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LAW OF THE INDEPENDENT ADAPTIVE EVOLU...
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See also:LAW OF THE See also:INDEPENDENT ADAPTIVE See also:EVOLUTION OF PARTS . FIG. II.—See also:Diagram demonstrating that there are an indefinite number of combinations of various adaptive types of limbs and feet with various adaptive types of See also:teeth, and that there is no fixed law of correlation between the two See also:series of adaptations. All these principles are consistent with See also:Francis See also:Galton's law of particulate See also:inheritance in See also:heredity, and with the See also:modern See also:doctrine of " unity of characters " held by students of Mendelian phenomena. Sudden versus See also:Gradual Evolution of Parts.—There is a broad and most interesting See also:analogy between the evolution of parts of animals and of See also:groups of animals studied as a whole. Thus we observe persistent See also:organs and persistent types of animals, analogous organs and analogous types of animals, and this analogy applico still further to the See also:rival and more or less contradictory hypotheses of the sudden as distinguished from the gradual See also:appearance of new parts or organs of animals, and the sudden appearance of new types of animals. The first exponent of the theory of sudden appearance of new parts and new types, to our knowledge, was See also:Geoffroy St Hilaire, who suggested saltatory evolution through the See also:direct See also:action of • the environment on development, as explaining the abrupt transitions in the Mesozoic Crocodilia and tho origin of the birds from the See also:reptiles. See also:Waagen's law of mutation or the appearance of new parts or organs so gradually than they can be perceived only by following them through > lccessive geologic See also:time stages, appears to be directly contradictory to the saltation principle; it is certainly one of the most firmly established principles of palaeontology, and it constitutes the contribution See also:par excellence of this See also:branch of See also:zoology to the law of evolution, since it is obvious that it could not possibly have been deduced from comparison ofliving animals but only through the See also:long See also:perspective gained by comparison of animals succeeding each other in time. The essence of Waagen's law is orthogenesis, or evolution in a definite direction, and, if there does exist an See also:internal hereditary principle controlling such orthogenetic evolution, there does not appear to be any essential See also:contradiction between its gradual operation in the " mutations of Waagen " and its occasional hurried operation in the " mutations of de Vries," which are by their See also:definition discontinuous or saltatory (See also:Osborn, 1907). 1. Origin from See also:Primitive or See also:Stem Forms.—As already observed, the same principles apply to groups of animals as to organs and groups of organs; an See also:organ originates in a primitive and unspecialized See also:stage, a See also:group of animals originates in a primitive or stem See also:form. It was See also:early perceived by See also:Huxley, See also:Cope and many others that See also:Cuvier's broad belief in a universal progression was erroneous, and there See also:developed the distinction between " persistent primitive types " (Huxley) and " progressive types." The theoretical existence of primitive or stem forms was clearly perceived by See also:Darwin, but the steps by which the stem form might be restored were first clearly enunciated by Huxley in 188o (" On the Application of Evolution to the Arrangement of the See also:Vertebrata and more particularly of the See also:Mammalia," Scient. Mem. iv. 457) namely, by See also:sharp separation of the See also:primary or stem characters from the secondary or adaptive characters in all the known descendants or branches of a theoretical See also:original form. The sum of the primitive characters approximately restores the primitive form; and the gaps in palaeontological See also:evidence are supplied by See also:analysis of the available zoological, embryological and anatomical evidence. Thus Huxley, with true prophetic See also:instinct, found that the sum of primitive characters of all the higher placental mammals points to a stem form of a generalized insectivore type, a prophecy which has been fully confirmed by the latest See also:research. On the other See also:hand, Huxley's summation of the primitive characters of all the mammals led him to an amphibian stem type, a prophecy which has proved faulty because based on erroneous analysis and comparison. More or less independently, Huxley, Kowalevsky and Cope restored the stem ancestor of the hoofed animals, or ungulates, a restoration which has been nearly fulfilled by the See also:discovery, in 1873, of the generalized type See also:Phenacodus of See also:northern See also:Wyoming. Similar anticipations and verifications among the invertebrates have been made by See also:Hyatt, See also:Beecher, See also:Jackson and others. In certain cases the See also:character stem forms actually survive in unspecialized types. Thus the analysis of See also:George See also:Baur of the ancestral form of the lizards, mosasaurs, dinosaurs, crocodiles and phytosaurs led both to the generalized Palaeohatteria of the See also:Permian and indirectly to the surviving Tuatera See also:lizard of New See also:Zealand. 2. Adaptations to Alternations of See also:Habitat. Law of Irreversibility of Evolution.—In the long vicissitudes of time-and procession of See also:continental changes, animals have been subjected to alternations of habitat either through their own migrations or through the " See also:migration of the environment itself," to employ See also:Van den Broeck's epigrammatic description of the profound and sometimes sudden environmental changes which may take See also:place in a single locality. The traces of alternations of adaptations corresponding to these alternations of habitat are recorded both in palaeontology and See also:anatomy, although often after the obscure analogy of the earlier and later writings of a See also:palimpsest. Huxley in 188o briefly suggested the arboreal origin, or primordial See also:tree-habitat of all the marsupials, a See also:suggestion abundantly confirmed by the detailed studies of Dollo and of See also:Bensley, according to which we may imagine the marsupials to have passed through (I) a former terrestrial phase, followed by (2) a primary arboreal phase—illustrated in the tree phalangers—followed by (3) 5, secondary terrestrial phase—illustrated in the kangaroos and wallabies—followed by (4) a secondary arboreal phase—illustrated in the tree kangaroos. Additional information and CommentsThere are no comments yet for this article.
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