]S0 DR. B. A. BENSLEY 0^^ THE EVOLUTION 



Such a generalized type while not represented in the Australian group is, except for a 

 more primitive condition of the lower incisor formula, exactly represented hy the 

 American Didelphyidae, and the present section is devoted to a consideration of the 

 A'arious members of this family both in their relations to the Australian fauna and to 

 one another. 



Reviewing the opinions already expressed with respect to the relationships of the 

 -Vustraiian fauna, we tind them to be of the most diverse kiad. Writing in 1871 Owen 

 remarked: — "Among these initial forms of Marsupialia [referring to the Mesozoic 

 Mammalia] we may see in Amphitherium the prototype of Mynnecobius ; Peralestes has 

 culminated in Sm'cophilus ; Triconodon in Thylacinus ; Plagiaulax is to Thylacoleo 

 ^^'hat the Weasel is to the Lion. But derivative change has not advanced to the 

 long-limbed saltatory type of Marsupial ; nor has any evidence yet been had of a 

 Mesozoic predecessor of the climbing Koala, the volant Petaurist, or the burrowing 

 Wombat. 



" The Marsupial type . . . has in America progressed to and been succeeded by the 

 more specialized form of Didelphys. 



" If Australia possessed Marsupials as far back in time as did America and Europe, 

 analogy would lead us to suppose that the primitive diminutive multimolar insectivorous 

 type prevailed. It has not there yet become extinct ; but it seems to have been reduced 

 to the solitary exceptional form of Ilyrmecoblus." 



Wallace, in his ' Geographical Distribution of Animals ' (1876, v. p. 2), has expressed a 

 similar view of a connection of the Australian fauna with northern Mesozoic forms : — "As, 

 liowever, no other form but that of the Didelphyidoe occurs there [in Europe] during 

 the Tertiary period, we must suppose that it was at a far more remote epoch that the 

 ancestral forms of all the other marsupials entered Australia ; and the curious little 

 mammals of the Oolite and Trias offer valuable indications as to the time when this 

 really took place. ... It was probably far back in the Secondary period that some portion 

 of the Australian region was in actual connection with the northern continent, and 

 became stocked with the ancestral forms of marsupials." 



To the same order belong the views of Falconer and others who have sought to 

 establish a relationship between the Australian diprotodont forms and the Plagiaulacidae, 

 and that cf Cope (1882, 1884), who actually referred the form Thylacoleo to tlie family 

 Plagiaulacidse, at the same time connecting the family with the Macropodidae through a 

 hypothetical ancestor Trltomodon. 



The ' Systematische Phylogenie ' of Haeckel (1895) contains much more definite views 

 of a connection of the Mesozoic Mammalia A\ith existing Marsupials. This writer 

 recognizes a Jurassic group of Prodidelphia giving rise to the Marsupials along poly- 

 protodont and diprotodont Hues. 



Apparently the only writers who have anticipated what is probably the true relationship 

 of the Australian fauna are Winge (1893) and Lydekker (1896), both of whom favour 

 a more direct connection Avith the existing American Didelphyida3 than with Mesozoic 

 forms. In a general scheme of the marsupial families Winge indicates three lines of 

 development — one of them represented by a group composed of Dlprotodon, Thylacoleo, 



