Internal causes of extinction are to be found in exaggeration of body size, in the hypertrophy or over-specialization of certain organs, in the irreversibility of evolution, and possibly, although this has not been demonstrated, in a progressive reduction of variability. In a full analysis of this problem of internal and external causes in relation to the Tertiary Mammalia, H. F. Osborn (“Causes of Extinction of the Mammalia,” Amer. Naturalist, 1906, pp. 769–795, 829–859) finds that foremost in the long series of causes which lead to extinction are the grander environmental changes, such as physiographic changes, diminished or contracted land areas, substitution of insular for continental conditions; changes of climate and secular lowering of temperature accompanied by deforestation and checking of the food supply; changes influencing the mating period as well as fertility; changes causing increased humidity, which in turn favours enemies among insect life. Similarly secular elevations of temperature, either accompanied by moisture or desiccation, by increasing droughts or by disturbance of the balance of nature, have been followed by great waves of extinction of the Mammalia. In the sphere of living environment, the varied evolution of plant life, the periods of forestation and deforestation, the introduction of deleterious plants simultaneously with harsh conditions of life and enforced migration, as well as of mechanically dangerous plants, are among the well-ascertained causes of diminution and extinction. The evolution of insect life in driving animals from feeding ranges and in the spread of disease probably has been a prime cause of extinction. Food competition among mammals, especially intensified on islands, and the introduction of Carnivora constitute another class of causes. Great waves of extinction have followed the long periods of the slow evolution of relatively inadaptive types of tooth and foot structure, as first demonstrated by Waldemar Kowalevsky; thus mammals are repeatedly observed in a cul-de-sac of structure from which there is no escape in an adaptive direction. Among still other causes are great bulk, which proves fatal under certain new conditions; relatively slow breeding; extreme specialization and development of dominant organs, such as horns and tusks, on which for a time selection centres to the detriment of more useful characters. Little proof is afforded among the mammals of extinction through arrested evolution or through the limiting of variation, although such laws undoubtedly exist. One of the chief deductions is that there are special dangers in numerical diminution of herds, which may arise from a chief or original cause and be followed by a conspiracy of other causes which are cumulative in effect. This survey of the phenomena of extinction in one great class of animals certainly establishes the existence of an almost infinite variety of causes, some of which are internal, some external in origin, operating on animals of different kinds.
VIII.—Underlying Biological Principles as they appear to the Palaeontologist
It follows from the above brief summary that palaeontology affords a distinct and highly suggestive field of purely biological research; that is, of the causes of evolution underlying the observable modes which we have been describing. The net result of observation is not favourable to the essentially Darwinian view that the adaptive arises out of the fortuitous by selection, but is rather favourable to the hypothesis of the existence of some quite unknown intrinsic law of life which we are at present totally unable to comprehend or even conceive. We have shown that the direct observation of the origin of new characters in palaeontology brings them within that domain of natural law and order to which the evolution of the physical universe conforms. The nature of this law, which, upon the whole, appears to be purposive or teleological in its operations, is altogether a mystery which may or may not be illumined by future research. In other words, the origin, or first appearance of new characters, which is the essence of evolution, is an orderly process so far as the vertebrate and invertebrate palaeontologist observes it. The selection of organisms through the crucial test of fitness and the shaping of the organic world is an orderly process when contemplated on a grand scale, but of another kind; here the test of fitness is supreme. The only inkling of possible underlying principles in this orderly process is that there appears to be in respect to certain characters a potentiality or a predisposition through hereditary kinship to evolve in certain definite directions. Yet there is strong evidence against the existence of any law in the nature of an internal perfecting tendency which would operate independently of external conditions. In other words, a balance appears to be always sustained between the internal (hereditary and ontogenetic) and the external (environmental and selectional) factors of evolution.
Bibliography.—Among the older works on the history of palaeontology are the treatises of Giovanni Battista Brocchi (1772–1826), Conchiologia fossile Subappenina . . . Disc. sui progressi dello studio . . . 1843 (Milan); of Étienne Jules d’Archiac, Histoire du progrès de la géologie de 1834 à 1862 (Paris, Soc. Géol. de France, 1847–1860); of Charles Lyell in his Principles of Geology. A clear narrative of the work of many of the earlier contributors is found in Founders of Geology, by Sir Archibald Geikie (London, 1897–1905). The most comprehensive and up-to-date reference work on the history of geology and palaeontology is Geschichte der Geologie und Paläontologie, by Karl Alfred von Zittel (Munich and Leipzig, 1899), the final life-work of this great authority, translated into English in part by Maria M. Ogilvie-Gordon, entitled “History of Geology and Palaeontology to the end of the 19th Century.” The succession of life from the earliest times as it was known at the close of the last century was treated by the same author in his Handbuch der Paläontologie (5 vols., Munich and Leipzig, 1876–1893). Abbreviated editions of this work have appeared from the author, Grundzüge der Paläontologie (Palaeozoologie) (Munich and Leipzig, 1895, 2nd ed., 1903), and in English form in Charles R. Eastman’s Text-Book of Palaeontology (1900–1902). A classic but unfinished work describing the methods of invertebrate palaeontology is Die Stämme des Thierreichs (Vienna, 1889), by Melchior Neumayr. In France admirable recent works are Eléments de Paléontologie, by Felix Bernard (Paris, 1895), and the still more recent philosophical treatise by Charles Depéret, Les Transformations du monde animal (Paris, 1907). Huxley’s researches, and especially his share in the development of the philosophy of palaeontology, will be found in his essays, The Scientific Memoirs of Thomas Henry Huxley (4 vols., London, 1898–1902). The whole subject is treated systematically in Nicholson and Lydekker’s A Manual of Palaeontology (2 vols., Edinburgh and London, 1889), and A. Smith Woodward’s Outlines of Vertebrate Palaeontology (Cambridge, 1898).
Among American contributions to vertebrate palaeontology, the development of Cope’s theories is to be found in the volumes of his collected essays, The Origin of the Fittest (New York, 1887), and The Primary Factors of Organic Evolution (Chicago, 1896). A brief summary of the rise of vertebrate palaeontology is found in the address of O. Marsh, entitled “History and Methods of Palaeontological Discovery” (American Association for the Advancement of Science, 1879). The chief presentations of the methods of the American school of invertebrate palaeontologists are to be found in A. Hyatt’s great memoir “Genesis of the Arietidae” (Smithsonian Contr. to Knowledge, 673, 1889), in Hyatt’s “Phylogeny of an Acquired Characteristic” (Philosophical Soc. Proc., vol. xxxii, 1894), and in Geological Biology, by H. S. Williams (New York, 1895).
In preparing the present article the author has drawn freely on his own addresses: see H. F. Osborn, “The Rise of the Mammalia in North America” (Proc. Amer. Assn. Adv. Science, vol. xlii., 893), “Ten Years' Progress in the Mammalian Palaeontology of North America” (Comptes rendus du 6e Congrès intern. de zoologie, session de Bern, 1904), “The Present Problems of Palaeontology” (Address before Section of Zool. International Congress of Arts and Science, St Louis, Sept. 1904), “The Causes of Extinction of Mammalia” (Amer. Naturalist, xl. 769–795, 829–859, 1906). (H. F. O.)
PALAEOSPONDYLUS, a small fish-like organism, of which the skeleton is found fossil in the Middle Old Red Sandstone of Achanarras, near Thurso, Caithness. It was thus named (Gr. ancient vertebra) by Dr R. H. Traquair in 1890, in allusion to its well-developed vertebral rings; and its structure was
