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EARTH SCIENCES
See also:Geology and Palaeophysiography.—Fossils are not See also:absolute timekeepers, because we have little See also:idea of the See also:rate of See also:evolution; they are only relative timekeepers, which enable us to check off the See also:period of deposition of one formation with that of another. See also:Huxley questioned the See also:time value of fossils, but See also:recent See also:research has tended to show that identity of See also:species and of mutations is, on the whole, a See also:guide to synchroneity, though the See also:general range of vertebrate and invertebrate See also:life as well as of plant life is generally necessary for the See also:establishment of approximate synchronism. Since fossils afford an immediate and generally a decisive See also:clue to the mode of deposition of rocks, whether marine, lacustrine, fluviatile, See also:flood See also:plain or aeolian, they See also:lead us naturally into palaeophysiography. Instances of marine and lacustrine See also:analysis have been cited above. The analysis of See also:continental faunas into those inhabiting See also:rivers, lowlands, forests, plains or uplands, affords a See also: All conclusions derived from the various forms of See also:animal and plant life should be scrutinized closely and compared. The brilliant theories of the palaeobotanist, Oswald Heer, as to the See also:extension of a sub-tropical climate to See also:Europe and even to extreme See also:northern latitudes in Tertiary time, which have appealed to the See also:imagination and found their way so widely into literature, are now challenged by J. W. See also:Gregory (See also:Climatic See also:Variations, their Extent and Causes, See also:International See also:Geological See also:Congress, See also: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 See also:elevation and See also:distribution of the See also:land and See also:sea masses depend for their See also:solution chiefly upon the palaeontologist. With See also:good See also:reason geographers have given reluctant consent to some of the bold restorations of See also:ancient continental outlines by palaeontologists; yet some of the greatest achievements of recent science have been in this See also: 1831), followed by Frech, Cann, de See also:Lapparent and others. Neumayr was the first to See also:attempt to restore the grander earth outlines of the earth as a whole in See also:Jurassic times. Suess outlined the ancient relations of See also:Africa and See also:Asia through his " See also:Gondwana Land," a land See also:mass practically identical with the " Lemuria " of zoologists. South See also:American palaeogeography has been traced by von Ihring into a northern land mass, " Archelenis," and a See also:southern mass, " Archiplata," the latter at times See also:united with an See also:antarctic continent. Following the See also:pioneer studies of See also:Dana, the American palaeontologists and stratographers See also:Bailey See also:Willis, See also: With this has been connected the theory of " centres of origin " or of the geographic regions where the chief characters of great See also:groups have been established. Among invertebrates Barrande's See also:doctrine of centres of origin was applied by See also:Hyatt to the See also:genesis of the Arietidae (1889); after studying thousands of individuals from the See also:principal deposits of Europe he decided that the cradles of the various branches of this See also:family were the basins of the Cate d'Or and southern See also: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 See also:historical work. This branch of the science has reached the highest development in its application to the history of the extinct See also:mammalia of the Tertiary through the See also:original work of See also:Cope and See also:Henri Filhol, which has been brought to a much higher degree of exactness recently through the studies of H. F. See also: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 See also:binomial terminology and its subsequent trinomial refinement for species, sub-species, and varieties was adapted to See also:express the See also:differences between animals as they exist to-See also:day, distributed contemporaneously over the See also:surface of the earth, and that it is wholly inadapted to express either the See also:minute gradations of successive generic series or the branchings of a genetically connected See also:chain of life. Such gradations, termed " mutations " by See also:Waagen, are distinguished, as observed, in single characters; they are the 586 nuances, or grades of difference, which are the more See also:gradual the more finely we dissect the geologic See also: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 See also: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. See also:Embryology and Ontogeny.—In following the See also:discovery of the See also: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 See also:Horse in Each See also:Hind See also:Foot See also:Teeth See also:Quaternary or See also:Age of See also:Man Recent Three Toes J See also:Stile toes not touching the ground Three Toes See also:Side toes not touching the ground One Toe Splints of Viand 4thdigits See also:Long-Crowned, See also: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 See also:Reptiles Fore See also:Fool See also:Pleistocene See also:Pliocene One Toe Splints of 2"-°and edigits Oligocene Four Toes This revolution may be accomplished by adding the See also:term " mutation ascending " or " mutation descending " for the minute steps of transformation, and the term phylum, as employed in Germany, for the See also:minor and See also:major branches of genetic series. See also: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 See also:place in a See also: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 See also: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 See also:Plate III., fig. 8; also fig. 9) in a See also:lower See also:horizon a See also:cusp is adumbrated in shadowy See also:form, in a slightly higher horizon it is visible, in a still higher horizon it is full-grown; and we See also:honour this final See also: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 See also:Owen, Cope, See also: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 See also:part of the organism enjoys its gradual and See also:independent evolution. In the See also:face of the continuous series of characters and types revealed by palaeontology, the Linnaean terminology,the individual See also:order of development and the ancestral order of evolution. The mutual relations of palaeontology and embryo-logy and See also:comparative See also:anatomy as means of determining the ancestry of animals are most interesting. In tracing the phylogeny, or ancestral history of See also: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 See also:array because placed in a graded series, may be, nevertheless, in an order inverse to that of the actual historical See also:succession. The most marked See also:case of such See also:inversion in comparative anatomy is that of Carl See also:Gegenbaur (1826-1903), who in arranging the fins of fishes in support of his theory that the fin of the Australian See also:lung-See also:fish (Ceratodus) was the most See also: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 See also:tripod of evidence, derived from the available facts of embryology, comparative anatomy and palaeontology. VI.—THE PALAEONTOLOGIST AS HISTORIAN The modes of See also: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 See also:tree of _animal descent. Were the geologic See also: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 See also:protoplasm. Dealing with interrupted evidence, however, it becomes necessary to exercise the closest analysis and See also:synthesis as part of his general See also: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 " See also:analogy," " See also:parallelism," " convergence " and " homoplasy." Of these two kinds of genetic and adaptive resemblance, homogeny is the warp composed of the See also:vertical, hereditary strands, which connect animals with their ancestors and their successors, while analogy is the woof, composed of the See also:horizontal strands which tie animals together by their superficial resemblances. This wide distinction between similarity of descent and similarity of See also:adaptation applies to every See also: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 See also:grand See also:scale. Analogy, in its See also: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 See also: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 See also:shark, the ichthyosaur, (After a See also:drawing by Charles R. See also:Knight, made under the direction of See also:Professor Osborn.) The See also:external similarity in the fore See also:paddle and back fin of these three marine animals is absolute, although they are totally unrelated to each other, and have a totally different See also: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 See also:lobe of tail upturned. B, Ichthyosaur (See also:Ichthyosaurus quadricissus), with fin-like paddles, long lobe of tail down-turned. C, See also:Dolphin (Sotalia fluviatilis), with horizontal tail, fin or See also:fluke. and the dolphin (fig. so) superficially resemble each other, but if the See also:outer form be removed this resemblance proves to be a See also:mere See also:veneer of adaptation, because their internal skeletal parts are as radically different as are their genetic relations, founded on See also:heredity. Analogy also produces equally remarkable internal or skeletal transformations. The ingenuity of nature, however, in adapting animals is not See also: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 See also: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 See also: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 See also: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 See also: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 " See also:special homology " of Owen, is the supreme test of kinship or of hereditary relationship, and thus the basis of all See also:sound reasoning in phylogeny. The two See also: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 See also:function, and the recognition of former similarities of adaptation (see below) are the true bases for the See also: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 See also:laws See also: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 See also: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 See also:familiar. For example, among the land vertebrates the feet (associated with the structure of the limbs and See also:trunk) may take one of many lines of adaptation to different See also:media or See also:habitat, either aquatic, terrestrial, arboreal or aerial; while the teeth (associated with the structure of the See also:skull and jaws) also may take one of many lines of adaptation to` different kinds of See also: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 See also: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 See also:cursorial limbs that at first in See also: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 See also:sloth-like claws, such as Chalicotherium. This latter animal is closely related to one which Cuvier termed See also:Pangolin gigantesque, and had he restored it according to his " law of correlation " he would have pictured a See also:giant " scaly See also:anteater," a type as wide as the poles from the actual form of Chalicotherium, which in See also: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 See also: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 See also:shell of an ammonite, for example, may change markedly in form without a corresponding change in suture, or See also:vice versa. Similarly, there is no correlation in the rate of evolution either of adjoining or of separated parts; the See also:middle See also: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. Additional information and CommentsThere are no comments yet for this article.
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