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Rh contrast with contemporaneous or “geographic varieties,” which he considered inconstant and of slight systematic value. More recent analysis has shown, however, that certain modifications observed within the same stratigraphic level are really grades of mutations which show divergences comparable to those found in successive levels. The collective term “mutation,” as now employed by palaeontologists, signifies a type modified to a slight degree in one or more of its characters along a progressive or definite line of phyletic development. The term “mutation” also applies to a single new character and for distinction may be known as “the mutation of Waagen.” This definitely directed evolution, or development in a few determinable directions, has since been termed “orthogenetic evolution,” and is recognized by all workers in invertebrate palaeontology and phylogeny as fundamental because the facts of invertebrate palaeontology admit of no other interpretation.

Among the many who followed the method of attack first outlined by Hyatt, or who independently discovered his method, only a few can be mentioned here—namely, Waagen (1869), Neumayr (1871), Württemberger (1880), Branco (1880), Mojsisovics (1882), Buckman (1887), Karpinsky (1889), Jackson (1890), Beecher (1890), Perrin-Smith (1897), Clarke (1898) and Grabau (1904). Melchior Neumayr, the great Austrian palaeontologist, especially extended the philosophic foundations of modern invertebrate palaeontology, and traced a number of continuous genetic series (formenreihe) in successive horizons. He also demonstrated that mutations have this special or distinctive character, that they repeat in the same direction without oscillation or retrogression. He expressed great reserve as to the causes of these mutations. He was the first to attempt a comprehensive treatment of all invertebrates from the genetic point of view; but unfortunately his great work, entitled Die Stämme des Thierreichs (Vienna and Prague, 1889), was uncompleted.

The absolute agreement in the results independently obtained by these various investigators, the interpretation of individual development as the guide to phyletic development, the demonstration of continuous genetic series, each mutation falling into its proper place and all showing a definite direction, constitute contributions to biological philosophy of the first importance, which have been little known or appreciated by zoologists because of their pubUcation in monographs of very special character.

Vertebrate Palaeontology after Darwin.—The impulse which Darwin gave to vertebrate palaeontology was immediate and unbounded, finding expression especially in the writings of Thomas Henry Huxley (1825-1895) in England, of Jean Albert Gaudry (b. 1827) in France, in America of Edward Drinker Cope (1840-1897) and Othniel Charles Marsh (1831-1899). Fine examples of the spirit of the period as applied to extinct Mammalia are Gaudry's Animaux fossiles et géologie de l'Attique (1862) on the Upper Miocene fauna of Pikermi near Athens, and the remarkable memoirs of Vladimir Onufrievich Kowalevsky (1842-1883), published in 1873. These works swept aside the dry traditional fossil lore which had been accumulating in France and Germany. They breathed the new spirit of the recognition of adaptation and descent. In 1867-1872 Milne Edwards published his memoirs on the Miocene birds of central France. Huxley's development of the method of palaeontology should be studied in his collected memoirs (Scientific Memoirs of Thomas Henry Huxley, 4 vols., 1898). In Kowalevsky's Versuch einer natürlichen Classification der Fossilen Hufthiere (1873) we find a model union of detailed inductive study with theory and working hypothesis. All these writers attacked the problem of descent, and published preliminary phylogenies of such animals as the horse, rhinoceros and elephant, which time has proved to be of only general value and not at all comparable to the exact phylogenetic series which were being established by invertebrate palaeontologists. Phyletic gaps began to be filled in this general way, however, by discovery, especially through remarkable

discoveries in North America by Leidy, Cope and Marsh, and the ensuing phylogenies gave enormous prestige to palaeontology.

Cope's philosophic contributions to palaeontology began in 1868 (see essays in The Origin of the Fittest, New York, 1887, and The Primary Factors of Organic Evolution, Chicago, 1896) with the independent discovery and demonstration among vertebrates of the laws of acceleration and retardation. To the law of “recapitulation” he unfortunately applied Hyatt's term “parallelism,” a term which is used now in another sense. He especially pointed out the laws of the “extinction of the specialized” and “survival of the non-specialized” forms of life, and challenged Darwin's principle of selection as an explanation of the origin of adaptations by saying that the “survival of the fittest” does not explain the “origin of the fittest.” He personally sought to demonstrate such origin, first, in the existence of a specific internal growth force, which he termed bathmic force, and second in the direct inheritance of acquired mechanical modifications of the teeth and feet. He thus revived Lamarck's views and helped to found the so-called neo-Lamarckian school in America. To this school A. Hyatt, W. H. Dall and many other invertebrate palaeontologists subscribed.

''History of Discovery. Vertebrates.''—In discovery the theatre of interest has shifted from continent to continent, often in a sensational manner. After a long period of gradual revelation of the ancient life of Europe, extending eastward to Greece, eastern Asia and to Australia, attention became centred on North America, especially on Rocky Mountain exploration. New and unheard-of orders of amphibians, reptiles and mammals came to the surface of knowledge, revolutionizing thought, demonstrating the evolution theory, and solving some of the most important problems of descent. Especially noteworthy was the discovery of birds with teeth both in Europe (Archaeopteryx) and in North America (Hesperornis), of Eocene stages in the history of the horse, and of the giant dinosauria of the Jurassic and Cretaceous in North America. Then the stage of novelty suddenly shifted to South America, where after the pioneer labours of Darwin, Owen and Burmeister, the field of our knowledge was suddenly and vastly extended by explorations by the brothers Ameghino (Carlos and Florentino). We were in the midst of more thorough examination of the ancient world of Patagonia, of the Pampean region and of its submerged sister continent Antarctica, when the scene shifted to North Africa through the discoveries of Hugh J. L. Beadnell and Charles W. Andrews. These latter discoveries supply us with the ancestry of the elephants and many other forms. They round out our knowledge of Tertiary history, but leave the problems of the Cretaceous mammals and of their relations to Tertiary mammals still unsolved. Similarly, the Mesozoic reptiles have been traced successively to various parts of the world from France, Germany, England, to North America and South America, to Australia and New Zealand and to northern Russia, from Cretaceous times back into the Permian, and by latest reports into the Carboniferous.

Discovery of Invertebrates.—The most striking feature of exploration for invertebrates, next to the world-wide extent to which exploration has been carried on and results applied, is the early appearance of life. Until comparatively recent times the molluscs were considered as appearing on the limits of the Cambrian and Ordovician; but Charles D. Walcott has described a tiny lamellibranch (Modioloides) from the inferior Cambrian, and he reports the gastropod (?) genus Chuaria from the pre-Cambrian. Cephalopod molluscs have been traced back to the straight-shelled nautiloids of the genus Volborthella, while true ammonites have been found in the inferior Permian of the Continent and by American palaeontologists in the true coal measures. Similarly, early forms of the crustacean sub-class Merostomata have been traced to the pre-Cambrian of North America.

Recent discoveries of vertebrates are of the same significance, the most primitive fishes being traced to the Ordovician or base of the Silurian, which proves that we shall discover more