Among These Are The Great Wingless Cassowary, Two Species
Of Heavy Brush Turkeys, And Two Of Short Winged Thrushes; Which
Could Certainly Not Have Passed Over The 150 Miles Of Open Sea To
The Coast Of New Guinea.
This barrier is equally effectual in the
case of many other birds which live only in the depths of the
forest, as the kinghunters (Dacelo gaudichaudi), the fly-catching
wrens (Todopsis), the great crown pigeon (Goura coronata), and
the small wood doves (Ptilonopus perlatus, P. aurantiifrons, and
P. coronulatus).
Now, to show the real effect of such barrier,
let us take the island of Ceram, which is exactly the same
distance from New Guinea, but separated from it by a deep sea.
Cut of about seventy land-birds inhabiting Ceram, only fifteen
are found in New Guinea, and none of these are terrestrial or
forest-haunting species. The cassowary is distinct; the
kingfishers, parrots, pigeons, flycatchers, honeysuckers,
thrushes, and cuckoos, are almost always quite distinct species.
More than this, at least twenty 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 iŁ 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
representatives of all classes of animals, because many
terrestrial mammals and some reptiles have the means of passing
over short distances of sea. But in these cases the number of
species that have thus migrated will be very small, and there
will be great deficiencies even in birds and flying insects,
which we should imagine could easily cross over. The island of
Timor (as I have already shown in Chapter XIII) bears this
relation to Australia; for while it contains several birds and
insects of Australian forms, no Australian mammal or reptile is
found in it, and a great number of the most abundant and
characteristic forms of Australian birds and insects are entirely
absent. Contrast this with the British Islands, in, which a large
proportion of the plants, insects, reptiles, and Mammalia of the
adjacent parts of the continent are fully represented, while
there are no remarkable deficiencies of extensive groups, such as
always occur when there is reason to believe there has been no
such connexion. The case of Sumatra, Borneo, and Java, and the
Asiatic continent is equally clear; many large Mammalia,
terrestrial birds, and reptiles being common to all, while a
large number more are of closely allied forms. Now, geology has
taught us that this representation by allied forms in the same
locality implies lapse of time, and we therefore infer that in
Great Britain, where almost every species is absolutely identical
with those on the Continent, the separation has been very recent;
while in Sumatra and Java, where a considerable number of the
continental species are represented by allied forms, the
separation was more remote.
From these examples we may see how important a supplement to
geological evidence is the study of the geographical distribution
of animals and plants, in determining the former condition of the
earth's surface; and how impossible it is to understand the
former without taking the latter into account. The productions of
the Aru Islands offer the strangest evidence, that at no very
distant epoch they formed a part of New Guinea; and the peculiar
physical features which I have described, indicate that they must
have stood at very nearly the same level then as they do now,
having been separated by the subsidence of the great plain which
formerly connected them with it.
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