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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 See also:

habitat as applied to the See also:interpretation of the See also:anatomy of the marsupials, of many kinds of fishes, of such See also: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 See also:carapace of the aberrant See also:leather-back See also:sea-turtles (Sphargidae) by prefacing an initial See also:land phase, in which the typical See also: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 See also:fourth resumed or secondary marine phase, in which this polygonal armature began to degenerate. Each of these alternate See also: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 See also:great importance in phylogenetic study. A very important evolutionary principle is that in such secondary returns to See also:primary phases lost See also:organs are never recovered, but new organs are acquired; hence the force of Dollo's dictum that See also: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 See also:adaptation. 3. Adaptive Radiations of See also:Groups, See also:Continental and See also:Local.—Starting with the See also:stem forms the descendants of which have passed through either persistent or changed habitats, we reach the underlying See also:idea of the branching See also:law of See also:Lamarck or the law of divergence of See also:Darwin, and find it perhaps most clearly ex-pressed in the words "adaptive See also:radiation" (See also:Osborn), which convey the idea of radii in many directions. Among See also:extinct See also:Tertiary mammals we can actually trace the giving off of these radii in all directions, for taking See also:advantage of every possibility to secure See also:food, to See also:escape enemies and to reproduce See also: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 See also:history of continental life presents a picture of contemporaneous radiations in different parts of the See also:world and of a See also:succession of radiations in the same parts. We observe the contemporaneous and largely See also:independent radiations of the hoofed animals in See also:South See also:America, in See also:Africa and in the great See also:ancient See also:continent comprising See also:Europe, See also:Asia and See also:North America; we observe the Cretaceous radiation of hoofed animals in the See also:northern hemisphere, followed by a second radiation of hoofed animals in the same region, in some cases one surviving See also:spur of an old radiation becoming the centre of a new one. As a See also:rule, the larger the geographic See also:theatre the grander the radiation. Successive discoveries have revealed certain See also:grand centres, such as (r) the marsupial radiation of See also:Australia, (2) the little-known Cretaceous radiation of placental mammals in the northern hemisphere, which was probably connected in See also:part with the peopling of South America, (3) the Tertiary placental radiation in the northern hemisphere, partly connected with Africa, (4) the See also: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 See also:

separate contributions of the land masses of North America, South America, Europe, Asia, Africa, and of Australia to the mammalian See also:fauna of the world, a result which can be obtained through palaeontology only. 4. Adaptive Local Radiation.—On a smaller See also:scale are the local adaptive radiations which occur through segregation of See also:habit and local See also:isolation in the same See also:general geographic region wherever physiographic and See also:climatic See also:differences are sufficient to produce local differences in food See also:supply or other local factors of See also:change. This local divergence may proceed as rapidly as through wide See also:geographical segregation or isolation. This principle has been demonstrated recently among Tertiary rhinoceroses and titanotheres, in which remains of four or five genetic See also:series in the same geologic deposits have been discovered. We have See also:proof that in the Upper See also:Miocene of See also:Colorado there existed a See also:forest-living See also:horse,or more persistent See also: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 See also:desert type (see See also:Plate IV., See also:figs. 12, 13, 14, 15). In times of drought these animals undoubtedly resorted to the same See also: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 See also:group of animals. The polyphyletic law was See also:early demonstrated among invertebrates by See also: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 See also:

Middle Devonian of North America, while more than twenty divergent lines have been observed by Grabau among the See also:species of the gastropod genus Fusus in Tertiary and See also:recent times. Vertebrate palaeontologists were slow to grasp this principle; while the early speculative phylogenies of the horse of See also:Huxley and See also: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 See also:direct phylum which leads to the See also:modern horse has become a See also:matter of far more difficult See also:search. As early as 1862 See also:Gaudry set forth this very polyphyletic principle in his See also:tabular phylogenies, but failed to carry it to its logical application. It is now applied throughout the See also: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 See also:American'Miocene and See also:Pliocene camels; in See also:short, the polyphyletic See also:condition is the rule rather than the exception. It is displayed to-See also: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 See also: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 See also: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 See also:Jurassic See also: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 See also:

form that we have learned only recently to distinguish similarities of analogous habit from the similitudes of real kinship. From whatever See also:order of See also:Mammalia or Reptilia an See also:animal may be derived, prolonged aquatic adaptation will See also:model its See also:outer, and finally its inner, structure according to certain advantageous designs. The requirements of an elongate See also:body moving through the resistant See also: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 See also:Dean). We owe especially to Willy Kiikenthal, See also:Eberhard See also:Fraas, S.W. Williston and R. C. Osburn a See also:summary of those modifications of form to which aquatic life invariably leads. The law of See also:analogy also operates in retrogression. A. See also:Smith See also:Woodward has observed that the decline of many groups of fishes is heralded by the tendency to assume elongate and finally See also:eel-shaped forms, as seen independently, for example, among the declining Acanthodians or palaeozoic sharks, among the modern crossopterygian Polypterus and Calamoichthys of the See also:Nile, in the modern dipneustan Lepidosiren and Protopterus, in the Triassic chondrostean Belonorhynchus, as well as in the See also: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 See also:

power to coil, observed in the terminals of many declining series of gastropods from the See also:Cambrian to the See also: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 See also: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 See also: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, See also: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 See also:minute continuous change is by far the most universal; but many palaeontologists as well as zoologists and botanists hold a contrary See also: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 See also:acceleration or retardation of separate organs. Thus H. S. See also:Williams observes (Geological See also:Biology, p. 268) that the evolution of those fundamental characters which See also:mark differences between separate classes, orders, sub-orders, and even families of organisms, took See also:place in relatively short periods of time.

Among the brachiopods the See also:

chief expansion of each type is at a relatively early period in their life-history. See also: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 "See also: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," See also:Annals and Mag. Natural Hist., 1906, p. 317) notes that the fundamental advances in the growth of See also:fish life have always been sudden, beginning with excessive vigour at the end of See also:long periods of apparent stagnation; while each advance has becn•marked by the fixed and definite acquisition of some new anatomical See also:character or " expression point," a See also: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 See also: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 See also:evidence that the acquisition either of the jaw or of the lateral fins had not been in itself an extremely gradual See also:process. Thus both invertebrate and vertebrate palaeontologists have reached independently the conclusion that the evolution of groups is not continuously at a See also:uniform See also:rate, but that there are, especially in the beginnings of new phyla or at the time of acquisition of new organs, sudden See also: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. See also:

Hypothesis of the Sudden See also: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 See also: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 See also: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 See also: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 See also:borne in mind, first, that wherever a new animal suddenly appears or a new character suddenly arises in a fossil See also:horizon we must consider whether such appearance may be due to the non-See also:discovery of transitional links with older forms, or to the sudden invasion of a new type or new See also: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 See also:zoology and See also:embryology but never can be demonstrated by palaeontology, because of the incompleteness of the geological See also: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 See also:

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 See also:Lower Cambrian. Among the ammonites the loss of power to coil the See also:shell is one feature of racial old See also:age, and in others old age is accompanied by closer coiling and loss of See also: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. See also:Brocchi and See also:Daniel See also:Rosa (1899) have See also: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. See also: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 See also: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 See also: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.

See also:

Internal causes of extinction are to be found in exaggeration of body See also:size, in the See also: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 See also:analysis of this problem of internal and See also: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 See also: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 See also:climate and See also: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 See also:insect life. Similarly secular elevations of temperature, either accompanied by moisture or See also:desiccation, by increasing droughts or by disturbance of the See also:balance of nature, have been followed by great waves of extinction of the Mammalia. In the See also:sphere of living environment, the varied evolution of plant life, the periods of forestation and deforestation, the introduction of deleterious See also:plants simultaneously with harsh conditions of life and enforced See also:migration, as well as of mechanically dangerous plants, are among the well-ascertained causes of diminution and extinction. The evolution of insect life in See also:driving animals from feeding ranges and in the spread of disease probably has been a See also:prime cause of extinction. Food competition among mammals, especially intensified on islands, and the introduction of See also: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 See also:foot structure, as first demonstrated by Waldemar Kowalevsky; thus mammals are repeatedly observed in a cul-de-See also: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 See also:

laws undoubtedly exist. One of the chief deductions is that there are See also:special dangers in numerical diminution of herds, which may arise from a chief or See also:original cause and be followed by a See also: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 See also: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 See also:research; that is, of the causes of evolution underlying the observable modes which we have been describing. The See also: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 See also: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 See also:physical universe See also:con-forms. The nature of this law, which, upon the whole, appears to be purposive or teleological in its operations, is. altogether a See also: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 See also: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 See also:

York, 1887), and The Primary Factors of Organic Evolution (See also: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 See also:Advancement of See also: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 " See also:Genesis of the Arietidae " (Smithsonian Contr. to Knowledge, 673, 1889), in Hyatt's " Phylogeny of an Acquired Characteristic " (Philosophical See also:Soc. Proc., vol. xxxii. 1894), and in Geological Biology, by H. S. Williams (New York, 1895). In preparing the present See also:article the author has See also: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 See also:

Bern, 1904), " The Present Problems of Palaeontology " (Address before See also:Section of Zool. See also:International See also:Congress of Arts and Science, St Louis, See also:Sept. 1904), " The Causes of Extinction of Mammalia " (Amer. Naturalist, xl. 769–795 829–859, 1906). (H. F.

End of Article: CURSORIAL

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