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Originally appearing in Volume V20, Page 591 of the 1911 Encyclopedia Britannica.
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CURSORIAL Digitigrade contributed most brilliant discussions of the theory of alter-nations of habitat as applied to the interpretation of the anatomy of the marsupials, of many kinds of fishes, of such reptiles as the herbivorous dinosaurs of the Upper Cretaceous. He has applied the theory with especial ingenuity to the interpretation of the circular bony plates in the carapace of the aberrant leather-back sea-turtles (Sphargidae) by prefacing an initial land phase, in which the typical armature of land tortoises was acquired, a first marine or pelagic phase, in which this armature was lost, a third littoral or seashore phase, in which a new polygonal armature was acquired, and a fourth resumed or secondary marine phase, in which this polygonal armature began to degenerate. Each of these alternate life phases may leave some profound modification, which is partially obscured but seldom wholly lost; thus the tracing of the evidences of former adaptations is of great importance in phylogenetic study. A very important evolutionary principle is that in such secondary returns to primary phases lost organs are never recovered, but new organs are acquired; hence the force of Dollo's dictum that evolution is irreversible from the point of view of structure, while frequently reversible, or recurrent, in point of view of the conditions of environment and adaptation. 3. Adaptive Radiations of Groups, Continental and Local.—Starting with the stem forms the descendants of which have passed through either persistent or changed habitats, we reach the underlying idea of the branching law of Lamarck or the law of divergence of Darwin, and find it perhaps most clearly ex-pressed in the words "adaptive radiation" (Osborn), which convey the idea of radii in many directions. Among extinct Tertiary mammals we can actually trace the giving off of these radii in all directions, for taking advantage of every possibility to secure food, to escape enemies and to reproduce kind; further, among such well-known quadrupeds as the horses, rhinoceroses and titanotheres, the modifications involved in these radiations can be clearly traced. Thus the history of continental life presents a picture of contemporaneous radiations in different parts of the world and of a succession of radiations in the same parts. We observe the contemporaneous and largely independent radiations of the hoofed animals in South America, in Africa and in the great ancient continent comprising Europe, Asia and North America; we observe the Cretaceous radiation of hoofed animals in the northern hemisphere, followed by a second radiation of hoofed animals in the same region, in some cases one surviving spur of an old radiation becoming the centre of a new one. As a rule, the larger the geographic theatre the grander the radiation. Successive discoveries have revealed certain grand centres, such as (r) the marsupial radiation of Australia, (2) the little-known Cretaceous radiation of placental mammals in the northern hemisphere, which was probably connected in part with the peopling of South America, (3) the Tertiary placental radiation in the northern hemisphere, partly connected with Africa, (4) the main Tertiary radiation in South America. Each of these radiations produced a greater or less number of analogous groups, and while originally independent the animals thus evolving as autochthonous types finally mingled together as migrant or invading types. We are thus working out gradually the separate contributions of the land masses of North America, South America, Europe, Asia, Africa, and of Australia to the mammalian fauna of the world, a result which can be obtained through palaeontology only. 4. Adaptive Local Radiation.—On a smaller scale are the local adaptive radiations which occur through segregation of habit and local isolation in the same general geographic region wherever physiographic and climatic differences are sufficient to produce local differences in food supply or other local factors of change. This local divergence may proceed as rapidly as through wide geographical segregation or isolation. This principle has been demonstrated recently among Tertiary rhinoceroses and titanotheres, in which remains of four or five genetic series in the same geologic deposits have been discovered. We have proof that in the Upper Miocene of Colorado there existed a forest-living horse,or more persistent primitive type, which was contemporaneous with and is found in the same deposits with the plains-living horse (Neohipparion) of the most advanced or specialized desert type (see Plate IV., figs. 12, 13, 14, 15). In times of drought these animals undoubtedly resorted to the same water-courses for drink, and thus their fossilized remains are found associated. 5. The Law of Polyphyletic Evolution. The Sequence of Phyla or Genetic Series.—There results from continental and local adaptive radiations the presence in the same geographical region of numerous distinct lines in a given group of animals. The polyphyletic law was early demonstrated among invertebrates by Neumayr (1889) when he showed that the ammonite genus Phylloceras follows not one but five distinct lines of evolution of unequal duration. The brachiopods, generally classed collectively as Spirifer mucronatus, follow at least five distinct lines of evolution in the Middle Devonian of North America, while more than twenty divergent lines have been observed by Grabau among the species of the gastropod genus Fusus in Tertiary and recent times. Vertebrate palaeontologists were slow to grasp this principle; while the early speculative phylogenies of the horse of Huxley and Marsh, for example, were mostly displayed monophyletically, or in single lines of descent, it is now recognized that the horses which were placed by Marsh in a single series are really to he ranged in a great number of contemporaneous but separate series, each but partially known, and that the direct phylum which leads to the modern horse has become a matter of far more difficult search. As early as 1862 Gaudry set forth this very polyphyletic principle in his tabular phylogenies, but failed to carry it to its logical application. It is now applied throughout the Vertebrata of both Mesozoic and Cenozoic times. Among marine Mesozoic reptiles, each of the groups broadly known as ichthyosaurs, plesiosaurs, mosasaurs and crocodiles were polyphyletic in a marked degree. Among land animals striking illustrations of this local polyphyletic law are found in the existence of seven or eight contemporary series of rhinoceroses, five or six contemporary series of horses, and an equally numerous contemporary series of American'Miocene and Pliocene camels; in short, the polyphyletic condition is the rule rather than the exception. It is displayed to-day among the antelopes arid to a limited degree among the zebras and rhinoceroses of Africa, a continent which exhibits a survival of the Miocene and Pliocene conditions of the northern hemisphere. 6. Development of Analogous Progressive and Retrogressive Groups.—Because of the repetition of analogous physiographic and climatic conditions in regions widely separated both in time and in space, we discover that continental and local adaptive radiations result in the creation of analogous groups of radii among all the vertebrates and invertebrates. Illustrations of this law were set forth by Cope as early as 1861 (see " Origin of Genera," reprinted in the Origin of the Fittest, pp. 95-206) in pointing out the extraordinary parallelisms between unrelated groups of amphibians, reptiles and mammals. In the Jurassic period there were no less than six orders of reptiles Which independently abandoned terrestrial life and acquired more or less perfect adaptation to sea life. Nature, limited in her resources for adaptation, fashioned so many of these animals in like form that we have learned only recently to distinguish similarities of analogous habit from the similitudes of real kinship. From whatever order of Mammalia or Reptilia an animal may be derived, prolonged aquatic adaptation will model its outer, and finally its inner, structure according to certain advantageous designs. The requirements of an elongate body moving through the resistant medium of water are met by the evolution of similar entrant and exit curves, and the bodies of most swiftly moving aquatic animals evolve into forms resembling the hulls of modern sailing yachts (Bashford Dean). We owe especially to Willy Kiikenthal, Eberhard Fraas, S.W. Williston and R. C. Osburn a summary of those modifications of form to which aquatic life invariably leads. The law of analogy also operates in retrogression. A. Smith Woodward has observed that the decline of many groups of fishes is heralded by the tendency to assume elongate and finally eel-shaped forms, as seen independently, for example, among the declining Acanthodians or palaeozoic sharks, among the modern crossopterygian Polypterus and Calamoichthys of the Nile, in the modern dipneustan Lepidosiren and Protopterus, in the Triassic chondrostean Belonorhynchus, as well as in the bow-fin (Amia) and the garpike (Lepidosteus). Among invertebrates similar analogous groups also develop. This is especially marked in retrogressive, though also well-known in progressive series. The loss of the power to coil, observed in the terminals of many declining series of gastropods from the Cambrian to the present time, and the similar loss of power among Natiloidea and Ammonoidea of many genetic series, as well as the ostraean form assumed by various declining series of pelecypods and by some brachiopods, may be cited as examples. 7. Periods of Gradual Evolution of Groups.—It is certainly a very striking fact that wherever we have been able to trace genetic series, either of invertebrates or vertebrates, in closely sequent geological horizons, or life zones, we find strong proof of evolution through extremely gradual mutation simultaneously affecting many parts of each organism, as set forth above. This proof has been reached quite independently by a very large number of observers studying a still greater variety of animals. Such diverse organisms as brachiopods, ammonites, horses and rhinoceroses absolutely conform to this law in all those rare localities where we have been able to observe closely sequent stages. The inference is almost irresistible that the law of gradual transformation through minute continuous change is by far the most universal; but many palaeontologists as well as zoologists and botanists hold a contrary opinion. 8. Periods of Rapid Evolution of Groups.—The above law of gradual evolution is perfectly consistent with a second principle, namely, that at certain times evolution is much more rapid than at others, and that organisms are accelerated or retarded in development in a manner broadly analogous to the acceleration or retardation of separate organs. Thus H. S. Williams observes (Geological Biology, p. 268) that the evolution of those fundamental characters which mark differences between separate classes, orders, sub-orders, and even families of organisms, took place in relatively short periods of time. Among the brachiopods the chief expansion of each type is at a relatively early period in their life-history. Hyatt (1883) observed of the ammonites that each group originated suddenly and spread out with great rapidity. Deperet notes that the genus Neumayria, an ammonite of the Kimmeridgian, suddenly branches out into an "explosion" of forms. Deperet also observes the contrast between periods of quiescence and limited variability and periods of sudden efflorescence. A. Smith Woodward (" Relations of Palaeontology to Biology," Annals and Mag. Natural Hist., 1906, p. 317) notes that the fundamental advances in the growth of fish life have always been sudden, beginning with excessive vigour at the end of long periods of apparent stagnation; while each advance has becn•marked by the fixed and definite acquisition of some new anatomical character or " expression point," a term first used by Cope. One of the causes of these sudden advances is undoubtedly to be found in the acquisition of a new and extremely useful character. Thus the perfect jaw and the perfect pair of lateral fins when first acquired among the fishes favoured a very rapid and for a time unchecked development. It by no means follows, however, from this incontrovertible evidence that the acquisition either of the jaw or of the lateral fins had not been in itself an extremely gradual process. Thus both invertebrate and vertebrate palaeontologists have reached independently the conclusion that the evolution of groups is not continuously at a uniform rate, but that there are, especially in the beginnings of new phyla or at the time of acquisition of new organs, sudden variations in the rate of evolution which have been termed variously " rhythmic," "pulsating," " efflorescent," "intermittent " and even " explosive " (Deperet). This varying rate of evolution has (illogically, we believe) been compared with and advanced in support of the "mutation lawof De Vries,"or the theory of saltatory evolution, which we may next consider. 9. Hypothesis of the Sudden Appearance of New Parts or Organs.—The rarity of really continuous series has naturally led palaeontologists to support the hypothesis of brusque transitions of structure. As we have seen, this hypothesis was fathered by Geoffroy St Hilaire in 183o from his studies of Mesozoic Crocodilia, was sustained by Haldemann, and quite recently has been revived by such eminent palaeontologists as Louis Dollo and A. Smith Woodward. The evidence for it is not to be confused with that for the law of rapid efflorescence of groups just considered. It should be remembered that palaeontology is the most unfavourable field of all for observation and demonstration of sudden saltations or mutations of character, because of the limited materials available for comparison and the rarity of genetic series. It should be borne in mind, first, that wherever a new animal suddenly appears or a new character suddenly arises in a fossil horizon we must consider whether such appearance may be due to the non-discovery of transitional links with older forms, or to the sudden invasion of a new type or new organ which has gradually evolved elsewhere. The rapid variation of certain groups of animals or the acceleration of certain organs is also not evidence of the sudden appearance of new adaptive characters. Such sudden appearances may be demonstrated possibly in zoology and embryology but never can be demonstrated by palaeontology, because of the incompleteness of the geological record. ro. Decline or Senescence of Groups.—Periods of gradual evolution and of efflorescence may be followed by stationary or senescent conditions. In his history of the Arietidae Hyatt points out that toward the close of the Cretaceous this entire group of ammonites appears to have been affected with some malady; the unrolled forms multiply, the septa are simplified, the ornamentation becomes heavy, thick, and finally disappears in the adult; the entire group ends by dying out and leaving no descendants. This is not due to environmental conditions solely, because senescent branches of normal progressive groups are found in all geologic horizons, beginning, for gastropods, in the Lower Cambrian. Among the ammonites the loss of power to coil the shell is one feature of racial old age, and in others old age is accompanied by closer coiling and loss of surface ornamentation, such as spines, ribs, spirals; while in other forms an arresting of variability precedes extinction. Thus Williams has observed that if we find a species breeding perfectly true we can conceive it to have reached the end of its racial life period. Brocchi and Daniel Rosa (1899) have developed the hypothesis of the progressive reduction of variability. Such decline is by no means a universal law of life, however, because among many of the continental vertebrates at least we observe extinctions repeatedly occurring during the expression of maximum variability. Whereas among many ammonites and gastropods smooth ness of the shell, following upon an ornamental youthful condition, is generally a symptom of decline, among many other invertebrates and vertebrates, as C. E. Beecher (1856–1905) has pointed out (1898), many animals possessing hard parts tend toward the close of their racial history to produce a superfluity of dead matter, which accumulates in the form of spines among invertebrates, and of horns among the land vertebrates, reaching a maximum when the animals are really on the down-grade of development. rr. The Extinction of Groups.—We have seen that different lines vary in vitality and in longevity, that from the earliest times senescent branches are given off, that different lines vary in the rate of evolution, that extinction is often heralded by symptoms of racial old age, which, however, vary widely in different groups. In general we find an analogy between the development of groups and of organs; we discover that each phyletic branch of certain organisms traverses a geologic career comparable to the life of an individual, that we may often distinguish, especially among invertebrates, a phase of youth, a phase of maturity, a phase of senility or degeneration fore-shadowing be extinction of a type. 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. 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 con-forms. 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 thetest 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. 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 0. 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., 1893), " Ten Years' Progress in the Mammalian Palaeontology of North America " (Comptes rendus du 6e Congres 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.)
End of Article: CURSORIAL

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