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Show 658 PROF.'HUXLEY O N T H E [Dec. 14, of the fcetal with the maternal structures is subsequent to their loose union. Hence Eutheria, with deciduate placentae, are in a higher stage of evolution than those with non-deciduate placentae. In discussing the relations of the various existing groups of the higher Mammalia with one another, it would be a mistake to attempt to trace any direct genetic connexion between them. Each, as the case of the Equidae suggests, has probably had a peculiar line of ancestry; and, in these lines, Eutherian forms with deciduate placentation constitute the latest term, Eutherian forms with non-deciduate placentation the next latest, Metatherian forms the next, Prototherian forms the earliest among those animals which, according to existing definition, would be regarded as Mammals. The accompanying Table (p. 659) presents, at a glance, the arrangement of the Mammalia in accordance with the views which I have endeavoured to express. The sign O marks the places on the scheme which are occupied by known Mammals ; while X indicates the groups of which nothing is known, but the former existence of which is deducible from the law of evolution. I venture to express a confident expectation that investigation into the Mammalian fauna of the Mesozoic epoch will sooner or later fill up these blanks. But if deduction from the law of evolution is to be justified thus far, it may be trusted much further. If we may confidently expect that Eohippus had a pentadactyle claviculate ancestor, then we may expect, with no less confidence, that the Pro totheria proceeded from ancestors which were not mammals-in so far as they had no mammary glands, and in so far as the mandible was articulated with a quadrate bone, of which the malleus of the true mammal is the reduced representative. Probably also the corpus callosum had not appeared as a distinct structure. Our existing classifications have no place for this " submammalian" stage of evolution (already indicated by Haeckel under the name of Promammale). It would be separated from the Sauropsida by its two condyles, and by the retention of the left as the principal aortic arch; while it would probably be no less differentiated from the Amphibia by the presence of an amnion and the absence of branchiae at any period of life. I propose to term the representatives of this stage Hypotheria ; and I do not doubt that when we have a fuller knowledge of the terrestrial Vertebrata of the later palaeozoic epochs, forms belonging to this stage will be found among them. Now, if we take away from the Hypotheria the amnion and the corpus callosum, and add the functional branchiae-the existence of which in the ancestors of the Mammalia is as clearly indicated by their visceral arches and clefts, as the existence of complete clavicles in the ancestral Canidae is indicated by their vestiges in the dog- the Hypotheria, thus reduced, at once take their place among the A m phibia. For the presence of branchiae implies that of an incompletely divided ventricle and of numerous aortic arches, such as exist in the mammalian embryo, but are more or less completely suppressed in the course of its development. Thus I regard the Amphibian type as the representative of the |