Evolution of Animals 495 



formed the primitive group of small mammals most of which, 

 like some of the present day, varied in size from a small mouse 

 to a rat. In line with such evidence as Matthew has brought 

 forward (161: 811), some of these took to an arboreal existence, 

 but as indicated later they modified along many lines of response 

 to varied environment. The entire group however seems to 

 have represented amphibian derivatives of permian age, whose 

 remains are almost wholly unknown to us, at least as yet. 



During late permian or early triassic times an extensive 

 land connection seems to have formed from Central and South 

 America across the southern hemisphere to Australia, and 

 which permitted eastward migration of many plant genera 

 like Araucaria, Myrsiphyllum, and others. Simultaneously 

 also probably, specializing polyprotodonts, some of which 

 were developing diprotodont habits, spread into Australia 

 and there became an isolated zoological island from the time 

 of the later cretaceous and early eocene. But connections of 

 the southern land, probably by Central x\merica with northern 

 Africa and southern Europe, permitted Coenolestes and the 

 opossums to work north and again east, so that we have fossil 

 remains of a myrmecobius-like form in southwestern Europe, 

 and the living representative in western Australia; while the 

 eocene and lower miocene Peratherium or Didelphys of Europe 

 tells how widespread were and are the opossums. 



The marsupial migrants from America and outside parts 

 to Australia were evidently cut off from contact with the extra- 

 australian area in late cretaceous or early eocene times, and 

 so alongside more primitive polyprotodonts that have largely 

 retained primitive characters, like Myrmecohius, Peragale, and 

 Phascologale, others have evolved by condensation and speciali- 

 zation into diprotodont groups. For apart from Coenolestes 

 of Equador, and the extinct Epanorthida of Patagonia that 

 represent a group intermediate between the polyprotodonts 

 and diprotodonts, the latter seem largely, if not wholly, to 

 have evolved in the australian area. 



In addition to specializing characters already given for the 

 latter, it might also shortly be indicated that the syndactyly 



