420 TRANSACTIONS OF SECTION D. 



From time to time one investigator after another has fixed his attention on 

 some detail in mammalian anatomy, and magnified it into an insuperable obstruc- 

 tion against the possibility of a reptilian derivation. The occipital condyles, 

 the mesenteric vessels, the epiglottis, the mode of development of the heart, the 

 nature of the skin and its sense organs, the auditory ossicles, and the early 

 phases of the Eutherian blastocyst; these features, among others, have been used 

 time after time as arguments for an Amphibian, in opposition to a Reptilian, 

 ancestry for mammals. 



The ai'gument from the blastocyst has been utterly demolished by Professor 

 J. P. Hill's recently published researches on the early embryology of Dasyurvs ; 

 or, perhaps, it would be more accurate to say he has turned the tables upon 

 those who have been insisting upon the supreme importance of embryological 

 evidence by demonstrating the thoroughly sauropsidan derivation of the mam- 

 malian mode of blastocyst-formation. 



Paleontologists — Osborn, Broom, and others — have shown how the bicondylar 

 arrangement of the Amphibian occipital bone has persisted in many extinct 

 reptiles and especially in that particular group of Theriodonts which present 

 such a remarkable series of mammalian resemblances in their skeletons. 



The comparative anatomy of the brain in various Vertebrate groups affords 

 positive evidence that in the course of its evolution the cerebral cortex passed 

 through a particular stage, which is not met with except in the Sauropsida. The 

 process of differentiation of the mammalian hippocampal formation becomes 

 intelligible only when the preparatory stages represented in the Reptilian brain 

 are known. The Amphibian brain, on the other hand, so far from helping us 

 to understand the mammalian cortex, is a source of confusion, because its cor- 

 tical formation has become so specialised, or perhaps so degenerate, in comparison 

 with its forerunner, as witnessed in the Dipnoi, or its successor, as witnessed in 

 the Reptilia, that we must regard it as being off the path that led to the 

 Mammalia. 



In spite of the certainty that the mammalian brain passed through a reptilian 

 stage in its phylogeny, the brain of no living reptile fulfils the conditions 

 required in the actual ancestor of the Protomammalia. Each is diversely 

 specialised in some way. The brain of Sphenoclon represents a curious blend- 

 ing of primitive features with Lacertilian and Chelonian characteristics; but it 

 inclines too decidedly to the Lacertilian phylum to afford a type of the ancestral 

 reptilian brain. 



Here, however, the palaeontologists come to our aid, not in giving us any 

 further information regarding the brain, but in indicating an extinct group of 

 early reptiles, which had not undergone those specialisations that gave birth to 

 to Rhyncocephalia. Lacertilia, Chelonia, &c, but retained a more generalised 

 structure, and at the same time have developed traits definitely foreshadowing 

 the Mammalia. 



The general nature of this evidence as collected and set forth by Owen, 

 Peeley, Osborn, and others has long been known ; but the recent work of Broom 

 and D. M. P. Watson (of Manchester) has given us a much more intimate 

 acquaintance with the structure of the Triassic Cynodontia (or the large group 

 including it, which Broom has called ' Therapsida ') of South Africa; and it is 

 now difficult to resist accepting the obvious significance of their observations. 

 The Therapsida present a curious blend of primitive reptilian and primitive 

 mammalian features, many characters of the skull of the Rhyncocephalia, of the 

 Polyprotodont Marsupials, and of the Insectivora being reproduced with sur- 

 prising exactitude ; and in the limbs Prototherian peculiarities are often closely 

 reproduced, or rather foreshadowed. 



Comparative anatomy and embryology point to a primitive reptilian as the 

 parent of the Prototherian phylum. It is unnecessary for us to discuss a hypo- 

 thetical group in this search for mammalian origins. For even if we have not 

 found the actual ancestor, the group of extinct Cynodonts provides us with so 

 many forms presenting mammalian characters of skull and teeth, and limbs and 

 trunk, that it is no longer possible to refuse to recognise them as the representa- 

 tives of the order to which the ancestor of the Prototheria belonged. 



The impossibility pf deriving either Reptiles or Mammals from the true 

 Amphibia. 



In the course of its evolution from the Dipnoan stage the Amphibian brain 



