658 prof/huxley on the [Dec. 14, 



of the foetal with the maternal structures is subsequent to their loose 

 union. Hence Evtheria, with decidunte placental are in a higher 

 stage of evokUion than those with non-deciduate placentae. 



In discussing the relations of the various existing groups of the 

 higher Mammaha with one another, it would he a mistake to attempt 

 to trace any direct genetic connexion between them. Each, as the 

 case of the Equidfe suggests, has probably had a pecuHar Hue of 

 ancestry; and, in these lines, Eutherian forms with decidnate pla- 

 centation constitute the htest term, Eutherian forms with non- 

 deciduate placentation the next latest, Metatherian forms the next, 

 Prototherian forms the earliest among those animals which, ac- 

 cording to existing definition, would be regarded as Mammals. 



The accompanying Table (p. G.o9) 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 form.er existence of 

 which is dcducible from the law of evolution. 



I venture to express a confident expectation that investigation 

 into the Mammalian fauna of the ^lesozoic 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 

 Prototheria 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 

 Fromammale). 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 palaeo- 

 zoic 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 ]\Iammalia is as clearly indicated by 

 their visceral arches and clefts, as the existence of complete clavicles 

 in the ancestral Canidte is indicated by their vestiges in the dog — 

 the Hypotheria, thus reduced, at once take their place among the Am- 

 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 



