Jan. 6, 1.88 1] 



NATURE 



229 



great majority of the Metatheria, of which I doubt not a 

 great multitude will shortly be discovered in Mesozoic 

 formations, differed widely from our existing Marsupials ; 

 not only lacking the pouch, as do some existing " Alarsu- 

 pialia," but possessing undivided vagina^, and probably 

 bringing forth their young, not earlier than existing 

 Carnivores and Rodents do, the nutrition of the fcetus 

 during prolonged gestation being provided for, in all 

 probability, by an umbilical placental apparatus, and its 

 respiration by a non-placental allantois. 



In the remaining group of the Mammalia, hitherto 

 spoken of as the " higher Mammalia : " — 



1. The mammary glands are provided with teats." 



2. The cloaca has usually disappeared. Sometimes, 

 however (Beavers, Sloths), a shallow cloaca is present, 

 especially in the female. 



3. The openings of the ureters are always entocystic ; 

 but their position varies greatly, from close to the neck 

 (e.g. Sorex) to the anterior end of the bladder (e. g. Hyrax). 



4. There is a distinct vagina, which is almost always 

 undivided. The oviducts are difterentiated into uterine 

 and Fallopian portions. 



5. The penis is usually large, the bulb single or partially 

 divided, and the corpora cavernosa almost ahvays.directly 

 attached to the ischia. 



6. The vertebrae have epiphyses. 



7. The malleus is usually small, the incus relatively 

 large, the stapes stirrup shaped. 



8. The coracoid is almost always much reduced, and it 

 is ankylosed with the scapula. 



9. The iliac axis makes a small angle with the sacral 

 axis ; and there is no epipubis, or only a fibrous vestige 

 of it. 



10. The corpus callosum and the anterior commissure 

 vary widely. In such forms as Erinaceiis and Dasypus 

 they are almost Monotreme-like. 



1 1. The fcetus is connected with the uterus of the mother 

 by an allantoic placenta. The umbilical sac varies in 

 size, and in some lower forms (e.g. Lepiis) it is, at first, 

 highly vascular, and perhaps plays a quasi-placental part 

 during the early stages of development. 



It is obvious that, in all these respects, we have the 

 mammalian type in a higher stage of evolution than that 

 presented by the Prototheria and the Metatheria. Hence 

 we may term forms which have reached this stage the 

 EutSieria. 



It is a fact, curiously in accordance with what might be 

 expected on evolutionary principles, that while the exist- 

 ing members of the Prototheria and the Metatheria are 

 all extremely modified, there are certain forms of living 

 Eutheria which depart but little from the general type. 

 For example, if Cyininira possessed a diffuse placenta- 

 tion, it would be an excellent representative of an undif- 

 ferentiated Eutherian. Many years ago, in my lectures at 

 the Royal College of Surgeons, I particularly insisted on 

 the central position of the Insectivora among the higher 

 Mammalia ; and further study of this order and of the 

 Rodentia has only strengthened my conviction, that any 

 one who is acquainted with the range of variation of 

 structure in these groups, possesses the key to every pe- 

 culiarity which is met with in the Primates, the Carnivora, 

 and the Ungulata. Given the common plan of the 

 Insectivora and of the Rodentia, and granting that the 

 modifications of the structure of the limbs, of the brain, 

 .and of the alimentary and reproductive viscera, which 

 occur among them, may exi;t and accumulate elsewhere, 

 and the derivation of all the Eutheria from animals 

 which, except for their simpler placentation, would be 

 Insectivores, is a simple deduction from the law of 

 evolution. 



There is no known Monotreme which is not vastly 

 more different from the Prototherian type, and no Marsu- 



' The only exceplion known 10 me is ihe Cape Mole {.Chrysockloris), 

 which, according to Peters, has none. 



pial which has not far more widely departed from tte 

 Metatherian t)-pe, than Gyiniiura, or, indeed, Erinaceus, 

 have from the Eutherian type. 



The broadest physiological distinction between the 

 Prototheria, the Metatheria, and the Eutheria respectively 

 lies in the differences which the arrangements for pro- 

 longing the period of intra-uterine and extra-uterine 

 nutrition by the parent present in each. The possibility 

 of a higher differentiation of the species is apparently 

 closely connected with the length of this period. Simi- 

 larly, the broadest morphological distinction which can 

 be drawn among the Eutheria lies in their placentation. 

 All forms of deciduate placentation commence by being 

 non-deciduate, and the intimate connection of the fcetal 

 with the maternal structures is subsequent to their loose 

 union. Hence Eutheria, with deciduate placentas, are in 

 a higher stage of evolution than those with non-deciduate 

 placentje. 



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 

 connection between them. Each, as the case of the 

 Equidse suggests, has probably had a peculiar line of 

 ancestry ; and, in these lines, Eutherian forms with deci- 

 duate 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 exist- 

 ing definition, would be regarded as iVIammals. 



The accompanying Table (p. 230) 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 investi- 

 gation 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 ' e trusted much farther. If we may confidently 

 expect that Eo/iippus had a pentadactjle 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 or cartilage, of which the malleus of the 

 true mammal is the reduced representative. Probably 

 also the corpus callosum had not appeared as a distinct 

 structure. 



Cur existing classifications have no place for this "sub- 

 mammalian"' stage of evolution (already indicated by 

 Haeckel under the name of Proiiianuna/e). 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 branchiffi 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 ter- 

 restrial \'ertebrata of the later pahcozoic 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 

 Canidas is indicated by their vestiges in the dog— the 

 Hypotheria, thus reduced, at once take their place among 

 the Amphibia. For the presence of branchia; implies 

 that of an incompletely divided ventricle and of nume- 

 rous aortic arches, such as exist in the mammalian 

 embryo, but are more or less completely suppressed in 

 the course of its development. 



