46 
COMPENDIUM OF GEOGRAPHY AND TRAVEL 
account for the absence of so many important groups of 
mammalia. Two explanations are open to us. Either 
the former union with Borneo, and, perhaps, with For¬ 
mosa, was limited in extent and of short duration, so 
that only a few mammalian types ever entered the 
country; or, the union having been more complete and 
of sufficient duration, the islands became well stocked 
with mammals, but a great amount of subsidence has 
since so reduced the land area and altered the physical 
conditions, that numbers of them, especially those of the 
largest size, have become extinct. This latter hypothesis 
is supported by the fact, that almost everywhere are found 
lame tracts of elevated coral reefs containing* shells simi- 
o o 
lar to those now living in the adjacent seas—proving that 
at a comparatively recent period the islands have been 
partially submerged, and therefore less extensive than 
they are now. Mr. Bolfe, too, considers the present flora 
to have been differentiated when the islands were much 
more immersed than is now the case. We know that 
all volcanic countries are subject to elevations and sub¬ 
sidences, and it is highly probable that so pre-eminently 
volcanic a district as the Philippines has been repeatedly 
subject to partial elevations and depressions ; at one time 
effecting a union with adjacent lands, and thus favouring 
the introduction of new animals, at another submerging 
extensive areas, and thus leading to the extermination of 
many forms of life. Changes of this kind, if continued 
through the latter portion of the Tertiary period, would 
inevitably produce such a limited yet peculiar fauna as is 
now found to characterise these islands. Minute geo¬ 
logical investigation, combined with a more complete 
knowledge of the existing fauna and flora, will alone 
enable us to determine how far these suppositions are 
correct. There is not much doubt, however, that we have 
