he absence of terrestrial mammals in relation to the remoteness
of islands from continents, there is also a relation, to a certain
extent independent of distance, between the depth of the sea separating
an island from the neighbouring mainland, and the presence in both of
the same mammiferous species or of allied species in a more or less
modified condition. Mr. Windsor Earl has made some striking observations
on this head in regard to the great Malay Archipelago, which is
traversed near Celebes by a space of deep ocean; and this space
separates two widely distinct mammalian faunas. On either side the
islands are situated on moderately deep submarine banks, and they are
inhabited by closely allied or identical quadrupeds. No doubt some few
anomalies occur in this great archipelago, and there is much difficulty
in forming a judgment in some cases owing to the probable naturalisation
of certain mammals through man's agency; but we shall soon have much
light thrown on the natural history of this archipelago by the admirable
zeal and researches of Mr. Wallace. I have not as yet had time to follow
up this subject in all other quarters of the world; but as far as I have
gone, the relation generally holds good. We see Britain separated by a
shallow channel from Europe, and the mammals are the same on both
sides; we meet with analogous facts on many islands separated by similar
channels from Australia. The West Indian Islands stand on a deeply
submerged bank, nearly 1000 fathoms in depth, and here we find American
forms, but the species and even the genera are distinct. As the amount
of modification in all cases depends to a certain degree on the lapse
of time, and as during changes of level it is obvious that islands
separated by shallow channels are more likely to have been continuously
united within a recent period to the mainland than islands separated
by deeper channels, we can understand the frequent relation between the
depth of the sea and the degree of affinity of the mammalian inhabitants
of islands with those of a neighbouring continent,--an inexplicable
relation on the view of independent acts of creation.

All the foregoing remarks on the inhabitants of oceanic
islands,--namely, the scarcity of kinds--the richness in endemic forms
in particular classes or sections of classes,--the absence of whole
groups, as of batrachians, and of terrestrial mammals notwithstanding
the presence of aerial bats,--the singular proport