ORIGIN OF DISTRIBUTION 
There is this to be said, however, at this point: It is a possibility that 
this whole matter of the spread of primitive marsupials (and of some other 
of the older types) has been viewed ‘‘ wrong end to.” Vari- antarctic 
ous facts and indications point to the probability that the Origin? 
Southern Hemisphere received its animal life first from original centers of 
birth and distribution in the South Polar region, then mild and fruitful 
in climate. If this were so, then the adjacent Australian country would 
have become populous first, and the spread of marsupials would have 
been onward into Asia and Europe, until the more distant were cut off by 
the water gap opened early in the Cretaceous era along Wallace’s Line; 
and the early disappearance of this race from the mainland would be 
more readily understood than on the supposition that it originated there. 
Such a hypothesis (direct evidence of which must always remain hidden for 
the most part under the antarctic ice) would also explain the curious fact 
that the only marsupials now living elsewhere than in Australasia are in 
South America, one species, our opossum, ranging north to the central 
United States. Fossil remains of primitive marsupials abound in the early 
Tertiary rocks of Patagonia, and by the time of the Oligocene opossums 
had spread over all North America and Europe. The notable faunal and 
floral resemblance in general beween Australia and South America, in- 
creasing as it is traced backward in geologic history, can be explained only 
by some sort of former land connection now submerged, presumably ant- 
arctic. The great objection to this hypothesis is a negative one; namely, 
that thus far no fossil forms of Mesozoic age have been found in Australia. 
It is in suggesting such large thoughts as this that the marsupials 
become interesting, rather than in what they nowadays are or do. 
The marsupials present another engaging aspect in show- 
ing themselves to be an epitome of the whole mammalian 
world, and so illustrating on a large scale, and in Adapta- 
the most conspicuous way, how the necessity and  "°"™* 
habit of making a living in a particular manner brings about 
a suitable modification of structure, by such methods of adap- 
tation as Morgan *° has elucidated. In Australia these ani- 
mals found themselves confined in a comparatively small, 
sharply limited space, including both plains and forests, with 
a fair uniformity of climate and products. There was an 
abundance of insect, reptilian, and bird life; but no other mam- 
493 
