genera, which are common to New Guinea and Aru, do not extend into Ceram,
indicating with a force which every naturalist will appreciate, that the two latter
countries have received their faunas in a radically different manner. Again, a true
kangaroo is found in Aru, and the same species occurs in Mysol, which is
equally Papuan in its productions, while either the same, or one closely allied to
it, inhabits New Guinea; but no such animal is found in Ceram, which is only
sixty miles from Mysol. Another small marsupial animal (Perameles doreyanus)
is common to Aru and New Guinea. The insects show exactly the same results.
The butterflies of Aru are all either New Guinea species, or very slightly
modified forms; whereas those of Ceram are more distinct than are the birds of
the two countries.
It is now generally admitted that we may safely reason on such facts as those,
which supply a link in the defective geological record. The upward and
downward movements which any country has undergone, and the succession of
such movements, can be determined with much accuracy; but geology alone can
tell us nothing of lands which have entirely disappeared beneath the ocean. Here
physical geography and the distribution of animals and plants are of the greatest
service. By ascertaining the depth of the seas separating one country from
another, we can form some judgment of the changes which are taking place. If
there are other evidences of subsidence, a shallow sea implies a former
connexion of the adjacent lands; but if this evidence is wanting, or if there is
reason to suspect a rising of the land, then the shallow sea may be the result of
that rising, and may indicate that the two countries will be joined at some future
time, but not that they have previously been so. The nature of the animals and
plants inhabiting these countries will, however, almost always enable us to
determine this question. Mr. Darwin has shown us how we may determine in
almost every case, whether an island has ever been connected with a continent or
larger land, by the presence or absence of terrestrial Mammalia and reptiles.
What he terms "oceanic islands" possess neither of these groups of animals,
though they may have a luxuriant vegetation, and a fair number of birds, insects,
and landshells; and we therefore conclude that they have originated in mid-
ocean, and have never been connected with the nearest masses of land. St.
Helena, Madeira, and New Zealand are examples of oceanic islands. They
possess all other classes of life, because these have means of dispersion over
wide spaces of sea, which terrestrial mammals and birds have not, as is fully
explained in Sir Charles Lyell's "Principles of Geology," and Mr. Darwin's
"Origin of Species." On the other hand, an island may never have been actually
connected with the adjacent continents or islands, and yet may possess