201 BR. B. A. BEXSLEY ON THE EVOLUTION 



These relations point to the origin of the Phascolomyidse and Diprotodontidse from a 

 common stem-form, which is in turn traceable to the Phalangerinse. The absence of 

 annectant types makes it impossible to fix the affinities of the two families with any 

 existing pliaiangerine genera. 



The Question of the Time and Mode of Origin of the Australian Radiation. 



Partial reference has already been made to the views of different writers as to the 

 time and conditions under which the Australian fauna originated in dealing with the 

 identification of the stem-form. 



With regard to the direction from Avhich the ancestral forms entered Australia there 

 appear to be only two possibilities — that they came either from the North, as supposed 

 byWallace (1875) and Lydekker (1896), or from South America, as thought by Ameghino 

 (1891) and Spencer (1896). 



According to Lydekker's view the Dasyuridse and Didelphyida? may have originated 

 from common ancestors inhabiting South-eastern Asia, from which region the former 

 family migrated into Australia, while the latter one dispersed in two directions into 

 Europe and North America. In advocating a South-American origin of the family 

 Spencer takes exceptioa to this view on the general grounds of the paucity of pol^ 

 protodont types in New Guinea, through which the ancestral forms would have pas^ ^^ 

 the difficulty of explaining the absence of Didelphyidte in Australia, and the la 

 evidence concerning the former existence of Marsupials in Asia. aestion 



With reference to the first of these objections, it may be pointed out that the 'g^j^g^ 

 does not concern so much the relative numbers of polyprotodont forms in Ne^jj^^^.^^^^ 

 and Australia, since, in view of the greater area and greater possibilities of dis^^ ^j^^^ .^ 

 difi'erentiation of the latter region, we may expect to find, as we ^^tualh'' ' ^^.^^ .^ 

 contains a greatly predominating portion of the polyprotodont fauna. T' ^^^ ^_^.^,j^ 

 rather whether or not New Guinea contains prototypal forms. This/^^ addition to 

 answered in tlie affirmative. On referring to the Dasyuridse we find ^^y j'/wrbeckiaua 

 the speciaUzed forms Fhascogale IJorits, P. dorsalis, P. Wallacei, and ^,^ occurs there as 

 which are confined to the Papuan region, the prototypal form P. flcm^ Peramelidce all 

 well as in the eastern and south-eastern parts of Australia. Of ^j^^.^^ genera of the 

 the species presenting prototypal characters are Papuan. Of tlj-;,p^a^ ^.^^^^^3^^^^.^^ 

 Phalangerinse which present primitive characters, Dromicia has on^^^^^^^^^ ^^ ^j^^^ reo^ion. 

 {D. caudata), Acrobates one {A. pulchellus), while Dlstcechurus i^ ^^^ Guinea, and^his 

 It is true that migrations may have taken place from Australiy^g^ ^^^^^ ^^ evidence of 

 factor should be kept in mind as bearing on the question, uL ^ ^^^.^^-^ ^^^^^^ ^^ ^^^^ 

 such migration in the distribution of Fhalanger, Dorcopsis, au/ 



Small Wallabies. /rning the former presence 



Spencer's further objection as to the lack of evidence ^od ^j^^ ij^^ications of faunal 



of Marsupials in Asia is, of course, unanswerable. We Ji'-^v^ii^termigration over what is 



community between Asia and North America, poir*^ifig) 



