TRANSACTIONS OP SECTION D. 425 



mammals what they are, as well as in supplying evidence to show whence they 

 came, has been wholly ignored. 



Evidence of teeth (Wilson and Hill) and foetal membranes (Hill) clearly 

 demonstrate that Marsupials are a degenerate stock sprung from a diphyodont, 

 placental ancestry. The evidence of the early stages of development, such as the 

 mode of development of the blastocyst and the presence of a shell-membrane 

 (Hill), and the arrangement of the hippocampal formation and cerebral commis- 

 sures and many other structural features, indicate that in most respects the 

 Marsupials have retained, in far greater measure than the Eutheria, the features 

 distinctive of the common ancestor of both groups. While recognising that all 

 living Marsupials are specialised in greater or less degree so that no one of 

 them could be looked upon as ancestral to the Eutheria, it must be admitted that 

 the more highly specialised Eutheria in the course of their phylogeny must have 

 passed through a stage not very different from that represented by the modern 

 Perameles. If we are justified in applying the term 'Metatherian ' to the hypo- 

 thetical diphyodont, generalised-limbed, diminutive creature from which Per- 

 ameles must have been derived, then we must recognise a Metatherian stage in 

 the ancestry of the Eutheria. The similarity of structure in many of the more 

 generalised Insectivora and Edentata, on the one hand, and the Polyprotodont 

 Marsupials on the other, is so close, and the fact that structural modifications 

 (especially in the brain) that are begun in the Marsupials are carried a stage 

 further in the Eutheria can be explained only by admitting (a) the intimacy of 

 the bond of kinship that links them, and (b) in recognising the ancestral Meta- 

 therian as a member of the Eutherian phylum. 



The early zoologists included the Monotremes amongst the Mammalia 

 because they had a hairy coat and mammary glands. Further research has com- 

 pleted the demonstration of their kinship to other mammals and established the 

 monophyletic derivation of the whole mammalian group. Not only is the skin 

 and its hairy and glandular epithelium typically mammalian, but also the 

 alimentary canal and liver, the diaphragm, the auditory ossicles and their mode 

 of development and the organ of Jacobson. In the brain the complex specialisa- 

 tion of the hippocampal formation and its curious fascia dentata, so peculiarly 

 distinctive of the Mammalia, is carried to a degree of differentiation at least as 

 great as in other mammals ; the characteristically mammalian neopallium is 

 present and emits a system of projection fibres forming both pyramidal and 

 cerebro-pontine groups as in other mammals. 



Hill has recently shown that the early phases of development in the Marsu- 

 pials are a secondary modification of those through which the Monotremes and 

 their reptilian ancestors pass in ontogeny. In many other features, such as the 

 shoulder girdle (Broom), laryngeal cartilages (Weber), and azygos veins (Bed- 

 dard), the ontogenetic development of certain Marsupials recapitulates that of 

 the Monotremes. 



In Perameles, the Marsupial which retains the allantoic placenta of its ances- 

 tors and many other primitive features in its structures, we find the plumpness 

 of the cephalic end of the hippocampal arc, which is lost to a greater extent in 

 most other Marsupials and all Eutheria, but is reminiscent of the Prototherian 

 condition. 



The living Monotremes are separated by a very wide gap from the closely 

 related Meta- and Eutheria. At a very early stage in the history of the Mam- 

 malia, soon after the acquisition of skin, hair, milk-glands, and the appearance 

 of the typical hippocampus and neopallium, the Prototheria divided into two 

 phyla, one of which retained its generalised features and the other specialised; 

 From the former the common Metatherian ancestors of all the Metatheria and 

 Eutheria sprang by gradual transformation : from the latter the highly differen- 

 tiated structure of the living Monotremes was derived, creatures which display 

 a very high degree of specialisation, in association with the fixation of certain 

 extremely primitive phases of mammalian structure, to display to us what the 

 primitive mammal just emerged from the reptilian stage was like. 



All mammals were sprung from an oviparous Prototherian stock, vastly 

 different from the living Monotremata, but still deserving the name Prototheria. 

 There is an overwhelming mass of evidence of varied nature, anatomical, 

 embryological, and paheontological, to prove that the mammalian phylum sprang 

 from the Reptilia. 



