TRANSACTIONS OF SECTION D. 623 



Our conceptions of the interrelationsliips of the Maraupialia and Placentalia 

 have during the period we are considering been delimited beyond expectation, by 

 the discovery that an allantoic placenta in a polyprotodont marsupial, replaces the 

 vitelline, present in its allies.^* When it is remembered that in the formation of the 

 placenta of the rabbit, bat, and hedgehog, there is a provisional vitelline stage,^'-' it 

 is tempting to suggest that the evidence for the direct relationship of the two 

 mammalian sub-classes first named overlaps (there being a placental marsupial 

 on one hand, a marsupial placental on the other), much as we have come to 

 regard Archseopteryx as an avian reptile, the Odontornithes as reptilian birds. 

 These facts, moreover, prove that the type of placenta inherited by the Placentalia 

 must have been discoidal, and that from that all others were derived. _ 



Equally important concerning our knowledge of the Marsupialia is the 

 discovery, first made clear by Professor Symington, of this College, that Owen wa3 

 correct in denying them a corpus callosum.^" How Owen arrived at this conclusion 

 it is difiicult to conceive; but in these later days the history of discovery is 

 largely that of method ; and it is by the employment of chrome-silver, methylene- 

 blue, and other reagents, which in diflerentiating the fibre-tracts enable us to 

 delimit their course, that this conclusion has been proved. By the corpus 

 callosum we now understand a series of neo-pallial fibres which transect the 

 alveus and are present only in the Placentalia.'*^ 



There is no department of mammalogy in which recent work has been more 

 luminous than this which concerns the brain ; and, to mention but one result, 

 it may be said that in the renewed study of the commissures there has been 

 found a fibre-tract characteristic of the Diprotodontia alone, so situated as to 

 prove that they and the Placentalia must liave specialised on diverse lines from a 

 polyprotodont stock.'- Interesting this, the more, since the phalangers and 

 kangaroos are known to be polyprotodont when young.^' And when we add the 

 discovery that in the form of its hippocampal commissure the brain of the Elephant 

 Shrew, a lowly insectivore, alone among that of all Placentalia known realises the 

 marsupial state,** as does its accessory organ of smell, we have to admit the existence 

 of annectant conditions just where they should occur.** 



The morphological method is sound ! 



The master hand which has given us this result has also reinvestigated the 

 Lemurs. From an exhaustive study of the brain or its cast of all species of the 

 order, living and extinct, there has come the proof that the distinctive characters 

 of the lemuroid brain are intelligible only on a knowledge of the pithecoid type ; 

 that its structural simplicity in the so-called lower lemurs is due to retrogressive 

 change, in some species proved to be ontogenetic ; and that the Tarsier, I'ecently 

 claimed to be an insectivore, is a lemur of lemurs.*'^ It is impossible to over- 

 estimate the importance of this conclusion, which receives confirmation in recent 

 palaeontological work ; *' and there is demanded a reinvestigation of those early 

 described Tertiary fossil forms placed on the Ungulo-lemuroid border line, as also 

 a reconsideration of current views on the evolution of the primates and of man.*^ 



In dismissing the Mammalia, we recall the capture during the period we review 

 of three new genera, a fourth, the so-called Neomylodon,^^ having proved by its 

 skull to be Grypotherium darwinii, already known.^*' The African Okapi, an 

 object of sensation beyond its deserts, has found its place at last. To have been 

 dubbed a donkey, a zebra, and a primitive hornless giraffe, is distinction indeed ; 

 and we cannot refrain from contrasting the nonsensical statement that its 

 discovery is ' the most important since Archseopteryx ' with the truth that it is a 

 giraffine, horned in the male, annectant between two groups well known.'"' As a 

 discovery it does not compare with that of the Mole-marsupial,^- and it falls into 

 insignificance beside that of the South American diprotodont Coenolestes, the 

 survivor of a family which there flourished in Middle Tertiary times.''^ 



Passing to Birds and Reptiles, it will be convenient to consider them together. 

 A knowledge of their anatomy has extended on all hands, and in respect to nothing 

 more instructively than their organs of respiration. Surprise must be expressed 

 at the discovery, in the chelonian, of a mode of advancing complication of the lung 

 suggestive of that of birds. On looking into this, I find that Huxley, who 



