238. PHORA BIFASCIATA, n. s. _Foem._ Atra, subtus flavescenti-alba,
antennis fulvis, abdomine lanceolato, fasciis duabus apice pedibus
halteribusque flavescenti-albis, pedibus posticis nigris basi
flavescenti-albis, tarsis intermediis nigricantibus, alis cinereis.
_Female._ Deep black, yellowish white beneath; antennæ tawny; abdomen
lanceolate, much longer than the thorax; sides elevated, a broad basal
yellowish white band, and a narrower one beyond the middle, tip also
yellowish white; anterior legs and halteres yellowish white, middle
tarsi blackish, hind femora with the basal half yellowish white;
wings cinereous, veins black, pale at the base; costal vein ending
at a little beyond half the length of the wing; radial cubital,
præbrachial, and pobrachial veins parallel and equally distinct.
Length of the body 2-2-1/2 lines; of the wings 5-6 lines.
* * * * *
On the Zoological Geography of the Malay Archipelago. By ALFRED R.
WALLACE, Esq. Communicated by CHARLES DARWIN, Esq., F.R.S. & L.S.
[Read Nov. 3rd, 1859.]
In Mr. Sclater's paper on the Geographical Distribution of Birds,
read before the Linnean Society, and published in the 'Proceedings'
for February 1858, he has pointed out that the western islands of the
Archipelago belong to the Indian, and the eastern to the Australian
region of Ornithology. My researches in these countries lead me to
believe that the same division will hold good in every branch of
Zoology; and the object of my present communication is to mark out
the precise limits of each region, and to call attention to some
inferences of great general importance as regards the study of the
laws of organic distribution.
The Australian and Indian regions of Zoology are very strongly
contrasted. In one the Marsupial order constitutes the great mass of
the mammalia,--in the other not a solitary marsupial animal exists.
Marsupials of at least two genera (_Cuscus_ and _Belideus_) are found
all over the Moluccas and in Celebes; but none have been detected
in the adjacent islands of Java and Borneo. Of all the varied forms
of _Quadrumana_, _Carnivora_, _Insectivora_ and _Ruminantia_ which
abound in the western half of the Archipelago, the only genera found
in the Moluccas are _Paradoxurus_ and _Cervus_. The _Sciuridæ_, so
numerous in the western islands, are represented in Celebes by only
two or three species, while not one is found further east. Birds
furnish equally remarkable illustrations. The Australian region is
the richest in the world in Parrots; the Asiatic is (of tropical
regions) the poorest. Three entire families of the Psittacine order
are peculiar to the former region, and two of them, the Cockatoos
and the Lories, extend up to its extreme limits, without a solitary
species passing into the Indian islands of the Archipelago. The genus
_Paloeornis_ is, on the other hand, confined with equal strictness to
the Indian region. In the Rasorial order, the _Phasianidæ_ are Indian,
the _Megapodiidæ_ Australian; but in this case one species of each
family just passes the limits into the adjacent region. The genus
_Tropidorhynchus_, highly characteristic of the Australian region,
and everywhere abundant as well in the Moluccas and New Guinea as in
Australia, is quite unknown in Java and Borneo. On the other hand,
the entire families of _Bucconidæ_, _Trogonidæ_ and _Phyllornithidæ_,
and the genera _Pericrocotus_, _Picnonotus_, _Trichophorus_, _Ixos_,
in fact, almost all the vast family of Thrushes and a host of
other genera, cease abruptly at the eastern side of Borneo, Java,
and Bali. All these groups are _common birds_ in the great Indian
islands; they abound everywhere; they are the characteristic features
of the ornithology; and it is most striking to a naturalist, on
passing the narrow straits of Macassar and Lombock, suddenly to miss
them entirely, together with the _Quadrumana_ and _Felidæ_, the
_Insectivora_ and _Rodentia_, whose varied species people the forests
of Sumatra, Java, and Borneo.
To define exactly the limits of the two regions where they are
(geographically) most intimately connected, I may mention that during
a few days' stay in the island of Bali I found birds of the genera
_Copsychus_, _Megalaima_, _Tiga_, _Ploceus_, and _Sturnopastor_,
all characteristic of the Indian region and abundant in Malacca,
Java, and Borneo; while on crossing over to Lombock, during three
months collecting there, not one of them was ever seen; neither have
they occurred in Celebes nor in any of the more eastern islands I
have visited. Taking this in connexion with the fact of _Cacatua_,
_Tropidorhynchus_, and _Megapodius_ having their western limit in
Lombock, we may consider it established that the Strait of Lombock
(only 15 miles wide) marks the limits and abruptly separates two of
the great Zoological regions of the globe. The Philippine Islands are
in some respects of doubtful location, resembling and differing from
both regions. They are deficient in the varied Mammals of Borneo,
but they contain no Marsupials. The Psittaci are scarce, as in the
Indian region; the Lories are altogether absent, but there is one
representative of the Cockatoos. Woodpeckers, Trogons, and the genera
_Ixos_, _Copsychus_, and _Ploceus_ are highly characteristic of India.
_Tanysiptera_ and _Megapodius_, again, are Australian forms, but
these seem represented by only solitary species. The islands possess
also a few peculiar genera. We must on the whole place the Philippine
Islands in the Indian region, but with the remark that they are
deficient in some of its most striking features. They possess several
isolated forms of the Australian region, but by no means sufficient to
constitute a real transition thereto.
Leaving the Philippines out of the question for the present, the
western and eastern islands of the Archipelago, as here divided,
belong to regions more distinct and contrasted than any other of the
great zoological divisions of the globe. South America and Africa,
separated by the Atlantic, do not differ so widely as Asia and
Australia; Asia with its abundance and variety of large Mammals and
no Marsupials, and Australia with scarcely anything but Marsupials;
Asia with its gorgeous _Phasianidæ_, Australia with its dull-coloured
_Megapodiidæ_; Asia the poorest tropical region in Parrots, Australia
the richest; and all these striking characteristics are almost
unimpaired at the very limits of their respective districts; so that
in a few hours we may experience an amount of zoological difference
which only weeks or even months of travel will give us in any other
part of the world!
Moreover there is nothing in the aspect or physical character of
the islands to lead us to expect such a difference; their physical
and geological differences do not coincide with the zoological
differences. There is a striking homogeneity in the TWO _halves_ of
the Archipelago. The great volcanic chain runs through both parts;
Borneo is the counterpart of New Guinea; the Philippines closely
resemble the equally fertile and equally volcanic Moluccas; while
in eastern Java begins to be felt the more arid climate of Timor
and Australia. But these resemblances are accompanied by an extreme
zoological diversity, the Asiatic and Australian regions finding in
Borneo and New Guinea respectively their highest development.
But it may be said: "The separation between these two regions is not
so absolute. There _is_ some transition. There _are_ species and
genera common to the eastern and western islands." This is true, yet
(in my opinion) proves no transition in the proper sense of the word;
and the nature and amount of the resemblance only shows more strongly
the absolute and original distinctness of the two divisions. The
exception here clearly proves the rule.
Let us investigate these cases of supposed transition. In the western
islands almost the only instance of a group peculiar to Australia and
the eastern islands is the _Megapodius_ in North-west Borneo. Not one
of the Australian forms of Mammalia passes the limits of the region.
On the other hand, Quadrumana occur in Celebes, Batchian, Lombock,
and perhaps Timor; Deer have reached Celebes, Timor, Buru, Ceram, and
Gilolo, but not New Guinea; Pigs have extended to New Guinea, probably
the true eastern limit of the genus _Sus_; Squirrels are found in
Celebes, Lombock, and Sumbawa: among birds, _Gallus_ occurs in Celebes
and Sumbawa, Woodpeckers reach Celebes, and Hornbills extend to the
North-west of New Guinea. These cases of identity or resemblance in
the animals of the two regions we may group into three classes; 1st,
identical species; 2nd, closely allied or representative species; and
3rd, species of peculiar and isolated genera. The common Grey Monkey
(_Macacus cynomolgus_) has reached Lombock, and perhaps Timor, but
not Celebes. The Deer of the Moluccas seems to be a variety of the
_Cervus rufus_ of Java and Borneo. The Jungle Cock of Celebes and
Lombock is a Javanese species. _Hirundo javanica_, _Zosterops flavus_,
_Halcyon collaris_, _Eurystomus gularis_, _Macropygia phasianella_,
_Merops javanicus_, _Anthreptes lepida_, _Ptilonopus melanocephala_,
and some other birds appear the same in the adjacent islands of
the eastern and western divisions, and some of them range over the
whole Archipelago. But after reading Lyell on the various modes of
dispersion of animals, and looking at the proximity of the islands, we
shall feel astonished, not at such an amount of interchange of species
(most of which are birds of great powers of flight), but rather that
in the course of ages a much greater and almost complete fusion has
not taken place. Were the Atlantic gradually to narrow till only a
strait of twenty miles separated Africa from South America, can we
help believing that many birds and insects and some few mammals would
soon be interchanged? But such interchange would be a fortuitous
mixture of faunas essentially and absolutely dissimilar, not a natural
and regular transition from one to the other. In like manner the
cases of identical species in the eastern and western islands of the
Archipelago are due to the gradual and accidental commingling of
originally absolutely distinct faunas.
In our second class (representative species) we must place the Wild
Pigs, which seem to be of distinct but closely allied species in each
island; the Squirrels also of Celebes are of peculiar species, as are
the Woodpeckers and Hornbills, and two Celebes birds of the Asiatic
genera _Phænicophæus_ and _Acridotheres_. Now these and a few more of
like character are closely allied to other species inhabiting Java,
Borneo, or the Philippines. We have only therefore to suppose that the
species of the western passed over to the eastern islands at so remote
a period as on one side or the other to have become extinct, and to
have been replaced by an allied form, and we shall have produced
exactly the state of things now existing. Such extinction and such
replacement we know has been continually going on. Such has been the
regular course of nature for countless ages in every part of the earth
of which we have geological records; and unless we _are_ prepared
to show that the Indo-Australian Archipelago was an altogether
exceptional region, such must have been the course of nature here
also. If these islands have existed in their present form only during
one of the later divisions of the Tertiary period, and if interchange
of species at very rare and distant intervals has occurred, then the
fact of some identical and other closely allied species is a necessary
result, even if the two regions in question had been originally
peopled by absolutely distinct creations of organic beings, and there
had never been any closer connexion between them than _now_ exists.
The occurrence of a limited number of representative species in the
two divisions of the Archipelago does not therefore prove any true
transition from one to the other.
The examples of our third class--of peculiar genera having little
or no affinity with those of the adjacent islands--are almost
entirely confined to Celebes, and render that island a district _per
se_, in the highest degree interesting. _Cynopithecus_, a genus of
Baboons, the extraordinary Babirusa and the singular ruminant _Ansa
depressicornis_ have nothing in common with Asiatic mammals, but seem
more allied to those of Africa. A quadrumanous animal of the same
genus (perhaps identical) occurs in the little island of Batchian,
which forms the extreme eastern limit of the highest order of
mammalia. An allied species is also said to exist in the Philippines.
Now this occurrence of quadrumana in the Australian region proves
nothing whatever as regards a transition to the western islands,
which, among their numerous monkeys and apes, have nothing at all
resembling them. The species of Celebes and Batchian have the high
superorbital ridge, the long nasal bone, the dog-like figure, the
minute erect tail, the predaceous habits and the fearless disposition
of the true Baboons, and find their allies nowhere nearer than in
tropical Africa. The _Anoa_ seems also to point towards the same
region, so rich in varied forms of Antelopes.
In the class of birds, Celebes possesses a peculiar genus of Parrots
(_Prioniturus_), said to occur also in the Philippines; _Meropogon_,
intermediate between an Indian and an African form of Bee-eaters; and
the anomalous _Scissirostrum_, which Prince Bonaparte places next to a
Madagascar bird, and forms a distinct subfamily for the reception of
the two. Celebes also contains a species of _Coracias_, which is here
quite out of its normal area, the genus being otherwise confined to
Africa and continental India, not occurring in any other part of the
Archipelago. The Celebes bird is placed, in Bonaparte's 'Conspectus,'
between two African species, to which therefore I presume it is more
nearly allied than to those of India. Having just received Mr. Smith's
Catalogue of the Hymenoptera collected during my first residence in
Celebes, I find in it some facts of an equally singular nature. Of 103
species, only 16 are known to inhabit any of the western islands of
the Archipelago, while 18 are identical with species of continental
India, China, and the Philippine Islands, two are stated to be
identical with insects hitherto known only from tropical Africa, and
another is said to be most closely allied to one from the Cape.
These phenomena of distribution are, I believe, the most anomalous
yet known, and in fact altogether unique. I am aware of no other spot
upon the earth which contains a number of species, in several distinct
classes of animals, the nearest allies to which do not exist in any
of the countries which on every side surround it, but which are to be
found only in another primary division of the globe, separated from
them all by a vast expanse of ocean. In no other case are the species
of a genus or the genera of a family distributed in _two_ distinct
areas separated by countries in which they do not exist; so that it
has come to be considered a law in geographical distribution, "that
both species and groups inhabit continuous areas."
Facts such as these can only be explained by a bold acceptance of vast
changes in the surface of the earth. They teach us that this island of
Celebes is more ancient than most of the islands now surrounding it,
and obtained some part of its fauna before they came into existence.
They point to the time when a great continent occupied a portion
at least of what is now the Indian Ocean, of which the islands of
Mauritius, Bourbon, &c. may be fragments, while the Chagos Bank and
the Keeling Atolls indicate its former extension eastward to the
vicinity of what is now the Malayan Archipelago. The Celebes group
remains the last eastern fragment of this now submerged land, or of
some of its adjacent islands, indicating its peculiar origin by its
zoological isolation, and by still retaining a marked affinity with
the African fauna.
The great Pacific continent, of which Australia and New Guinea are
no doubt fragments, probably existed at a much earlier period, and
extended as far westward as the Moluccas. The extension of Asia as far
to the south and east as the Straits of Macassar and Lombock must have
occurred subsequent to the submergence of both these great southern
continents; and the breaking up and separation of the islands of
Sumatra, Java, and Borneo has been the last great geological change
these regions have undergone. That this has really taken place as
here indicated, we think is proved by the following considerations.
Not more than twenty (probably a smaller number) out of about one
hundred land birds of Celebes at present known are found in Java
or Borneo, and only one or two of twelve or fifteen Mammalia. Of
the Mammalia and birds of Borneo, however, at least three-fourths,
probably five-sixths, inhabit also Java, Sumatra, or the peninsula of
Malacca. Now, looking at the direction of the Macassar Straits running
nearly north and south, and remembering we are in the district of the
monsoons, a steady south-east and north-west wind blowing alternately
for about six months each, we shall at once see that Celebes is more
favourably situated than any other island to receive stray passengers
from Borneo, whether drifted across the sea or wafted through the
air. The distance too is less than between any of the other large
islands; there are no violent currents to neutralize the action of
the winds; and numerous islets in mid-channel offer stations which
might rescue many of the wanderers, and admit, after repose, of
fresh migrations. Between Java and Borneo the width of sea is much
greater, the intermediate islands are fewer, and the direction of
the monsoons _along_ and not _across_ the Java sea, accompanied by
alternating currents in the same direction, must render accidental
communication between the two islands exceedingly difficult; so that
where the facilities for intercommunication are greatest, the number
of species common to the two countries is least, and _vice versá_.
But again, the mass of the species of Borneo, Java, &c., even when not
_identical_ are _congeneric_, which, as before explained, indicates
_identity_ at an earlier epoch; whereas the great mass of the fauna
of Celebes is widely different from that of the western islands,
consisting mostly of genera, and even of entire families, altogether
foreign to them. This clearly points to a former total diversity
of forms and species,--existing similarities being the result of
intermixture, the extreme facilities for which we have pointed out. In
the case of the great western islands a former more complete identity
is indicated, the present differences having arisen from their
isolation during a considerable period, allowing time for that partial
extinction and introduction of species which is the regular course
of nature. If the very small number of western species in Celebes is
all that the most favourable conditions for transmission could bring
about, the complete similarity of the faunas of the western islands
could never (with far less favourable conditions) have been produced
by the same means. And what other means can we conceive but the former
connexion of those islands with each other and with the continent of
Asia?
In striking confirmation of this view we have physical evidence
of a very interesting nature. These countries are in fact _still
connected_, and that so completely that an elevation of only 300 feet
would nearly double the extent of tropical Asia. Over the whole of the
Java Sea, the Straits of Malacca, the Gulf of Siam, and the southern
part of the China Sea, ships can anchor in less than fifty fathoms.
A vast submarine plain unites together the apparently disjointed
parts of the Indian zoological region, and abruptly terminates,
exactly at its limits, in an unfathomable ocean. The deep sea of the
Moluccas comes up to the very coasts of Northern Borneo, to the Strait
of Lombock in the south, and to near the middle of the Strait of
Macassar. May we not therefore from these facts very fairly conclude
that, according to the system of alternate bands of elevation and
depression that seems very generally to prevail, the last great rising
movement of the volcanic range of Java and Sumatra was accompanied
by the depression that now separates them from Borneo and from the
continent?
It it worthy of remark that the various islands of the Moluccas,
though generally divided by a less extent of sea, have fewer species
in common; but the separating seas are in almost every case of immense
depth, indicating that the separation took place at a much earlier
period. The same principle is well illustrated by the distribution
of the genus _Paradisea_, two species of which (the common Birds
of Paradise) are found only in New Guinea and the islands of Aru,
Mysol, Waigiou, and Jobie, all of which are connected with New Guinea
by banks of soundings, while they do not extend to Ceram or the Ké
Islands, which are no further from New Guinea, but are separated from
it by deep sea. Again, the chain of small volcanic islands to the west
of Gilolo, though divided by channels of only ten or fifteen miles
wide, possess many distinct representative species of insects, and
even, in some cases, of birds also. The Baboons of Batchian have not
passed to Gilolo, a much larger island, only separated from it by a
channel ten miles wide, and in one part almost blocked up with small
islands.
Now looking at these phenomena of distribution, and especially at
those presented by the fauna of Celebes, it appears to me that a much
exaggerated effect, in producing the present distribution of animals,
has been imputed to the accidental transmission of individuals across
intervening seas; for we have here as it were a test or standard by
which we may measure the possible effect due to these causes, and we
find that, under conditions perhaps the most favourable that exist
on the globe, the percentage of species derived from this source is
extremely small. When my researches in the Archipelago are completed,
I hope to be able to determine with some accuracy this numerical
proportion in several cases; but in the mean time we will consider
20 per cent. as the probable maximum for birds and mammals which in
Celebes have been derived from Borneo or Java.
Let us now apply this standard to the case of Great Britain and
the Continent, in which the width of dividing sea and the extent
of opposing coasts are nearly the same, but in which the species
are almost all identical,--or to Ireland, more than 90 per cent. of
whose species are British,--and we shall at once see that no theory
of transmission across the present Straits is admissible, and shall
be compelled to resort to the idea of a very recent separation (long
since admitted), to account for these zoological phenomena.
It is, however, to the oceanic islands that we consider the
application of this test of the most importance. Let any one try to
realize the comparative facilities for the transmission of organized
beings across the Strait of Macassar from Borneo to Celebes, and
from South Europe or North Africa to the island of Madeira, at least
four times the distance, and a mere point in the ocean, and he
would probably consider that in a given period a hundred cases of
transmission would be more likely to occur in the former case than
one in the latter. Yet of the comparatively rich insect-fauna of
Madeira, 40 per cent. are continental species; and of the flowering
plants more than 60 per cent. The Canary Islands offer nearly similar
results. Nothing but a former connexion with the Continent will
explain such an amount of specific identity (the weight of which will
be very much increased if we take into account the representative
species); and the direction of the Atlas range towards Teneriffe, and
of the Sierra Nevada towards Madeira, are material indications of such
a connexion.
The Galapagos are no further from South America than Madeira is from
Europe, and, being of greater extent, are far more liable to receive
chance immigrants; yet they have hardly a species identical with any
inhabiting the American continent. These islands therefore may well
have originated in mid-ocean; or if they ever were connected with the
mainland, it was at so distant a period that the natural extinction
and renewal of species has left not one in common. The character of
their fauna, however, is more what we should expect to arise from the
chance introduction of a very few species at distant intervals; it
is very poor; it contains but few genera, and those scattered among
unconnected families; its genera often contain several closely allied
species, indicating a single antitype.
The fauna and flora of Madeira and of the Canaries, on the other hand,
have none of this chance character. They are comparatively rich in
genera and species; most of the principal groups and families are
more or less represented; and, in fact, these islands do not differ
materially, as to the general character of their animal and vegetable
productions, from any isolated mountain in Europe or North Africa of
about equal extent.
On exactly the same principles, the very large number of species
of plants, insects, and birds, in Europe and North America, either
absolutely identical or represented by very closely allied species,
most assuredly indicates that some means of land communication
in temperate or sub-arctic latitudes existed at no very distant
geological epoch; and though many naturalists are inclined to
regard all such views as vague and unprofitable speculations, we
are convinced they will soon take their place among the legitimate
deductions of science.
Geology can detect but a portion of the changes the surface of the
earth has undergone. It can reveal the past history and mutations
of what is now dry land; but the ocean tells nothing of her bygone
history. Zoology and Botany here come to the aid of their sister
science, and by means of the humble weeds and despised insects
inhabiting its now distant shores, can discover some of those past
changes which the ocean itself refuses to reveal. They can indicate,
approximately at least, where and at what period former continents
must have existed, from what countries islands must have been
separated, and at how distant an epoch the rupture took place. By the
invaluable indications which Mr. Darwin has deduced from the structure
of coral reefs, by the surveys of the ocean-bed now in progress,
and by a more extensive and detailed knowledge of the geographical
distribution of animals and plants, the naturalist may soon hope to
obtain some idea of the continents which have now disappeared beneath
the ocean, and of the general distribution of land and sea at former
geological epochs.
Most writers on geographical distribution have completely overlooked
its connexion with well-established geological facts, and have thereby
created difficulties where none exist. The peculiar and apparently
endemic faunæ and floræ of the oceanic islands (such as the Galapagos
and St. Helena) have been dwelt upon as something anomalous and
inexplicable. It has been imagined that the more simple condition
of such islands would be to have their productions identical with
those of the nearest land, and that their actual condition is an
incomprehensible mystery. The very reverse of this is however the
case. We really require no speculative hypothesis, no new theory, to
explain these phenomena; they are the logical results of well-known
laws of nature. The regular and unceasing extinction of species,
and their replacement by allied forms, is now no hypothesis, but an
established fact; and it necessarily produces such peculiar faunæ and
floræ in all but recently formed or newly disrupted islands, subject
of course to more or less modification according to the facilities
for the transmission of fresh species from adjacent continents. Such
phenomena therefore are far from uncommon. Madagascar, Mauritius,
the Moluccas, New Zealand, New Caledonia, the Pacific Islands, Juan
Fernandez, the West India Islands, and many others, all present such
peculiarities in greater or less development. It is the instances
of identity of species in distant countries that presents the real
difficulty. What was supposed to be the more normal state of things is
really exceptional, and requires some hypothesis for its explanation.
The phenomena of distribution in the Malay Archipelago, to which I
have here called attention, teach us that, however narrow may be
the strait separating an island from its continent, it is still an
impassable barrier against the passage of any considerable number and
variety of land animals; and that in all cases in which such islands
possess a tolerably rich and varied fauna of species mostly identical,
or closely allied with those of the adjacent country, we are forced to
the conclusion that a geologically recent disruption has taken place.
Great Britain, Ireland, Sicily, Sumatra, Java and Borneo, the Aru
Islands, the Canaries and Madeira, are cases to which the reasoning is
fully applicable.
In his introductory Essay on the Flora of New Zealand, Dr. Hooker
has most convincingly applied this principle to show the former
connexion of New Zealand and other southern islands with the southern
extremity of America; and I will take this opportunity of calling the
attention of zoologists to the very satisfactory manner in which this
view clears away many difficulties in the distribution of animals.
The most obvious of these is the occurrence of Marsupials in America
only, beyond the Australian region. They evidently entered by the
same route as the plants of New Zealand and Tasmania which occur in
South temperate America, but having greater powers of dispersion,
a greater plasticity of organization, have extended themselves
over the whole continent though with so few modifications of form
and structure as to point to a unity of origin at a comparatively
recent period. It is among insects, however, that the resemblances
approach in number and degree to those exhibited by plants. Among
Butterflies the beautiful _Heliconidæ_ are strictly confined to
South America, with the exception of a single genus (_Hamadryas_)
found in the Australian region from New Zealand to New Guinea.
In Coleoptera many families and genera are characteristic of the
two countries; such are _Pseudomorphidæ_ among the Geodephaga,
_Lamprimidæ_ and _Syndesidæ_ among the Lucani, _Anoplognathidæ_ among
the Lamellicornes, _Stigmoderidæ_ among the Buprestes, _Natalis_ among
the Cleridæ, besides a great number of representative genera. This
peculiar distribution has hitherto only excited astonishment, and has
confounded all ideas of unity in the distribution of organic beings;
but we now see that they are in exact accordance with the phenomena
presented by the flora of the same regions, as developed in the
greatest detail by the researches of Dr. Hooker.
It is somewhat singular, however, that not one _identical species_
of insect should yet have been discovered, while no less than 89
species of flowering plants are found both in New Zealand and South
America. The relations of the animals and of the plants of these
countries must necessarily depend on the same physical changes which
the Southern hemisphere has undergone; and we are therefore led to
conclude that insects are much less persistent in their specific
forms than flowering plants, while among Mammalia and land birds (in
which no genus even is common to the countries in question) species
must die and be replaced much more rapidly than in either. And this
is exactly in accordance with the fact (well established by geology)
that at a time when the shells of the European seas were almost all
identical with species now living, the European Mammalia were almost
all different. The duration of life of species would seem to be in
an inverse proportion to their complexity of organization and vital
activity.
In the brief sketch I have now given of this interesting subject, such
obvious and striking facts alone have been adduced as a traveller's
note-book can supply. The argument must therefore lose much of its
weight from the absence of detail and accumulated examples. There
is, however, such a very general accordance in the phenomena of
distribution as separately deduced from the various classes or
kingdoms of the organic world, that whenever one class of animals or
plants exhibits in a clearly marked manner certain relations between
two countries, the other classes will certainly show similar ones,
though it may be in a greater or a less degree. Birds and insects will
teach us the same truths; and even animals and plants, though existing
under such different conditions, and multiplied and dispersed by such
a generally distinct process, will never give conflicting testimony,
however much they may differ as regards the amount of relationship
between distant regions indicated by them, and consequently
notwithstanding the greater or less weight either may have in the
determining of questions of this nature.
This is my apology for offering to the Linnean Society the present
imperfect outline in anticipation of the more detailed proofs and
illustrations which I hope to bring forward on a future occasion.
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