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Show 1880.] ARRANGEMENT OF THE MAMMALIA. 651 vvould have to be placed in one order of the class Mammalia and their descendants in another. It may be suggested that it might be as well to wait until the primordial Hippoid is discovered before discussing the difficulties which will be created by its appearance. But the truth is, that the problem is already pressing in another shape. Numerous " Lemurs," with marked ungulate characters, are being discovered in the older Tertiaries of the United States and elsewhere ; and no one can study the more ancient mammals with which we are already acquainted, without being constantly struck with the " Insectivorous " characters which they present. In fact, there is nothing in the dentition of either Primates, Carnivores, or Ungulates which is not foreshadowed in the Insectivora; and I am not aware that there is any means of deciding whether a given fossil skeleton, with skull, teeth, and limbs almost complete, ought to be ranged with the Lemurs, the Insecti-vores, the Carnivores, or the Ungulates. In whatever order of Mammals a sufficiently long series of forms has come to light, they illustrate the threefold law of evolution as clearly, though perhaps not so strikingly, as the Equine series does. Carnivores, Artiodactyles, and Perissodactyles all tend, as we trace them back through the Tertiary epoch, towards less modified forms which will fit into none of the recognized orders, but come closer to the Insectivora than to any other. It would, however, be most inconvenient and misleading to term these primordial forms " Insectivora," the mammals so called being themselves more or less specialized modifications of the same common type ; and only, in a partial and limited sense, representatives of that type. The root of the matter appears to me to be that the palaeontological facts which have come to light in the course of the last ten or fifteen years have completely broken down existing taxonomical conceptions, and that attempts to construct fresh classifications upon the old model are necessarily futile. The Cuvierian method, which most modern classifiers up to the time of the appearance of Haeckel's ' Generelle Morphologie ' have followed, has been of immense value in leading to the close investigation and the clear statement of the anatomical characters of animals. But its principle, the construction of sharp logical categories defined by such characters, was sapped when Von Baer showed that, in estimating the likenesses and unlikenesses of animals, development must be fully taken into account; and if the importance of individual development is admitted, that of ancestral development necessarily follows. If the end of all zoological classification is the clear and concise expression of the morphological resemblances and differences of animals, then all such resemblances must have a taxonomic value. But they fall under three heads:-(1) those of adult individuals; (2) those of successive stages of embryological development or individual evolution; (3) those of successive stages of the evolution of the species, or ancestral evolution. An arrangement is " natural" (that is, logically justifiable in view 43* |