Similarly, there is no correlation in the rate of evolution either of adjoining or of separated parts; the middle digit of the foot of the three-toed horse is accelerated in development, while the lateral digits on either side are retarded. Many examples might be cited among invertebrates also.
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Fig. 11.—Diagram demonstrating that there are an indefinite number of combinations of various adaptive types of limbs and feet with various adaptive types of teeth, and that there is no fixed law of correlation between the two series of adaptations. |
All these principles are consistent with Francis Galton’s law of particulate inheritance in heredity, and with the modern doctrine of “unity of characters” held by students of Mendelian phenomena.
Sudden versus Gradual Evolution of Parts.—There is a broad and most interesting analogy between the evolution of parts of animals and of groups of animals studied as a whole. Thus we observe persistent organs and persistent types of animals, analogous organs and analogous types of animals, and this analogy applies still further to the rival and more or less contradictory hypotheses of the sudden as distinguished from the gradual 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 Geoffroy St Hilaire, who suggested saltatory evolution through the direct action of the environment on development, as explaining the abrupt transitions in the Mesozoic Crocodilia and the origin of the birds from the reptiles.
Waagen’s law of mutation, or the appearance of new parts or organs so gradually that they can be perceived only by following them through successive geologic 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 par excellence of this branch of zoology to the law of evolution, since it is obvious that it could not possibly have been deduced from comparison of living animals but only through the long 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 internal hereditary principle controlling such orthogenetic evolution, there does not appear to be any essential 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 definition discontinuous or saltatory (Osborn, 1907).
VII.—Modes of Change in Animals as a Whole or in Groups of Animals, and Methods of Analysing Them.
1. Origin from Primitive or Stem Forms.—As already observed, the same principles apply to groups of animals as to organs and groups of organs; an organ originates in a primitive and unspecialized stage, a group of animals originates in a primitive or stem form. It was early perceived by Huxley, Cope and many others that Cuvier’s broad belief in a universal progression was erroneous, and there developed the distinction between “persistent primitive types” (Huxley) and “progressive types.” The theoretical existence of primitive or stem forms was clearly perceived by Darwin, but the steps by which the stem form might be restored were first clearly enunciated by Huxley in 1880 (“On the Application of Evolution to the Arrangement of the Vertebrata and more particularly of the Mammalia,” Scient. Mem. iv. 457) namely, by sharp separation of the primary or stem characters from the secondary or adaptive characters in all the known descendants or branches of a theoretical original form. The sum of the primitive characters approximately restores the primitive form; and the gaps in palaeontological evidence are supplied by analysis of the available zoological, embryological and anatomical evidence. Thus Huxley, with true prophetic 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 research. On the other 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 discovery, in 1873, of the generalized type Phenacodus of northern Wyoming. Similar anticipations and verifications among the invertebrates have been made by Hyatt, Beecher, Jackson and others.
In certain cases the character stem forms actually survive in unspecialized types. Thus the analysis of George Baur of the ancestral form of the lizards, mosasaurs, dinosaurs, crocodiles and phytosaurs led both to the generalized Palaeohatteria of the Permian and indirectly to the surviving Tuatera lizard of New Zealand.
2. Adaptations to Alternations of Habitat. Law of Irreversibility of Evolution.—In the long vicissitudes of time and procession of continental changes, animals have been subjected to alternations of habitat either through their own migrations or through the “migration of the environment itself,” to employ Van den Broeck’s epigrammatic description of the profound and sometimes sudden environmental changes which may take place in a single locality. The traces of alternations of adaptations corresponding to these alternations of habitat are recorded both in palaeontology and anatomy, although often after the obscure analogy of the earlier and later writings of a palimpsest. Huxley in 1880 briefly suggested the arboreal origin, or primordial tree-habitat of all the marsupials, a suggestion abundantly confirmed by the detailed studies of Dollo and of Bensley, according to which we may imagine the marsupials to have passed through (1) a former terrestrial phase, followed by (2) a primary arboreal phase—illustrated in the tree phalangers—followed by (3) a secondary terrestrial phase—illustrated in the kangaroos and wallabies—followed by (4) a secondary arboreal phase—illustrated in the tree kangaroos. Louis Dollo especially has
