I8S0.] ARRANGEMENT OF THE MAMMALIA. 65" 



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 anky- 

 losed 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 Erinaceus and Dasi/pus they are almost Mono- 

 treme-like. 



1 1 . The foetus 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. Lepus) 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 Pro- 

 totheria raid the Metatheria. Hence we may term forms which 

 have reached this stage the Eutheria. 



It is a fact, cnriously in accordance with what might be expected 

 on evolutionary principles, that while the existing 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 Gymnura possessed a diffuse pla- 

 ceutatiou, it would be an excellent representative of an undiffer- 

 entiated 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 peculiarity 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 

 exist 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 Marsupial which has not far 

 more widely departed from the Metatherian type, than Gymnura 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 prolonging the period of intra-uterine 

 and extra-uterine nutrition by the parent present in each. The pos- 

 sibility of a higher differentiation of the species is apparently 

 closely connected with the length of this period. Similarly, the 

 broadest morphological distinction which can be drawn among the 

 Eutheria lies in their placentation. All forms of deciduate placen- 

 tation commence by being non-deciduate ; and the intimate connexion 



