SPECIES AND SPACE.
The geographical distribution of animals presents difficulties.--These
not insurmountable in themselves; harmonize with other
difficulties.--Fresh-water fishes.--Forms common to Africa and India;
to Africa and South America; to China and Australia; to North America
and China; to New Zealand and South America; to South America and
Tasmania; to South America and Australia.--Pleurodont
lizards.--Insectivorous mammals.--Similarity of European and South
American frogs--Analogy between European salmon and fishes of New
Zealand, &c. An ancient Antarctic continent probable.--Other modes of
accounting for facts of distribution.--Independent origin of closely
similar forms.--Conclusion.
The study of the distribution of animals over the earth's surface presents
us with many facts having certain not unimportant bearings on the question
of specific origin. Amongst these are instances which, at least at first
sight, appear to conflict with the Darwinian theory of "Natural Selection."
It is not, however, here contended that such facts do by any means
constitute by themselves obstacles which cannot be got over. Indeed it
would be difficult to imagine any obstacles of the kind which could not be
surmounted by an indefinite number of terrestrial modifications of
surface--submergences and emergences--junctions and separations of
continents in all directions and combinations of any desired degree of
frequency. All this being supplemented by the intercalation of armies of
enemies, multitudes of ancestors of all kinds, and myriads of connecting
forms, whose _raison d'être_ may be simply their utility or necessity {145}
for the support of the theory of "Natural Selection."
Nevertheless, when brought in merely to supplement and accentuate
considerations and arguments derived from other sources, in that case
difficulties connected with the geographical distribution of animals are
not without significance, and are worthy of mention even though, by
themselves, they constitute but feeble and more or less easily explicable
puzzles which could not alone suffice either to sustain or to defeat any
theory of specific origination.
Many facts as to the present distribution of animal life over the world are
very readily explicable by the hypothesis of slight elevations and
depressions of larger and smaller parts of its surface, but there are
others the existence of which it is much more difficult so to explain.
The distribution either of animals possessing the power of flight, or of
inhabitants of the ocean, is, of course, easily to be accounted for; the
difficulty, if there is really any, must mainly be with strictly
terrestrial animals of moderate or small powers of locomotion and with
inhabitants of fresh water. Mr. Darwin himself observes,[142] "In regard to
fish, I believe that the same species never occur in the fresh waters of
distant continents." Now, the Author is enabled, by the labours and through
the kindness of Dr. Günther, to show that this belief cannot be maintained;
he having been so obliging as to call attention to the following facts with
regard to fish-distribution. These facts show that though only one species
which is absolutely and exclusively an inhabitant of fresh water is as yet
known to be found in distant continents, yet that in several other
instances the same species _is_ found in the fresh water of distant
continents, and that very often the same _genus_ is so distributed.
The genus _Mastacembelus_ belongs to a family of fresh-water Indian {146}
fishes. Eight species of this genus are described by Dr. Günther in his
catalogue.[143] These forms extend from Java and Borneo on the one hand, to
Aleppo on the other. Nevertheless, a new species (_M. cryptacanthus_) has
been described by the same author,[144] which is an inhabitant of the
Camaroon country of _Western_ Africa. He observes, "The occurrence of
Indian forms on the West Coast of Africa, such as _Periophthalmus_,
_Psettus_, _Mastacembelus_, is of the highest interest, and an almost new
fact in our knowledge of the geographical distribution of fishes."
_Ophiocephalus_, again, is a truly Indian genus, there being no less than
twenty-five species,[145] all from the fresh waters of the East Indies. Yet
Dr. Günther informs me that there is a species in the Upper Nile and in
West Africa.
The acanthopterygian family (_Labyrinthici_) contains nine freshwater
genera, and these are distributed between the East Indies and South and
Central Africa.
The Carp fishes (Cyprinoids) are found in India, Africa, and Madagascar,
but there are none in South America.
Thus existing fresh-water fishes point to an immediate connexion between
Africa and India, harmonizing with what we learn from Miocene mammalian
remains.
On the other hand, the Characinidæ (a family of the physostomous fishes)
are found in Africa and South America, and not in India, and even its
component groups are so distributed,--namely, the _Tetragonopterina_[146]
and the _Hydrocyonina_.[147]
Again, we have similar phenomena in that almost exclusively fresh-water
group the Siluroids.
Thus the genera _Clarias_[148] and _Heterobranchus_[149] are found {147}
both in Africa and the East Indies. _Plotosus_ is found in Africa, India,
and Australia, and the species _P. anguillaris_[150] has been brought from
both China and Moreton Bay. Here, therefore, we have the same species in
two distinct geographical regions. It is however a coast fish, which,
though entering rivers, yet lives in the sea.
_Eutropius_[151] is an African genus, but _E. obtusirostris_ comes from
India. On the other hand, _Amiurus_ is a North American form; but one
species, _A. cantonensis_,[152] comes from China.
The genus _Galaxias_[153] has at least one species common to New Zealand
and South America, and one common to South America and Tasmania. In this
genus we thus have an absolutely and completely fresh-water form _of the
very same species_ distributed between different and distinct geographical
regions.
Of the lower fishes, a lamprey, _Mordacia mordax_,[154] is common to South
Australia and Chile; while another form of the same family, namely,
_Geotria chilensis_,[155] is found not only in South America and Australia,
but in New Zealand also. These fishes, however, probably pass part of their
lives in the sea.
We thus certainly have several species which _are_ common to the fresh
waters of distant continents, although it cannot be certainly affirmed that
they are exclusively and entirely fresh-water fishes throughout all their
lives except in the case of _Galaxias_.
Existing forms point to a close union between South America and Africa on
the one hand, and between South America, Australia, Tasmania, and New
Zealand on the other; but these unions were not synchronous any more than
the unions indicated between India and Australia, China and Australia,
China and North America, and India and Africa.
Pleurodont lizards are such as have the teeth attached by their sides {148}
to the inner surface of the jaw, in contradistinction to acrodont lizards,
which have the bases of their teeth anchylosed to the summit of the margin
of the jaw. Now pleurodont iguanian lizards abound in the South American
region; but nowhere else, and are not as yet known to inhabit any part of
the present continent of Africa. Yet pleurodont lizards, strange to say,
are found in Madagascar. This is the more remarkable, inasmuch as we have
no evidence yet of the existence in Madagascar of fresh-water fishes common
to Africa and South America.
[Illustration: INNER SIDE OF LOWER JAW OF PLEURODONT LIZARD.
(Showing the teeth attached to the inner surface of its side.)]
Again, that remarkable island Madagascar is the home of very singular and
special insectivorous beasts of the genera Centetes, Ericulus, and
Echinops; while the only other member of the group to which they belong is
Solenodon, which is a resident in the West Indian Islands, Cuba and Hayti.
The connexion, however, between the West Indies and Madagascar must surely
have been at a time when the great lemurine group was absent; for it is
difficult to understand the spread of such a form as Solenodon, and at the
same time the non-extension of the active lemurs, or their utter
extirpation, in such a congenial locality as the West Indian Archipelago.
The close connexion of South America and Australia is demonstrated (on the
Darwinian theory), not only from the marsupial fauna of both, but also from
the frogs and toads which respectively inhabit those regions. A truly
remarkable similarity and parallelism exist, however, between certain of
the same animals inhabiting South Western America and Europe. Thus Dr.{149}
Günther has described[156] a frog from Chile by the name of cacotus, which
singularly resembles the European bombinator.
[Illustration: SOLENODON.]
Again of the salmons, two genera from South America, New Zealand, and
Australia, are analogous to European salmons.
In addition to this may be mentioned a quotation from Professor Dana, given
by Mr. Darwin,[157] to the effect that "it is certainly a wonderful fact
that New Zealand should have a closer resemblance in its crustacea to Great
Britain, its antipode, than to any other part of the world:" and Mr. Darwin
adds "Sir J. Richardson also speaks of the reappearance on the shores of
New Zealand, Tasmania, &c. of northern forms of fish. Dr. Hooker informs me
that twenty-five species of algæ are common to New Zealand and to {150}
Europe, but have not been found in the intermediate tropical seas."
Many more examples of the kind could easily be brought, but these must
suffice. As to the last-mentioned cases Mr. Darwin explains them by the
influence of the glacial epoch, which he would extend actually across the
equator, and thus account, amongst other things, for the appearance in
Chile of frogs having close genetic relations with European forms. But it
is difficult to understand the persistence and preservation of such
exceptional forms with the extirpation of all the others which probably
accompanied them, if so great a migration of northern kinds had been
occasioned by the glacial epoch.
Mr. Darwin candidly says,[158] "I am far from supposing that all
difficulties in regard to the distribution and affinities of the identical
and allied species, which now live so widely separated in the north and
south, and sometimes on the intermediate mountain-ranges, are removed." ...
"We cannot say why certain species and not others have migrated; why
certain species have been modified and have given rise to new forms, whilst
others have remained unaltered." Again he adds, "Various difficulties also
remain to be solved; for instance, the occurrence, as shown by Dr. Hooker,
of the same plants at points so enormously remote as Kerguelen Land, New
Zealand, and Fuegia; but icebergs, as suggested by Lyell, may have been
concerned in their dispersal. The existence, at these and other distant
points of the southern hemisphere, of species which, though distinct,
belong to genera exclusively confined to the south, is a more remarkable
case. Some of these species are so distinct that we cannot suppose that
there has been time since the commencement of the last glacial period for
their migration and subsequent modification to the necessary degree." Mr.
Darwin goes on to account for these facts by the probable existence of a
rich antarctic flora in a warm period anterior to the last glacial {151}
epoch. There are indeed many reasons for thinking that a southern
continent, rich in living forms, once existed. One such reason is the way
in which struthious birds are, or have been, distributed around the
antarctic region: as the ostrich in Africa, the rhea in South America, the
emeu in Australia, the apteryx, dinornis, &c. in New Zealand, the epiornis
in Madagascar. Still the existence of such a land would not alone explain
the various geographical cross relations which have been given above. It
would not, for example, account for the resemblance between the crustacea
or fishes of New Zealand and of England. It would, however, go far to
explain the identity (specific or generic) between fresh water and other
forms now simultaneously existing in Australia and South America, or in
either or both of these, and New Zealand.
Again, mutations of elevation small and gradual (but frequent and
intermitting), through enormous periods of time--waves, as it were, of land
rolling many times in many directions--might be made to explain many
difficulties as to geographical distribution, and any cases that remained
would probably be capable of explanation, as being isolated but allied
animal forms, now separated indeed, but being merely remnants of extensive
groups which, at an earlier period, were spread over the surface of the
earth. Thus none of the facts here given are any serious difficulty to the
doctrine of "evolution," but it is contended in this book that if other
considerations render it improbable that the manifestation of the
successive forms of life has been brought about by minute, indefinite, and
fortuitous variations, then these facts as to geographical distribution
intensify that improbability, and are so far worthy of attention.
All geographical difficulties of the kind would be evaded if we could
concede the probability of the independent origin, in different localities,
of the same organic forms in animals high in the scale of nature. {152}
Similar causes must produce similar results, and new reasons have been
lately adduced for believing, as regards the _lowest organisms_, that the
same forms can arise and manifest themselves independently. The difficulty
as to higher animals is, however, much greater, as (on the theory of
evolution) one acting force must always be the ancestral history in each
case, and this force must always tend to go on acting in the same groove
and direction in the future as it has in the past. So that it is difficult
to conceive that individuals, the ancestral history of which is very
different, can be acted upon by all influences, external and internal, in
such diverse ways and proportions that the results (unequals being added to
unequals) shall be equal and similar. Still, though highly improbable, this
cannot be said to be impossible; and if there _is_ an innate law of any
kind helping to determine specific evolution, this may more or less, or
entirely, neutralize or even reverse the effect of ancestral habit. Thus,
it is quite conceivable that a pleurodont lizard might have arisen in
Madagascar in perfect independence of the similarly-formed American
lacertilia: just as certain teeth of carnivorous and insectivorous
marsupial animals have been seen most closely to resemble those of
carnivorous and insectivorous placental beasts; just as, again, the paddles
of the Cetacea resemble, in the fact of a multiplication in the number of
the phalanges, the many-jointed feet of extinct marine reptiles, and as the
beak of the cuttle-fish or of the tadpole resembles that of birds. We have
already seen (in Chapter III.) that it is impossible, upon any hypothesis,
to escape admitting the independent origins of closely similar forms, It
may be that they are both more frequent and more important than is
generally thought.
That closely similar structures may arise without a genetic relationship
has been lately well urged by Mr. Ray Lankester.[159] He has brought {153}
this notion forward even as regards the bones of the skull in osseous
fishes and in mammals. He has done so on the ground that the probable
common ancestor of mammals and of osseous fishes was a vertebrate animal of
so low a type that it could not be supposed to have possessed a skull
differentiated into distinct bony elements--even if it was bony at all. If
this was so, then the cranial bones must have had an independent origin in
each class, and in this case we have the most strikingly harmonious and
parallel results from independent actions. For the bones of the skull in an
osseous fish are so closely conformed to those of a mammal, that "both
types of skull exhibit many bones in common," though "in each type some of
these bones acquire special arrangements and very different
magnitudes."[160] And no investigator of homologies doubts that a
considerable number of the bones which form the skull of any osseous fish
are distinctly homologous with the cranial bones of man. The occipital, the
parietal, and frontal, the bones which surround the internal ear, the
vomer, the premaxilla, and the quadrate bones, may be given as examples.
Now, if such close relations of homology can be brought about independently
of any but the most remote genetic affinity, it would be rash to affirm
dogmatically that there is any impossibility in the independent origin of
such forms as centetes and solenodon, or of genetically distinct
batrachians, as similar to each other as are some of the frogs of South
America and of Europe. At the same time such phenomena must at present be
considered as very improbable, from the action of ancestral habit, as
before stated.
We have seen, then, that the geographical distribution of animals presents
difficulties, though not insuperable ones, for the Darwinian hypothesis.
If, however, other reasons against it appear of any weight--if, especially,
there is reason to believe that geological time has not been {154}
sufficient for it, then it will be well to bear in mind the facts here
enumerated. These facts, however, are not opposed to the doctrine of
evolution; and if it could be established that closely similar forms had
really arisen in complete independence one of the other, they would rather
tend to strengthen and to support that theory. [Page 155]
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