OUTSTANDING FACTS. 803 
MORE DETAILED PROBLEMS OF GEOGRAPHICAL 
DisTRIBUTION 
Leaving the general, and at present very obscure, problem 
of the evolution of faunas, let us briefly notice some of the 
more detailed questions of distribution. We shall content 
ourselves with stating (1) a few of the outstanding facts ; 
(2) the factors determining why some animals are here and 
others there ; and (3) the usually recognised zoo-geographical 
regions. 
Outstanding facts.—(a) Widely separated countries may 
' have an essentially similar fauna. Thus, there is much in 
common between Britain and Northern Japan, and there 
is so much agreement between the North European (Pale- 
arctic) and the North American (Nearctic) fauna, that.many 
unite the two regions in one (Holarctic). 
_ (8) Closely adjacent countries may have quite different 
faunas. Thus the Bahamas and Florida, Australia and 
New Zealand, are peopled by very different animals. Two 
little islands, Bali and Lombok, in the Malay Archipelago, 
which are separated by ‘‘Wallace’s Line,” a strait only 
fifteen miles wide at its narrowest part, differ from each 
other in their birds and quadrupeds more widely than 
Britain and Japan. 
(c) Regions with very different faunas are in many cases 
connected by transition areas. Thus a journey from the 
North of. Canada to Brazil would show a fairly gradual 
transition from an Arctic to a tropical fauna. 
(d) At the same time there are regions whose fauna is 
exceedingly distinctive and sharply defined. Thus the 
Mammalian fauna of Australia is distinctively Marsupial, 
and nowadays the American opossums and Canolestes are 
the only Marsupials beyond the Australasian limits. 
(e) Another striking fact is the “discontinuous distri- 
bution ” of certain types, by which we mean that examples 
of a type may occur in widely separated regions without 
there being any representatives in the intermediate area. 
The general explanation is that the type in question once 
enjoyed a wide distribution, as the rock record shows, and 
that the conditions favourable to survival have been found 
in widely separated places. Thus of the genus Tapir 
