318 GEOGRAPHICAL DISTRIBUTION 
commonly called Polynesia, avoiding, however, the northern outliers 
of the New-Zealand Region, and so return to encompass Australia 
proper and Tasmania. 
Though the characteristic Mammals of the Australian Region 
are highly remarkable, comprehending as they do the whole of the 
most peculiar (Ornithodelphia) of the three Subclasses and nearly all 
of a second (Didelphia), by far the largest portion of the area it covers 
is weak if not wanting in Mammalian life, and its zoological features 
are nearly as well exhibited by its Birds. Nor can mention be here 
omitted of the remarkable Ganoid Fish, Ceratodus, a genus which 
has come down to us unaltered from Mesozoic times,—all facts 
serving to shew that the isolation of Australia is probably the next 
oldest in the world to that of New Zealand, having possibly 
existed since the time when no Mammals higher than Marsupials 
had appeared on the face of the earth.t 
The prevalent zoological features of any Region are of two kinds 
—negative and positive. It is therefore just as much the business 
of the zoogeographer to ascertain what groups of animals are wanting 
in any particular locality (altogether independently of its extent) as 
to determine those which are forthcoming there. Of course, in 
the former case, it would be idle to regard as a valuable physical 
feature of a district the absence of groups which do not occur except 
in its immediate neighbourhood ; but when we find that certain 
groups, though abounding in some part of the vicinity, either 
suddenly cease from appearing or appear only in very reduced 
numbers, and occasionally in abnormal forms, the fact obviously has 
an important bearing. Now, as has been above stated, mere 
geographical considerations, taken from the situation and configura- 
tion of the islands of the Indian or Malay Archipelago, would 
indicate that they extended in an unbroken series from the Strait of 
Malacca to New Guinea, or even further to the eastward. Indeed, 
the very name Australasia, often applied to this part of the world, 
would induce the belief that all those countless islands were but a 
southern prolongation of the mainland of Asia—broken up, it is true. 
But so far from this being the case a very definite barrier is inter- 
posed. A strait, some 15 or 20 miles wide, dividing, as just stated, 
the two otherwise insignificant islands (Bali and Lombok), makes 
such a frontier as can hardly be shewn to exist elsewhere. The 
former belongs to the Indian Region, the latter to the Australian, 
and between them there is absolutely no true transition—that is, no 
species are common to both which cannot be easily accounted for 
by the various accidents and migrations that in the course of time 
1 It will be borne in mind that fossil remains shew that Marsupials once in- 
habited Europe. They are now restricted to Australia with the exception of one 
group, which inhabits the Neotropical Region, a single species ranging also over 
the temperate parts of North America. 
