—Fossils are not absolute timekeepers, because we have little idea of the rate of evolution; they are only relative timekeepers, which enable us to check off the period of deposition of one formation with that of another. Huxley questioned the time value of fossils, but recent research has tended to show that identity of species and of mutations is, on the whole, a guide to synchroneity, though the general range of vertebrate and invertebrate life as well as of plant life is generally necessary for the establishment of approximate synchronise. Since fossils afford an immediate and generally a decisive clue to the mode of deposition of rocks, whether marine, lacustrine, fluviatile, flood plain or aeolian, they lead us naturally into palaeophysiography. Instances of marine and lacustrine analysis have been cited above. The analysis of continental faunas into those inhabiting rivers, lowlands, forests, plains or uplands, affords a key to physiographic conditions all through the Tertiary. For example, the famous bone-beds of the Oligocene of South Dakota have been analysed by W. D. Matthew, and are shown to contain fluviatile or channel beds with water and river-living forms, and neighbouring flood-plain sediments containing remains of plains-living forms. Thus we may complete the former physiographic picture of a vast flood plain east of the Rocky Mountains, traversed by slowly meandering streams.

As already intimated, our knowledge of palaeometeorology, or of past climates, is derivable chiefly from fossils. Suggested two centuries ago by Robert Hooke, this use of fossils has in the hands of Barrande, Neumayr, the marquis de Saporta (1895), Oswald Heer (1809-1883), and an army of followers developed into a sub-science of vast importance and interest. It is true that a great variety of evidence is afforded by the composition of the rocks, that glaciers have left their traces in glacial scratchings and transported boulders, also that proofs of arid or semiarid conditions are found in the reddish colour of rocks in certain portions of the Palaeozoic, Trias and Eocene; but fossils afford the most precise and conclusive evidence as to the past history of climate, because of the fact that adaptations to temperature have remained constant for millions of years. All conclusions derived from the various forms of animal and plant life should be scrutinized closely and compared. The brilliant theories of the palaeobotanist, Oswald Heer, as to the extension of a sub-tropical climate to Europe and even to extreme northern latitudes in Tertiary time, which have appealed to the imagination and found their way so widely into literature, are now challenged by J. W. Gregory (Climatic Variations, their Extent and Causes, International Geological Congress, Mexico, 1906), who holds that the extent of climatic changes in past times has been greatly exaggerated.

It is to palaeogeography and zoogeography in their reciprocal relations that palaeontology has rendered the most unique services. Geographers are practically helpless as historians, and problems of the former elevation and distribution of the land and sea masses depend for their solution chiefly upon the palaeontologist. With good reason geographers have given reluctant consent to some of the bold restorations of ancient continental outlines by palaeontologists; yet some of the greatest achievements of recent science have been in this field. The concurrence of botanical (Hooker, 1847), zoological, and finally of palaeontological evidence for the reconstruction of the continent of Antarctica, is one of the greatest triumphs of biological investigation. To the evidence advanced by a great number of authors comes the clinching testimony of the existence of a number of varieties of Australian marsupials in Patagonia, as originally discovered by Ameghino and more exactly described by members of the Princeton Patagonian expedition staff; while

the fossil shells of the Eocene of Patagonia as analysed by Ortmann give evidence of the existence of a continuous shoreline, or at least of shallow-water areas, between Australia, New Zealand and South America. This line of hypothesis and demonstration is typical of the palaeogeographic methods generally—namely, that vertebrate palaeontologists, impressed by the sudden appearance of extinct forms of continental life, demand land connexion or migration tracts from common centres of origin and dispersal, while the invertebrate palaeontologist alone is able to restore ancient coast-lines and determine the extent and width of these tracts. Thus has been built up a distinct and most important branch. The great contributors to the palaeogeography of Europe are Neumayr and Eduard Suess (b. 1831), followed by Frech, Canu, de Lapparent and others. Neumayr was the first to attempt to restore the grander earth outlines of the earth as a whole in Jurassic times. Suess outlined the ancient relations of Africa and Asia through his “Gondwana Land,” a land mass practically identical with the “Lemuria” of zoologists. South American palaeogeography has been traced by von Ihring into a northern land mass, “Archelenis,” and a southern mass, “Archiplata,” the latter at times united with an antarctic continent. Following the pioneer studies of Dana, the American palaeontologists and stratographers Bailey Willis, John M. Clarke, Charles Schuchert and others have re-entered the study of the Palaeozoic geography of the North American continent with work of astonishing precision.

Zoogeography.—Closely connected with palaeogeography is zoogeography, the animal distribution of past periods. The science of zoogeography, founded by Humboldt, Edward Forbes, Huxley, P. L. Sclater, Alfred Russel Wallace and others, largely upon the present distribution of animal life, is now encountering through palaeontology a new and fascinating series of problems. In brief, it must connect living distribution with distribution in past time, and develop a system which will be in harmony with the main facts of zoology and palaeontology. The theory of past migrations from continent to continent, suggested by Cuvier to explain the replacement of the animal life which had become extinct through sudden geologic changes, was prophetic of one of the chief features of modern method—namely, the tracing of migrations. With this has been connected the theory of “centres of origin” or of the geographic regions where the chief characters of great groups have been established. Among invertebrates Barrande's doctrine of centres of origin was applied by Hyatt to the genesis of the Arietidae (1889); after studying thousands of individuals from the principal deposits of Europe he decided that the cradles of the various branches of this family were the basins of the Cote d'Or and southern Germany. Ortmann has traced the centre of dispersal of the fresh-water Crawfish genera Cambarus, Potamobius and Cambaroides to eastern Asia, where their common ancestors lived in Cretaceous time. Similarly, among vertebrates the method of restoring past centres of origin, largely originating with Edward Forbes, has developed into a most distinct and important branch of historical work. This branch of the science has reached the highest development in its application to the history of the extinct mammalia of the Tertiary through the original work of Cope and Henri Filhol, which has been brought to a much higher degree of exactness recently through the studies of H. F. Osborn, Charles Deperet, W. D. Matthew and H. G. Stehlin.

Systematic Zoology.—It is obvious that the Linnaean binomial terminology and its subsequent trinomial refinement for species, sub-species, and varieties was adapted to express the differences between animals as they exist to-day, distributed contemporaneously over the surface of the earth, and that it is wholly inadapted to express either the minute gradations of successive generic series or the branchings of a genetically connected chain of life. Such gradations, termed “mutations” by Waagen, are distinguished, as observed, in single characters; they are the