INTRODUCTION. 
remote although sub-species of varying degrees of distinctness are 
usually developed on each of the large land-masses. On the whole 
the avifauna of Java is the most distinct, indicating the early separa- 
tion of that island. The birds of Sumatra and the Malay Peninsula 
are very similar although each area has some characteristic forms, but 
more specialization is apparent in Borneo. 
Java shares with the Indochinese sub-region, but not with the 
remainder of Malaysia, a small group of species. This is usually 
explained by a theory which postulates a series of very complicated 
changes in land-surfaces for which, as in the case of some much 
simpler changes supposed by the zoo-geographer to have taken place 
in Malaysia, no geological evidence seems yet to have been brought 
forward.’ In this connection, however, another hypothesis demands 
consideration in which land-levels and connections are only of second- 
ary importance. There is a tendency for many migratory birds to 
establish themselves, and to become differentiated as resident races in 
the southern parts, In many cases, on the absolute southern periphery, 
of their range. 
In the case of tropical breeding races of Charadrius alexandrinus 
and Acrocephalus orientalis the range of the resident form is still 
invaded, in season, by the migratory subspecies. Among the passeres 
we sometimes find resident races in Sumatra, Borneo, aud Java, or 
some combination within this group, with migratory forms of the 
same species only ranging as far south as the Malay Peninsula. Here 
it is important to note that the present southern limit of the migrants 
varies with the species. These migrants ( eg . Oriohis, Dicrurus) seem 
to dislike the water-crossing and there is a suggestion in ail these 
species that a migratory habit, having established southern, differen- 
tiated groups, is dying out. 
* * * * 
In the systematic part of this book the Malaysian localities for 
the various species are arranged in four rows corresponding to four 
Provinces, Malayan, Sumatran, Javan and Bornean, and although this 
arrangement is largely natural, that is to say, in close agreement with 
both zoological and geographical facts it is also much tempered by a 
desire for a simple arrangement, and the four provinces are certainly 
not equal in value. Some of the small islands in particular require 
further study. 
1 Visual evidence of minor coastal changes in Malaysia to-day suggests 
an interesting point in connection with the great changes in land-surface 
that must have taken place in order to produce the erratic distribution of 
many species. Separation of such land-masses as the Malay Peninsula and 
Sumatra was probably preceded by a long period during which the inter- 
vening ground was a barrier to some species and not to others. For 
instance, an extensive mangrove swamp in the south of the Straits of 
Malacca would prevent the passage of leopards but not of monkeys. 
vit 
