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EARTH

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Originally appearing in Volume V20, Page 588 of the 1911 Encyclopedia Britannica.
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EARTH SCIENCES 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 synchronism. 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 semi-arid 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; whilethe fossil shells of the Eocene of Patagonia as analysed by Ortmann give evidence of the existence of a continuous shore-line, 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, Cann, 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 Cate 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. V.-RELATIONS OF PALAEONTOLOGY TO OTHER ZOOLOGICAL METHODS 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 586 nuances, or grades of difference, which are the more gradual the more finely we dissect the geologic column, while the terms species, sub-species and variety are generally based upon a sum of changes in several characters. Thus palaeontology has brought to light an entirely new nomenclatural problem, which can only be solved by resolutely adopting an entirely different principle. which is essentially based on a theory of interrupted or discontinuous characters, is inapplicable. Embryology and Ontogeny.—In following the discovery of the law of recapitulation among palaeontologists we have clearly stated the chief contribution of palaeontology to the science of ontogeny—namely, the correspondences and differences between Formations in Western United States and Characteristic Type of Horse in Each Hind Foot Teeth Quaternary or Age of Man Recent Three Toes J Stile toes not touching the ground Three Toes Side toes not touching the ground One Toe Splints of Viand 4thdigits Long-Crowned, Cement-covered Tertiary or Age of Mammals Three Toes Side toes touching the ground; splint of 5111digif Hypothetical Ancestors with Five Toes on Each Foot and Teeth like those of Monkeys etc. Reproduced by permission of the American Museum efNatural History Age of Reptiles Fore Fool Pleistocene Pliocene One Toe Splints of 2"-°and edigits Oligocene Four Toes This revolution may be accomplished by adding the term " mutation ascending " or " mutation descending " for the minute steps of transformation, and the term phylum, as employed in Germany, for the minor and major branches of genetic series. Bit by bit mutations are added to each other in different single characters until a sum or degree of mutations is reached which no zoologist would hesitate to place in a separate species or in a separate genus. The minute gradations observed by Hyatt, Waagen and all invertebrate palaeontologists, in the hard parts (shells) of molluscs, &c., are analogous to the equally minute gradations observed by vertebrate palaeontologists in the hard parts of rep-tiles and mammals. The mutations of Waagen may possibly, in fact, prove to be identical with the " definite variations " or " rectigradations " observed by Osborn in the teeth of mammals. For example, in the grinders of Eocene horses (see Plate III., fig. 8; also fig. 9) in a lower horizon a cusp is adumbrated in shadowy form, in a slightly higher horizon it is visible, in a still higher horizon it is full-grown; and we honour this final stage by assigning to the animal which bears it a new specific name. When a number of such characters accumulate, we further honour them by assigning a new generic name. This is exactly the nomenclature system laid down by Owen, Cope, Marsh and others, although established without any understanding of the law of mutation. But besides the innumerable characters which are visible and measurable; there are probably thousands which we cannot measure or which have not been discovered, since every part of the organism enjoys its gradual and independent evolution. In the face of the continuous series of characters and types revealed by palaeontology, the Linnaean terminology,the individual order of development and the ancestral order of evolution. The mutual relations of palaeontology and embryo-logy and comparative anatomy as means of determining the ancestry of animals are most interesting. In tracing the phylogeny, or ancestral history of organs, palaeontology affords the only absolute criterion on the successive evolution of organs in time as well as of (progressive) evolution in form. From comparative anatomy alone it is possible to arrange a series of living forms which, although. structurally a convincing array because placed in a graded series, may be, nevertheless, in an order inverse to that of the actual historical succession. The most marked case of such inversion in comparative anatomy is that of Carl Gegenbaur (1826-1903), who in arranging the fins of fishes in support of his theory that the fin of the Australian lung-fish (Ceratodus) was the most primitive (or Archipterygium), placed as the primordial type a fin which palaeontology has proved to be one of the latest types if not the last. It is equally true that palaeontological evidence has frequently failed where we most sorely needed it. The student must therefore resort to what may be called a tripod of evidence, derived from the available facts of embryology, comparative anatomy and palaeontology. VI.—THE PALAEONTOLOGIST AS HISTORIAN The modes of change among animals, and methods of analysing them.—As historian the palaeontologist always has before him as one of his most fascinating problems phylogeny, or the restoration of the great tree of _animal descent. Were the geologic record complete he would be able to trace the ancestry of man and of all other animals back to their very beginnings in the' primordial protoplasm. Dealing with interrupted evidence, however, it becomes necessary to exercise the closest analysis and synthesis as part of his general art as a restorer. The most fundamental distinction in analysis is that which must be made between homogeny, or true hereditary resemblance, and those multiple forms of adaptive resemblance which are variously known as cases of " analogy," " parallelism," " convergence " and " homoplasy." Of these two kinds of genetic and adaptive resemblance, homogeny is the warp composed of the vertical, hereditary strands, which connect animals with their ancestors and their successors, while analogy is the woof, composed of the horizontal strands which tie animals together by their superficial resemblances. This wide distinction between similarity of descent and similarity of adaptation applies to every organ, to all groups of organs, to animals as a whole, and to all groups of animals. It is the old distinction between homology and analogy on a grand scale. Analogy, in its power of transforming unlike and unrelated animals or unlike and unrelated parts of animals into likeness, has done such miracles that the inference of kinship is often almost irresistible. During the past century it was and even now is the very " will-o'-the-wisp " of evolution, always tending to lead the phylogenist astray. It is the first characteristic of analogy that it is superficial. Thus the shark, the ichthyosaur, (After a drawing by Charles R. Knight, made under the direction of Professor Osborn.) The external similarity in the fore paddle and back fin of these three marine animals is absolute, although they are totally unrelated to each other, and have a totally different internal or skeletal structure. It is one of the most striking cases known of the law of analagous evolution. A, Shark (Lamna cornubica), with long lobe of tail upturned. B, Ichthyosaur (Ichthyosaurus quadricissus), with fin-like paddles, long lobe of tail down-turned. C, Dolphin (Sotalia fluviatilis), with horizontal tail, fin or fluke. and the dolphin (fig. so) superficially resemble each other, but if the outer form be removed this resemblance proves to be a mere veneer of adaptation, because their internal skeletal parts are as radically different as are their genetic relations, founded on heredity. Analogy also produces equally remarkable internal or skeletal transformations. The ingenuity of nature, however, in adapting animals is not infinite, because the same devices are repeatedly employed by her to accomplish the same adaptive ends whether in fishes, reptiles, birds or mammals; thus she has repeated herself at least twenty-four times in the evolution of long-snouted rapacious swimming types of animals. The grandest application of analogy is that observed in the adaptations of groups of animals evolving on different continents, by which their various divisions tend to mimic those on other continents. _ Thus the collective fauna of ancient South America mimics the independently evolved collective fauna of North America, the collective fauna of modern Australia mimics the collective fauna of the Lower Eocene of North America. Exactly the same principles have developed on even a vaster scale among the Invertebrata. Among the ammonites of the Jurassic and Cretaceous periods types occur which in their external appearance so closely resemble each other that they could be taken for members of a single series, and not infrequently have been taken for species of the same genus and even for the same species; but their early stages of development and, in fact, their entire individual history prove them to be distinct and not infrequently to belong to widely separated genetic series. Homogeny, in contrast, the " special homology " of Owen, is the supreme test of kinship or of hereditary relationship, and thus the basis of all sound reasoning in phylogeny. The two joints of the thumb, for example, are homogenous throughout the whole series of the pentadactylate, or five-fingered animals, from the most primitive amphibian to man, The conclusion is that the sum of homogenous parts, which may be similar or dissimilar in external form according to their similarity or diversity of function, and the recognition of former similarities of adaptation (see below) are the true bases for the critical determination of kinship and phylogeny. Adaptation and the Independent Evolution of Parts.—Step by step there have been established in palaeontology a number of laws relating to the evolution of the parts of animals which closely coincide with similar laws discovered by zoologists. All are contained in the broad generalization that every part of an animal, however minute, has its separate and independent basis in the hereditary substance of the germ cells from which it is derived and may enjoy consequently a separate and independent history. The consequences of this principle when applied to the adaptations of animals bring us to the very antithesis of Cuvier's supposed "law of correlation," for we find that, while the end results of adaptation are such that all parts of an animal conspire to make the whole adaptive, there is no fixed correlation either in the form or rate of development of parts, and that it is there-fore impossible for the palaeontologist to predict the anatomy of an unknown animal from one of its parts only, unless the animal happens to belong to a type generally familiar. For example, among the land vertebrates the feet (associated with the structure of the limbs and trunk) may take one of many lines of adaptation to different media or habitat, either aquatic, terrestrial, arboreal or aerial; while the teeth (associated with the structure of the skull and jaws) also may take one of many lines of adaptation to` different kinds of food, whether herbivorous, insectivorous or carnivorous. Through this independent adaptation of different parts to their specific ends there have arisen among vertebrates an almost unlimited number of combinations of foot and tooth structure, the possibilities of which are illustrated in the accompanying diagram (see fig. 11 ; also Plate III., fig. 8). As instances of such combinations, some of the (probably herbivorous) Eocene monkeys with arboreal limbs have teeth so difficult to distinguish from those of the herbivorous ground-living Eocene horses with cursorial limbs that at first in France and also in America they were both classed with the hoofed animals. Again, directly opposed to Cuvier's principle, we have discovered carnivores with hoofs, such as Mesonyx, and herbivores with sloth-like claws, such as Chalicotherium. This latter animal is closely related to one which Cuvier termed Pangolin gigantesque, and had he restored it according to his " law of correlation " he would have pictured a giant " scaly anteater," a type as wide as the poles from the actual form of Chalicotherium, which in body, limbs and teeth is a modified ungulate herbivore, related remotely to the tapirs. In its, claws alone does it resemble the giant sloths. This independence of adaptation applies to every detail of structure; the six cusps of a grinding tooth may all evolve alike, or each may evolve independently and differently. Independent evolution of parts is well shown among invertebrates, where the shell of an ammonite, for example, may change markedly in form without a corresponding change in suture, or vice versa. Similarly, there is no correlation in the rate of evolution either of adjoining or of separated parts; the middle digit of the foot of the three-toed horse is accelerated in development, while the lateral digits on either side are retarded. Many examples might be cited among invertebrates also.
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