GEOLOGICAL REFLECTIONS.
We frequently hear it asserted, that, since the members of the Insecta
are so numerous and minute, when compared with those of other
departments of the organic world, the entomologist, whose province it
is to collect and classify them, can have but little time, if he
attempt the real advancement of his particular science, for
generalizations on a broad scale. Now, whilst there is necessarily
some reason in this remark (for the investigation of species is a work
of such labour and drudgery that it is apt to monopolize all the
leisure hours which the greater number of us are able to command), we
should recollect, on the other hand, that the soundest theorists have
ever been the most patient and accurate observers; and have, many of
them, spent whole years of their lives as humble students in Nature's
domain. We need not be afraid that an occupation amongst what is
microscopically small is liable to cramp the mind, and render it unfit
for wider processes of induction, since the very opposite of this
would seem to come nearer to the truth. The understanding which has
been well tutored by a system of close and steady observation, which
has been trained to seize upon differences amongst the objects of our
common experience, to balance the importance of generic and specific
characters, as tested in the acquisitions of our daily walks; and
which has been gradually brightened and matured by the habitual
exercise of its judgment on the most trifling phaenomena around us, has
usually gained strength enough to form conclusions from such data,
which will not only stand the test of analysis, but will be free from
those eccentricities of genius which too often mar the speculations of
less practical naturalists. The mind, moreover, having been chained
and fettered for a season to the mere detail of facts, breaks forth,
under such circumstances, with all the vigour with which the
contemplation of truth has gifted it, and takes its flight as it were
to a clearer sky; and, though a reaction may at times set in, hurrying
it away into regions beyond its sphere, it will assuredly return at
length, fraught with the soberness which its vocation has inspired,
and commence to build up its hypotheses, step by step, in harmony with
the material which it has amassed.
Yet though entomologists may be in reality as well qualified as any
other natural historians for drawing general conclusions from the
result of their researches, it is impossible to conceal the fact,
that, as a body, they have not ordinarily done so. Whether this has
happened through an accidental disinclination on their part to occupy
themselves in such matters, or (which is more probable) from their
whole time having been engrossed by the dry routine of their science,
I do not pretend to determine: be the solution, however, what it may,
the inference is practically the same,--that the Annulosa have not
hitherto been sufficiently regarded, in the great questions of
zoological geography. But especially have they been ignored during
that most significant of considerations which has been so ably brought
forward of late years by some of our keenest observers,--namely, the
distribution of animals, as affected by geological changes, on the
earth's surface.
It would be well if the collector of insects would devote at least a
tithe of his energies to the speculative branch of his subject.
Certain it is that much would probably be advanced, at first, on
slender premises; and would, as a consequence, fall to the ground,
leaving no record behind it. Yet such must inevitably be the case, at
the outset, in every region of inquiry; and we are prepared to expect
it. It does not however follow that _good_ would not be developed
also; whilst we are confident of the fact, that unless the trial be
made, it cannot possibly arise. No question has ever yet been mooted
without beneficial results: it has either been shown to be absurd, and
has received its death-blow on the spot, or else truth has been
elicited (indirectly perhaps), which has at once shed a new ray of
light on some of its obscurest bearings. And so, assuredly, it would
be in the present instance. We cannot doubt that there is much to be
discovered in the past history of insect dissemination, which would
tend, when rightly interpreted, to explain many of the occult
phaenomena of the present day; and we may be equally satisfied that
this cannot by any possibility be attempted without the assistance of
geology. Let us therefore glance hastily at a few of those more
undeniable convulsions which we are aware have, at various epochs,
taken place; and endeavour to catch a glimpse of how, in the common
course of things, that portion of the insect world would be affected
which was exposed to their influence.
First and foremost, perhaps, in importance, of all the changes which
it is self-evident have happened, may be mentioned _subsidence_.
Including, as it does, both the general lowering of some countries,
and the actual isolation of others, there are, I believe, no physical
crises to which we could point, through the instrumentality of which
the very _existence_ of the insect races (not to allude to their
diffusion) has been, by the nature of the case, more seriously
interfered with. We know that there are certain species of an alpine
and boreal character, which cannot live except in a climate of low
temperature,--guaranteed to them either by _elevation_ in one land, or
by a higher latitude in another: and let us picture the consequences
of the gradual sinking of a mountain chain, even to a small extent,
the _summits_ of which only just afforded the conditions of atmosphere
necessary for the continuance of creatures like these. Now this is an
example by no means far-fetched, and such as _must_ have occurred in
instances innumerable. But, what would be the many results of a
diminution in the level of our imaginary range? It needs no argument
to prove, that _one_ at least would be manifest in the total
extinction of those forms which could not adapt themselves to the
increased heat. Others, which were able with difficulty to endure the
alteration, would in all probability, even though they had now
emigrated to the loftiest peaks, flourish less vigorously than before;
and it is not unlikely, moreover, that they would become _somewhat
modified from their normal states_,--states which, be it recollected
(for this is an instructive lesson), would still exist in more
northern zones.
During my researches in mountain tracts, I have usually remarked, that
the highest points of land either teem with life, or else are
perfectly barren. My own experience would certainly tend to prove,
that, in a general sense, one or the other of these extremes does
almost constantly obtain. And, although I would not wish to dogmatize
on phaenomena which may in reality be explicable on other hypotheses,
it would perhaps be worth while to inquire whether the geological
movements of subsidence and elevation will not afford some clew to the
right interpretation of them. Be this, however, as it may, I can
answer, that in many countries, where there are strong indications of
the former, the alpine summits harbour an insect population to a
singular extent; whilst in others, where the latter is as distinctly
traceable, the upland ridges are comparatively untenanted. Now we have
already shown, that where the gradual lowering of a region has taken
place, there will be, of necessity, an undue accumulation of life on
its loftiest pinnacles,--for, even allowing a certain number of
species (which _even formerly_ were only just able to find a
sufficient altitude for their development) to have perished, we shall
have concentrated at that single elevation the residue of all those
which have survived _from the ancient elevations above it_. But, if,
on the other hand, an area, already peopled, be in parts greatly
upheaved, there will be _either_ a universal dying-out, from the cold,
of a large proportion of its inhabitants, or else an instinctive
striving amongst them to desert the higher grounds on which they have
been lifted up, and to descend to their normal altitudes: in both
cases, however, the present summits will display the same
feature,--namely, utter desolation.
Such are a few of the effects which elevation and subsidence, even on
a small scale, would seem (when tested by theory and practice) to
produce. It yet remains for us to suggest, that the latter, when
carried to its maximum, so as to cause the actual separation by the
sea of one district from another, is a contingency of immense
significance in regulating the distribution of the Annulose tribes.
Their outward contour and aspect we have shown in a previous chapter
to be very largely beneath the control of isolation, provided a
sufficient _time_ can be granted for the change: but their ultimate
absence from any particular place, through the impediment which it
offers to their migratory progress, we have not yet touched upon. Let
us conceive, therefore, an extensive continent; and, since the
insects which at present inhabit our earth must, if the doctrine of
specific centres be true, have been originally created in certain
definite spots, let us suppose a limited proportion of them to have
been first produced upon this tract. Self-dissemination, we will
assume, has been going on for centuries: those species which were
gifted with quick diffusive powers have become pretty evenly dispersed
over its surface; whilst those of naturally slow or sedentary habits
have peopled, comparatively, but small areas around the respective
localities of their birth. Such may have been the case, at some fixed
period, amongst the aboriginal beings of any country which we choose
to select as an illustration. But there is another element to be
considered. If this region be not insular, it will have received
colonists from foci of radiation situated beyond its bounds; and
these, therefore, according to their several capabilities for
progression, will have, likewise, in parts, overspread, or tenanted,
it. Now it is impossible to cite a more simple example than this. But
let us endeavour to realize what would be the necessary consequence of
the breaking up of such a district as that which we have imagined. If
a _general_ sinking should take place, causing its higher points to be
alone visible above the ocean, or merely a _partial_ one, so as to
admit of the sea encompassing portions of it which would remain
unaffected in their altitude; the result practically would be the
same,--namely, the constitution of a group of islands out of a once
continuous land. Then, as regards the animal population of this
tract, the main phaenomena are almost self-evident. Should any of its
isolated fragments chance to contain a portion of one of _those
limited areas_ which a species of slow progressive powers had
succeeded in colonizing, it would of course harbour (provided that the
other portion has disappeared) what would now be defined as _endemic_.
Numbers of these small areas, or, in other words, of the species which
had overspread them, would in all probability be lost for ever; whilst
the occurrence of any of the surviving ones in more than a single
island would manifestly depend on the proximity of the islands _inter
se_. Those forms which had diffused themselves over the whole original
continent would now be found in all the detachments of the cluster;
whilst others, which had wandered over the greater portion of it only,
might be traceable perhaps in every island _except a few_.
Such are the primary facts which suggest themselves, whilst discussing
the question of isolation as regulating the _distribution_ of the
Annulose tribes. Its _after effects_, on their external configuration
and development, we have examined in a preceding chapter of this
treatise; and we have also lately intimated what might be a few of the
presumptive consequences of a subsidence (in a general sense), _apart
from_ the still more important principle of isolation. Before,
however, we dismiss these brief and elementary reflexions on the
upward and downward movements which geology testifies to have
occurred, at various epochs, on the earth's surface, I shall perhaps
be pardoned if I digress so far from my immediate subject as to trace
out some of the actual results of isolation in the diffusion of the
Insecta (especially recognizable in the stoppage of a former migratory
progress) in a few of the northern Atlantic groups. I should premise,
however, that it is from the Coleoptera alone that I shall attempt to
draw my inferences; nevertheless, since that order is more extensive
than any of the others, and has moreover been closely investigated in
most of those islands, it may possibly afford us data of sufficient
comprehensiveness and accuracy for practical purposes.
To commence, then, with the Madeiras and Canaries; the first facts
which isolation discloses to us, concerning the statistics of a region
which was once continuous throughout that portion of the Atlantic, are
the _slowness_ and the _direction_ of the ancient migratory movements.
The former of these is rendered evident from the vast number of
endemic species which are at present contained, not merely in the two
groups combined, but in the several islands of which each of them is
composed. True it is, that these peculiar forms are, most of them,
apterous, and of naturally sluggish self-disseminating powers; yet,
still the circumstance remains, that these various creatures had not
overrun areas of any extent before the land of passage was
destroyed,--for otherwise they must have occurred, now, on islands and
rocks but slightly removed from each other, _which they do not_. The
latter of the above conclusions, namely, the _direction_ of the
migratory current, will become apparent in the sequel. We may premise
however, that, so far as the aborigines of this province are
concerned, their course will be found, upon the whole, to have been a
_northerly_ one.
As regards the slowness, and the direction, of the _quondam_ migration
(questions which can scarcely be treated apart from each other), some
light may be thrown on the subject from considerations like the
following. The Canaries are the head-quarters of the genus _Hegeter_;
Teneriffe may indeed be called the land of Hegeters. No less than
thirteen or fourteen species have been recorded as indigenous to those
islands; and there can be no reasonable doubt whatsoever that that
ancient region (when continuous and entire) was the primaeval centre,
or range, of that Heteromerous group. The Hegeters are an apterous
race, and of a sedentary temperament; hence, when the area (whether by
general or partial subsidence, it signifies not) was broken up, it is
not surprising that those local fragments of it should have become the
nucleus of reception, as it were, for the members of that genus.
Nevertheless, a few of these many representatives (of more discursive
capabilities perhaps than the rest) had found their way, before the
period of dissolution, to a considerable distance from their original
haunts. Thus, one of them (the _H. latebricola_, Woll.) had arrived at
what now constitutes the rocks of the Salvages; another (the _H.
elongatus_, Oliv.), at least, if not two, had colonized the Madeiras,
and is said (though I believe incorrectly) to have even reached the
present coast of Portugal. This latter species is clearly of a more
adaptive nature than its allies, inasmuch as it has, also, naturalized
itself (though this may be a more recent, and accidental,
circumstance) on the opposite shores of Africa. One thing, however, is
at any rate manifest,--that the Hegeters attain their maximum in the
Canaries, and that a few members only have been sent off, in a
northerly, or north-easterly, direction, from thence.
In like manner, the genus _Tarphius_ is distinctively Madeiran. I have
detected nearly twenty well-defined species of it in that group; yet,
out of so large a number, two only have occurred beyond the central
island. Now the _Tarphii_ are, also, wingless; and creatures of very
sluggish propensities,--scarcely ever stirring from the masses of
loose rotting timber which they so assimilate in hue, and to the under
sides of which they affix themselves, day and night. Although
difficult to investigate in their precise economy, it is extremely
probable (may I not say, certain?) that some important and peculiar
office is assigned to them in the remote upland districts to which
they exclusively belong: and there cannot be any question, to a person
who has studied them carefully on the spot, but that the region which
they now inhabit is the actual area of their primaeval appearance on
this earth. Many kindred species may of course have been lost, during
those gigantic subsidences which caused the Madeiras to be shaped out,
and to tell their tale above the waves as ruins of an ancient land;
yet our existing cluster of forms could not have wandered far at that
early period, from the Serras and ridges of their birth,--perhaps not
_so_ far indeed (considering the limited bounds within which they are
now confined, and that time should in reality have increased their
range rather than diminished it) as they have succeeded in doing at
the present day. Hence we may reasonably conclude, that Madeira proper
is an example of what we have alluded to in a preceding page,--namely,
of the accidental retention, during a vast downward movement, of a
nucleus of small specific areas of colonization, the colonizers of
which _had not extended elsewhere_. But I stated, that two of the
above-mentioned _Tarphii_ have occurred beyond the central mass. It is
in Porto Santo that they make their appearance; nevertheless, since
one of them is apparently peculiar to that island, it is only the _T.
Lowei_, Woll. (an insect of a different, and more active, nature than
the rest) which has violated that _local exclusiveness_ which would
seem to be almost a generic character, as it were, of its allies. That
species, however, both in its manners and aspect, recedes materially
from the remainder. Although, like them, nocturnal in its habits, it
is able to run with considerable velocity; and, instead of attaching
itself to the blocks of putrefying wood, which both fall and decay _in
situ_ on those elevated tracts, it hides within the bunches of
_Evernia scopulorum_ and _prunastri_ which clothe the trunks of living
trees, and fill up the crevices of the weather-beaten peaks. Hence,
when contrasted with its comrades, we can easily understand how the
varied processes of accidental transportation would operate to
increase the range of a creature which differs so essentially, in many
respects, from them. It is indeed, not unfrequently, brought down, at
the present day, by _human_ agencies from the mountain-slopes; for,
since the cutting of faggots is one of the few sources of livelihood
to a large proportion of the poor of Funchal, numerous insects of
subcortical and lichen-infesting tendencies are subject to be
naturalized (provided they can adapt themselves to the change) in
altitudes lower than their normal ones: so that there are many
chances, even _a priori_, in favour of the _T. Lowei_ having
overspread, whether by natural or artificial means, a wider area than
its congeners. I believe that there is no such thing as a _Tarphius_
in the Canarian Group: nevertheless, singularly enough, a
representative, which is more akin to the _T. Lowei_ than to any other
hitherto discovered (and which was imagined until lately to have been
the sole exponent of the genus), namely, the _T. gibbulus_, Germ.,
occurs in Sicily. From which data we arrive at this significant fact:
that, whilst Madeira proper is, without doubt, the original centre of
the _Tarphii_, two species (one of which is, likewise, Madeiran) are
found in Porto Santo, to the north-east of it; whilst a third makes
its appearance in an island of the Mediterranean.
The genus _Acalles_ presents a nucleus of species in the Canaries,
moulded on a very large pattern. A closely allied member, the _A.
Neptunus_, Woll. (which may perhaps be in reality but an insular
modification of the _A. argillosus_, Schoen., from Teneriffe), has been
detected on the rocks of the Salvages, to the north of them; whilst on
the Dezerta Grande, one of the most southern stations of the Madeiran
Group, we have a third, which displays far more in common with the
Canarian type than it does with that which obtains in Madeira
proper;--which last is gradually, in its turn, merged into the
ordinary European form. The genus _Pecteropus_, Woll., is another
instance in point. I possess three or four species from the Grand
Canary, Fuertaventura, and Teneriffe; and I believe it will be found,
on inquiry, to attain its maximum in that cluster. Unlike the others,
however, which we have just cited, it is powerfully winged; and we
should consequently expect to trace the evidences of its northward
progression with comparative perspicuity. Can we therefore do so? Yes:
in Madeira proper it has two representatives, and in Porto Santo (to
the north of it) one. And so with _Xenostrongylus_, Woll. (which is
likewise winged), we have two species, at least, in the Canaries; one
in the Madeiras; and a third, unless I am mistaken, in Sicily. The
genus _Ditylus_ is shadowed forth in the Canary Islands by two or
three singular representatives of a pallid, testaceous hue; and,
although the group is entirely absent in Madeira, a species (the _D.
fulvus_, Woll.) is found on the 'Great Piton' of the Salvages, so
nearly resembling, except in its smaller size, one of those from the
Canaries that I think it far from improbable that it is a fixed
insular state of that insect. _Deucalion_, also, may be quoted in
support of this twofold hypothesis, of the direction, and the
slowness, of the former migratory movements. It is an apterous genus,
and of eminently sluggish habits; and what is the consequence?--we
have a very remarkable species (the _D. oceanicum_, Woll.) on one of
the rocks of the Salvages, whilst another (the _D. Desertarum_, Woll.)
has been isolated on the two southernmost islands of the Madeiran
Group; and of so sedentary a nature is this last, that, although
physically unimpeded, it has not, even to this day, overrun the
diminutive areas on which, when the surrounding region was submerged,
it was originally saved from destruction. So strongly indeed was this
fact impressed upon me, when I first detected it, that I shall perhaps
be excused for recapitulating _in extenso_ the few reflexions which
then suggested themselves to my mind. "There is no genus, perhaps,
throughout all the Madeiran Coleoptera, more truly indigenous than
_Deucalion_. Confined apparently, so far as these islands are
concerned, to the remote and almost inaccessible ridges of the two
southern Dezertas, it would seem to bid defiance to the most
enthusiastic adventurer who would scale those dangerous heights. Its
excessive rarity, moreover, even when the localities are attained,
must ever impart to it a peculiar value in the eyes of a naturalist;
whilst its anomalous structure and sedentary[60] mode of life give it
an additional interest in connexion with that ancient continent, of
which these ocean ruins, on which for so many ages it has been cut
off, are the undoubted witnesses. Approximating in affinity to
_Parmena_ and _Dorcadion_, yet presenting a modification essentially
its own, it becomes doubly important in a geographical point of view;
and it was therefore with the greater pleasure that I lately
received a second representative, from the distant rocks of the
Salvages,--midway between Madeira and the Canaries. Differing widely
in specific minutiae, yet agreeing to an identity in everything
generic, they offer conjointly the strongest presumptive evidence to
the _quondam_ existence of many subsidiary links (long since lost, and
radiating in all probability from some intermediate type) during the
period when the whole of these islands were portions, and perhaps very
elevated ones, of a vast continuous land. * * * * * The _Deucalion
Desertarum_ is of the utmost rarity, the only two[61] specimens which
I have seen having been captured (the first by myself, in 1849; and
the second by the Rev. R. T. Lowe, in 1850) on the respective summits
of the Middle and Southern Dezertas. So local indeed does it seem to
be, that it, apparently, has not extended itself even over the Dezerta
Grande (where there are no external obstacles to bar its progress);
but retains the very position which in all probability constituted its
original centre of dissemination at the remote period of time when
this ancient continent received its allotted forms. Judging from the
slowness with which creatures of such habits must necessarily, under
any circumstances, be diffused, it is at least unlikely that the
present one could have circulated far, when the now submerged portions
of that region began to give way; and hence it is not impossible that
the Southern Dezerta, with the adjacent part (then united to it) of
the Central one, may have embraced the _whole area_ of its actual
primaeval range,--the remains of which (though they be now separated by
a channel) it still continues to occupy, and from which, even when
physically unimpeded, it has never roamed[62]."
Although it is not my province in this volume to draw inferences from
data which are not strictly entomological, I shall perhaps be pardoned
for adding a few words on the testimony which the Land Mollusca of the
Madeiras would seem to afford, in support of the general slowness of
the animal migrations over that primaeval continent. The researches of
the Rev. R. T. Lowe, and of myself, on every rock and island of the
group, have, it appears, so nearly exhausted the whole number of
species which lately remained to be found, that the conchological
statistics are perhaps, at the present time, more accurate than those
of any other department of the fauna: and, independently of the
modifications which have been manifestly brought about, in some few
instances, by isolation, since the periods of subsidence, it is truly
singular to remark how every detached portion of the entire cluster
harbours real species, which are now peculiarly its own. Thus (to
select an illustration from amongst the most anomalous of the endemic
forms), we have in Madeira proper, Porto Santo, and on the Southern
Dezerta, respectively, true representatives, in the _Helix tiarella_,
_coronata_, and _coronula_,--which in all probability still occupy the
positions (or nearly so) of their original _debut_ upon this earth.
Considering the sluggish, or sedentary, nature of the Terrestrial
Mollusks, it is extremely likely (nay, almost certain) that many
intermediate links, radiating from the same type, were lost for ever,
when the gigantic movements which rent this ancient region were in
course of operation: so that, if such were in reality the case, we
need not be surprised that one at least of this small geographical
nucleus should have been preserved on three of the existing islands of
the group. That these are actual species (saved alive from their
fellows, after the wholesale destructions in this Atlantic province
had been completed), and no results of insular development, is
demonstrated by the fact that two of them (for the third has
apparently become extinct) have not altered one iota since the _fossil
period_, which, in the opinion of Sir Charles Lyell, is anterior to
the dissolution of the intermediate land;--whereas, had they been mere
modifications of each other, induced by the local conditions and
influences to which they have been, through a long series of ages,
severally exposed, the difference between their recent contour and
that of their fossil homologues would have been doubtless at once
conspicuous. I gather, therefore, that like the _Tarphii_, to which we
have lately drawn attention, they are veritable surviving members of
an esoteric assemblage which found its birth-place on this
post-miocene (?) tract.
In a similar manner, the _H. undata_ in Madeira proper, the _H.
Vulcania_ on the Dezertas, and the _H. Porto-sanctana_ in Porto Santo,
are representative species,--each occupying the same position, and
being equally abundant, on their respective islands: and, although it
may be a problem whether the second of these is not an insular
modification of the first (or _vice versa_); yet, with the analogy of
the three already mentioned before us, I am inclined _a priori_ to
view it as distinct. These, also, occur in a subfossil state; and no
alteration appears to have been brought about, by either circumstances
or time. And so it is with numerous others (as the _H. latens_ in
Madeira, and the _H. obtecta_ in Porto Santo; the _H. squalida_ in
Madeira, and the _H. depauperata_ in Porto Santo; the _H. Delphinula_
in Madeira, and the _H. tectiformis_ in Porto Santo), which are no
less representative _inter se_. From which we are driven to
conclude;--first, that this _quondam_ continent was densely stocked at
the beginning with foci of radiation created expressly for itself[63];
and, secondly, that the areas which these various creatures had
overspread, before the land of passage was broken up, was extremely
limited,--or, which amounts to the same thing, that _their migratory
progress was unusually slow_.
Touching the two-fold question, of the _local engagement_ of this
Atlantic district with specific centres of diffusion, and the _extreme
slowness of their diffusive progress_, much instruction may be derived
from a contemplation of the conchological statistics. Porto Santo, for
instance, is a very small island (not more than seven miles in
length), yet the number of endemic species which it includes is so
perfectly astounding that it may be appropriately termed a _generic
area of radiation_. Nor does this primaeval excess of its aboriginal
beings strike us more forcibly than does the utter quiescence (if I
may so express it) which has been going on amongst them since the
remote era of their birth. Although a few have apparently died out[64]
since that epoch, consequent perhaps on the change of level and
diminished range which took place during the process of subsidence; we
are amazed to find that certain species which are now limited to
particular spots (even whilst unopposed by physical barriers) have
been absolutely peculiar to them from the first,--or, in other words,
that, whilst the fossil deposits extend throughout the lower regions
of the island, far and wide, it is only in those respective portions
of the beds which join on to the present "habitats" that the fossil
homologues of several of the species are to be met with. The _H.
Wollastoni_ is eminently a case in point. That most interesting of the
Madeiran mollusks was first detected by myself on the southern ascent
of the Pico de Conseilho, of Porto Santo, April 22, 1849; and the
subsequent explorations of the Rev. R. T. Lowe, in conjunction with my
own, have, I think, satisfactorily proved that it occurs nowhere else
except upon that single slope. Throughout the large expanse of
calcareous incrustations which are spread over the island elsewhere,
and on the adjoining Ilheo de Baixo, all of which teem with shells, I
think I may assert, without fear of contradiction, that the _H.
Wollastoni_ does not so much as exist. Yet at the Zimbral d'Areia,
which the Pico de Conseilho directly overhangs,--a rich tract for
these fossil remains,--as well as in the muddy composition of a cliff
near at hand, it literally abounds.
In like manner, we might recall many others which are peculiar,
_recent and fossil_, to the self-same precincts. Such, for example,
are the _H. calculus_ and _commixta_, which swarm on the summit of the
Ilheo de Baixo, in both states. The _H. attrita_, again, is the Pico
d'Anna Ferreira modification of the _H. polymorpha_; and it is only in
the beds towards the base of that mountain that its fossil homologue
is found. But what do these facts indicate? Surely they tell us
plainly of what we have already so often insisted upon,--namely, the
redundancy of this once continuous land with specific foci of its own,
and the sluggish or sedentary nature of those primaeval radiating
forms.
We must not however omit to notice, that some few of these endemic
_Helices_ appear to have been gifted (as we should _a priori_
anticipate) with more rapid capabilities for diffusion than the rest.
Thus, the _H. erubescens_ and _paupercula_ seem not only to have
colonized the entire province of which the Madeiras are detached
fragments, but to have even found their way to that distant portion of
it which now constitutes the Azores. The _H. polymorpha_ has also
penetrated the Madeiran region throughout; and being, like the _H.
erubescens_, peculiarly sensitive to the action of external
influences, we perceive, in consequence, that almost every island and
rock has now its own especial phasis of it. So greatly indeed is that
species beneath the control of local circumstances, that the very
districts of an island as insignificant as Porto Santo have each their
separate races to boast of. On the Pico d'Anna Ferreira it assumes a
form to which the name of _H. attrita_ has been applied; when on the
Ilheo de Baixo, it is the _H. papilio_; at the Zimbra d'Areia, on the
Pico de Conseilho, and in the Ribeira da Coxinha, it is the _H.
pulvinata_; and, in many other situations widely removed _inter se_,
it puts on the shape (variable, both in size and hue) to which the
title of _H. discina_ has been given. But, if we leave Porto Santo,
and follow this Protean _Helix_ into the other divisions of the group;
we meet with it on the Dezertas as the _H. senilis_ (those moreover
from the central island having a much more open umbilicus than is the
case in the northern and southern ones), whilst in Madeira proper it
constitutes the _H. lincta_ (with an additional pale variety for the
calcareous district of Canical),--and the _H. saccharata_, from the
Sao Lourenco promontory.
In the same way we might pursue the _H. erubescens_, and show that in
the sylvan regions, and on the low barren Ponta Sao Lourenco of
Madeira, on the Pico de Facho of Porto Santo, on the Ilheo Chao, on
the Central Dezerta, and on the Bugio (where it attains a gigantic
size), it has its distinct and permanent phases,--the evident results
of isolation, and other topographical influences, since the
subsidence of the intervening tracts. And in like manner, the
_Clausilia deltostoma_ is universal throughout the Madeiran
Archipelago,--displaying, however, in Porto Santo a fixed and strongly
ribbed state, peculiar to that island. Thus, if the examples which we
previously cited tend to establish the extreme slowness of the
migratory movements of the terrestrial mollusca across this former
continent, the present ones (which refer to a few exceptional species
of quicker self-diffusive powers) will show, no less than the
_insects_ to which I have lately called attention, that where
sufficient areas had been overspread (before the periods of
subsidence) for the creatures to have reached what now constitute the
various islands of the cluster, we at once detect traces of this fact,
through their more or less altered aspects,--the result of isolation,
and diminished range, during the enormous interval which has elapsed
since the successive convulsions which caused the partial destruction
of this Atlantic province were brought to a close.
To return, however, to the insects, after this long conchological
digression,--I need not multiply evidence, in corroboration of my
theory. Enough has been said to render intelligible the idea which I
wished to convey, concerning the _general direction_ of the migratory
current over that ancient tract, and the _extreme slowness of its
progress_,--the former of which I consider probable from the
north-easterly course in which creatures _generically identical_ were,
if we may so express it, "given-off;" whilst the circumstance of their
being for the most part _specifically dissimilar_ (or, in other words,
of the islands harbouring, many of them, species which are endemic)
would seem as it were to establish the latter.
We must not however forget, that it is only to the _aborigines_ of
this _quondam_ land that the above speculations apply. Assuming the
region not to have been insular, that is to say, to have been
connected, on its outer limits, with a European, or Mediterranean,
continent; it would necessarily follow, that a certain number of
colonists must have found their way over its area, and moreover _in an
opposite direction_ to the living stream (if we may so call it) which
had been long flowing in a north-easterly course across its surface.
Whatever be the length of the periods, however, during which these
counter migrations were going on, I think it sufficient to state that
I would refer them to epochs altogether different,--so that,
accompanied as they may have been by special geological phaenomena,
which, if known, would in all probability become at once explanatory,
we should be the less inclined to regard as absurd what might appear
at first sight difficult to understand. In the case of the British
Isles indeed, no less than five of these distinct migratory eras have
been assumed, and specified[65], by Professor Edward Forbes; therefore
(whatever value be attached to his able and interesting theory) I do
not consider it necessary to apologize for requiring _at least two_ in
behalf of this ancient Atlantic province. Not to insist upon those of
his faunas and floras which are of a less evident, or more
questionable, character, he has at any rate proved, I think, almost to
a demonstration, the _westward progress_ of the great mass of our
British animals and plants, over a then unbroken land (the upheaved
bed of the glacial sea), from the central Germanic plains; whilst the
accurate calculations of the late Mr. Thompson of Belfast, concerning
the reptile statistics of Ireland, England, and Belgium, respectively,
have succeeded in showing, with much presumptive reason, how the
formation of St. George's Channel, _before_ that of the German Ocean,
interrupted the march of these wanderers to the far West, and debarred
an immense proportion of them from an entry into Ireland,--which would
otherwise have colonized that country equally with our own.
As regards Professor Forbes's views of the creation of a vast
continent (reaching far into the Atlantic[66]) at the close of the
miocene epoch, through the upheaved bed of a shallow miocene sea,--a
region moreover of such an extent as to have connected the various
island groups between the Fucus bank and the shores of the Old World,
not only with each other, but with a Mediterranean province, Asturias,
and even the south-west of Ireland,--I must be content to pass them
by, hazarding only a few crude and desultory remarks. So large a
question, indeed, cannot be safely handled without a corresponding
amount of data, in all departments of natural science, to reason
from,--which I do not possess: still, if a speculation from
entomological premises, _per se_, be not altogether worthless, I would
point to the conclusions (lately adverted to) which my Madeiran
researches have forced upon me, concerning the _direction_ of the
former insect migrations,--inferences which are, from first to last,
of necessity erroneous, if the requisite medium for transit (into
South-European latitudes, at all events) be a mere conjecture or
romance. Such a notion, however, I would not for a moment
entertain,--for there is too much direct evidence in support of
distinct epochs of diffusion, to allow of any hypothesis, when
endeavouring to account for the phaenomena which we now behold, to
supersede the assumption of a once continuous tract. No matter if we
be compelled to suppose, whilst attempting to interpret what we see,
that the disseminating current has flowed in exactly opposite courses,
at different and remote periods, over the surface of that ancient
land,--seeing that the _fact_ (if such in reality it be) remains
untouched, that _the land itself is_ at any rate _there_. I am not,
however, prepared to assert that the opinion at which I had
independently arrived, from the insect statistics, does positively
require a northerly prolongation of that area beyond the line of the
central Mediterranean districts; yet, after making every possible
allowance for accidental introductions since the subsidences have
taken place, there is still left a large residuum which I am convinced
can never be explained (unless the doctrine of specific centres be a
myth) except through the means of ordinary and regular migration over
an unbroken continent. Nevertheless, though I would not presume, from
insufficient material, to insist upon an extension of this Atlantic
region into higher latitudes than those which I have just referred to,
I must express my individual belief that, the more the subject is
examined, with reference to the distribution of the Annulosa, the
less will Professor Forbes's idea suffer from the inquiry. In the
'Insecta Maderensia,' I have already thrown out a few scattered hints
which bear on this immediate consideration; and, since no subsequent
reason has induced me either to withdraw or modify them (but rather
the reverse), I will select the following,--extracted from my preface
to that work.
"Taking a cursory view of the Coleoptera here described, the fauna may
perhaps be pronounced as having a greater affinity with that of Sicily
than of any other country which has been hitherto properly
investigated. Apart from the large number of our genera (and even
species) which are diffused over more or less of the entire
Mediterranean basin, this is especially evinced in some of the most
characteristic forms,--such as _Apotomus_, _Xenostrongylus_,
_Tarphius_, _Cholovocera_, _Holoparamecus_, _Berginus_, _Litargus_,
_Thorictus_, and _Boromorphus_. There is, moreover, strange though it
may appear to be, some slight (though decided) collective assimilation
with what we observe in the south-western extremity of our own country
and of Ireland,--nearly all the species which are common to Madeira
and the British Isles being found in those particular regions; whilst
one point of coincidence at any rate, and of a very remarkable nature,
has been fully discussed under _Mesites_. Whether or not this partial
parallelism may be employed to further Professor E. Forbes's theory of
the _quondam_ approximation, by means of a continuous land, of the
Kerry and Gallician hills, and of a huge miocene continent extending
beyond the Azores, and including all these Atlantic clusters within
its embrace, I will not venture to suggest: nevertheless, it is
impossible to deny that, so far as the Madeiras betoken, everything
would go to favour this grand and comprehensive idea. Partaking in the
main of a Mediterranean fauna, the _northern tendency_ of which is in
the evident direction of the south-western portions of England and
Ireland, and with a profusion of endemic modifications of its own
(bearing witness to the engorgement of this ancient tract with centres
of radiation created expressly for itself), whilst geology proclaims
the fact that _subsidences_ on a stupendous scale have taken place, by
which means the ocean's groups were constituted; we seem to trace out
on every side records of the past, and to catch the glimpses, as it
were, of a _veritable_ Atlantis from beneath the waves of time[67]."
The _Mesites Maderensis_, Woll., to which I alluded in the above
quotation, is undoubtedly a strong case in point. Although
specifically dissimilar from the _M. Tardii_, its Irish counterpart,
it nevertheless approaches it so closely, that it might be literally
mistaken, _prima facie_, for that insect; and we know that it is one
of the plans on which Nature commonly proceeds, that species which are
not merely representative of (or analogous to) each other, but which
are actual homologues, or allies, should usually emanate at first from
foci not far removed _inter se_; or, at all events, if distant,
connected by an intervening land:--in other words, that _generic
areas_, no less than specific centres, of radiation, form a
substantial item of the comprehensive scheme on which the system of
created things was originally planned. We detect traces of this
primary law in each division, or class, of the organic world; nor is
its reality _as a law_ interfered with, through the occasional
exceptions which are liable, as in every other instance, to present
themselves. Such deviations are often easily to be accounted for,
whether by natural or artificial means; and do not affect the
subject, as a whole. Sometimes indeed they become at once intelligible
from the historical records connected with them, proving that human
agencies have been at work acting as transporting media, within a
period comparatively recent; whilst at others, the fact of the
creature having been endowed with self-diffusive powers to an
extravagant degree may succeed equally in rendering the phaenomena
explicable. But, even where neither of these solutions would seem to
suffice, we should still recollect that it is only in the mass that
such questions can be pronounced upon; and that, consequently, where
we are able to discover a rule which is _for the most part_ adhered
to, it is more philosophical to conclude that the departures from it
are the result of special disturbing causes (whatsoever they may have
been), than to permit them to undermine our faith in what would be
otherwise universally true. Thus, the botanist tells us of Ixias,
Stapelias, Mesembrianthemums, Pelargoniums, and Euphorbias, as
concentrated in Southern Africa; of Magnolias in Central America; of
Calceolarias on the Andes; of Myrtles, Banksias, Mimosas, and
_Eucalypti_, in Australia; and of the Bread-fruit Trees in the South
Sea Islands: the ornithologist points, _inter alia_, to the Toucans
and Humming-Birds from South America and the West Indies; whilst the
student of the higher animals informs us of the Kangaroos (indeed of
the whole of the subclass _Marsupialia_, except the genus _Didelphys_)
as peculiar to Australia and a few islands to the north of it; of
_Lemur_ _proper_ to Madagascar; of the Sloths, Armadillos, Tree
Porcupines, and of Alligators, and of the _Platyrrhini_ (amongst the
Monkeys), to South America; and of the Ourangs to the islands of the
Indian Archipelago.
And so it is with the Insecta; many of the larger groups of which (as
_Amycterus_ and _Paropsis_, in Australia; _Pachyrhynchus_ and
_Apocyrtus_, in the Philippine Islands; _Hipporhinus_, _Monochelus_,
_Dichelus_, and _Moluris_, in Southern Africa; _Macronota_, in Java;
and _Naupactus_, _Hypsonotus_, _Centrinus_, _Platyomus_, and
_Cyrtonota_, in South America) are confined to countries of
proportionate magnitude, whilst the smaller ones are more commonly (as
it were) shaped out for special provinces or regions, according as
local circumstances may require primary adaptations to harmonize with
them. Thus, whilst we frequently find an extensive genus diffused over
the greater portion of the known world, we perceive that even its
_structural_ characteristics are not uniform throughout, but afford
fixed geographical modifications (_not_, in this case, however, the
effect of development),--which have often, in their turn, obtained the
name of 'genera,' and have been described as such. Whether genera,
however, or not, they are undeniably small topographical assemblages,
satellites around their central types; and they may therefore be
safely regarded as genera, if we choose to view them in that light. Of
such a nature I have already pointed out[68] is _Saprinus_, as
compared with _Hister_; _Atlantis_ with _Laparocerus_; and _Oxyomus_
with _Aphodius_; and, I might also add, _Mesites_ with _Cossonus_. I
believe indeed that _Mesites_ will be found to attain its maximum on
the Pyrenees (I already possess two or three species, in abundance,
from that region); and, if such should be the case, we shall be able
to appreciate the significance of two representatives so closely
allied as the _M. Tardii_ and _Maderensis_,--one of which has been
given off in the direction of Ireland, and the other of the Madeiran
Archipelago.
But I will not digress further on the subject of this Atlantic
province; since, however much I may individually regard it as a
reality of the past (which the Coleopterous statistics have compelled
me to do), it must of necessity remain, as heretofore, a matter of
much controversy and doubt. I should indeed apologize for having
trespassed on the reader's attention, in wandering this far from the
immediate results of _subsidences_,--which I proposed, at the outset
of this chapter, to examine, with reference to the impeded diffusion
of the Annulose races. Nevertheless, concluding that a practical
illustration of the effects of one of those great downward movements
to which geology so repeatedly bears witness would not be irrelevant
to the _assumed consequences_ which I had previously ventured to
define, I have acted on that judgment; and, having finished my task,
will now proceed to notice, briefly, a few other considerations which
should not be omitted, when inquiring into insect distribution as
influenced by geological phaenomena.
Next in importance, perhaps, to the elevations and sinkings (traces of
one or the other of which are more or less manifest in almost every
region of the world), _natural barriers_ may be cited,--as presenting,
not unfrequently, insurmountable obstacles to the self-dissemination
of the insect tribes. By natural barriers, however, I would be
understood to imply natural _primary_ barriers,--or, in other words,
such as have continued as barriers ever since the present animals and
plants came into existence upon the earth. For, the _ocean_ (by way of
illustration) is a natural barrier; and yet it is not necessarily a
primary one, as may be readily gathered from the above remarks, in
which the results of _subsidences_ are discussed,--subsidences which
have had the effect of letting it in over portions of an _already
tenanted_, and unbroken, continent. Mountain-chains, also, are
barriers; but it may happen that they have not been so from the
beginning,--as in instances, for example, where they have been
gradually upraised during periods geologically recent. But both sea
and alpine ranges are barriers, when (as usually happens) they have
remained as such since the creation of the several species which now
inhabit our globe. Mr. Darwin has acknowledged this distinction,
whilst commenting upon the marked divergence of the faunas on the
eastern and western slopes of the Cordillera. "This fact," says he,
"is in perfect accordance with the geological history of the Andes;
for these mountains have existed as a great barrier since the present
races of animals have appeared; and therefore, unless we suppose the
same species to have been created in two different places, we ought
not to expect any closer similarity between the organic beings on the
opposite sides of the Andes, than on the opposite shores of the ocean.
In both cases, we must leave out of the question those kinds which
have been able to cross the barrier,--whether of solid rock or
salt-water[69]."
Conceding, therefore, this distinction between barriers of a primaeval
and more recent character, it is not difficult to understand why the
opposite sides of an alpine chain, as well as countries separated by
the sea, should display different phaenomena from each other. On the
contrary indeed, if we could feel satisfied that no means of
accidental transportation had operated to take them there, and that
the animals themselves were incapable of enduring great diversities of
temperature, and other contingencies; we should be startled to
discover creatures specifically identical in such regions,--so long at
least as the doctrine of unique centres of radiation formed part of
our zoological creed. We must not, however, be too hasty in
questioning (if I may be pardoned for the completion of a metaphor of
which I thoroughly disapprove) this article of our faith, through the
occurrence of similar beings in areas between which there exist
barriers, both primary and well-defined; for the methods of diffusion
are so complicated and numerous, that, even where human agency (that
most important of elements) is not concerned, what at first sight may
frequently appear to be impossible becomes clear enough when more
critically inquired into. Some species, we know, are gifted with
greater powers for horizontal and vertical progression than their
comrades, and can (though they are doubtless exceptions to the general
rule) pass through extremes of atmosphere sufficient to render even
lofty mountain summits no obstacles to _them_. Others, as the
_Calosoma Syncophanta_ of Europe, have been stated to traverse the
ocean unhurt[70]; and I believe that many do at times accidentally
arrive, in a half-drowned state, especially after boisterous weather,
across channels of considerable breadth. Mr. Kirby, on examining the
marine _rejectamenta_, during one of these apparent occurrences, along
the Suffolk coast, writes as follows: "Whether the insects I observed
upon the beach, wetted by the waves, had flown from our own shores,
and, falling into the water, had been brought back by the tide; or
whether they had succeeded in the attempt to pass from the continent
to us, by flying as far as they could, and then falling had been
brought by the waves, cannot certainly be ascertained; but Kalm's
observation inclines me to the latter opinion[71]." And Sir Charles
Lyell remarks:--"Exotic beetles are sometimes thrown on our shore,
which revive after being drenched in salt water[72]." Nor should we
forget that chance agencies of every description, which we are too apt
to overlook, are daily at work (and have been so since, at any rate,
the last creative epoch) to transport these variously organized beings
beyond their original spheres. Sometimes they are carried on, or
within, the bodies of larger animals, which is especially the case
with the parasitic tribes; at others on floating trunks of trees, and
casual substances of divers kinds, which are able to resist for a
definite period the destructive action of an element saturated with
salt. Unwilling victims, again, are ever and anon hurried to
comparatively distant lands by the very winds that blow; and not only
to distant lands, but over altitudes in which the severity of the cold
would quickly annihilate them, were they (as perhaps usually happens)
to be deposited there on their headlong and compulsory course. "As
almost all insects are winged[73]," says Sir Charles Lyell, "they can
readily spread themselves wherever their progress is not opposed by
uncongenial climates, or by seas, mountains, and other physical
impediments; and _these_ barriers they can sometimes surmount by
abandoning themselves to violent gales, which may in a few hours carry
them to very considerable distances. On the Andes some sphinxes and
flies have been observed by Humboldt, at the height of 19,180 feet
above the sea, and which appeared to him to have been involuntarily
carried into those regions by ascending currents of air[74]." With
respect to the accidental conveyance of numerous species across the
sea, it is not to the winds alone that we must look for an
explanation. Large and rapid rivers are liable to inundate their banks
and bring down insects in prodigious masses,--which are disgorged into
the ocean, and carried to a distance from the coast, in proportion to
the violence of the ejecting stream. When the body of water is
considerable, the sea becomes diluted to an unusual extent; and
creatures which must have otherwise perished, from the action of the
salt, are able to survive for a time, and may be deposited, by means
of rapid currents into which they are borne, on neighbouring islands
and continents. Even the _Hydradephaga_ are thus occasionally
transported; for Darwin mentions having captured a _Colymbetes_ off
Cape S^{ta} Maria (to the north of the Rio de la Plata), when
forty-five miles from the shore. And, in his 'Journal of Researches,'
he records the following remarkable facts, which bear upon this
immediate question. "On another occasion, when seventeen miles off
Cape Corrientes, I had a net overboard to catch pelagic animals. Upon
drawing it up, to my surprise I found a considerable number of beetles
in it, and, although in the open sea, they did not appear much injured
by the salt water. I lost some of the specimens; but those which I
preserved belonged to the genera _Colymbetes_, _Hydroporus_,
_Hydrobius_, _Notaphus_, _Cynucus_, _Adimonia_, and _Scarabaeus_. At
first I thought that these insects had been blown from the shore; but
upon reflecting that, out of the eight species, four were aquatic (and
two partly so) in their habits, it appeared to me most probable that
they were floated into the sea by a small stream which drains a lake
near Cape Corrientes. On any supposition, it is an interesting
circumstance to find live insects swimming in the open ocean seventeen
miles from the nearest point of land[75]."
Accidental means of dissemination, such as those to which I have just
alluded, and others to which we might appeal, will generally account,
and with much presumptive truth, for the many exceptional cases which
present themselves, during our investigation into the effects of
natural barriers, as visible in the distribution of the Annulose
races, on the earth's surface. I say "exceptional cases," because any
one who has laboured practically in mountain tracts cannot have failed
to recognize the marked difference which is often displayed by the
insect population on opposite sides of some alpine chain; whilst he
whose lot has been cast amidst island groups, will have become even
more conscious than the former of the permanency of those impediments
which have been placed (in this instance by the broad arms of the
mighty ocean) as checks upon a too rapid system of diffusion.
But if the sea and mountain ranges, when of a sufficient age _in
situ_, are amongst the most effectual of Nature's barriers against the
self-dispersion of the animate tribes; it follows that, if the two
could be (as it were) _united_, we should have found the greatest
obstacle which physical conditions can ordinarily present against the
wandering capabilities of the latter. The question therefore
arises,--Is it possible for them to _be_ so joined? Undoubtedly it is:
and hence we arrive at the conclusion, that a _mountain island_ should
afford us the _minimum of size, as regards the areas its species have
overspread_, which any country is able to furnish.
Madeira is a mountain island,--its highest peaks rising, although
resting on so small a base, to an altitude of more than 6000 feet. Yet
it is only partially a case in point; for, although it was a mountain
mass, and perhaps a very elevated one, when its endemic beings made
their first appearance upon its surface, we have already intimated
that it has become isolated _since_ that epoch: so that, whilst _one_
of the natural barriers against dispersion which it involves (namely,
mountain ridges) may be considered as primary; the _other_ (to wit,
the sea, as it now obtains) has played, as an agent of obstruction,
but a secondary part. Still, there is good reason to believe that the
ancient tract of which it is a portion was broken up at a
comparatively early date after the creation of those peculiar organic
forms which found their birthplace within its bounds; and that,
consequently, the latter could not have wandered far (if we except
those species on which unusual powers of diffusion were bestowed) when
the land of passage began to give way. Hence, even the sea, in this
particular instance, partakes almost of the character (no less than
the mountain heights) of an original impediment; and Madeira therefore
may be safely quoted as an example in which two barriers, of a primary
nature, are united; and where, consequently, we may anticipate those
ultra phaenomena of _areal limitation_ upon which we have been just
commenting.
But let us now inquire, whether the hypothesis at which we have
arrived will stand the test of experience; for unless it will do so,
we might have been spared the labour of propounding it. Madeira is a
country composed of narrow mountain ridges, which radiate from central
crests, and form the lateral boundaries of deep and precipitous
ravines. Modifications of this structural type are of course traceable
everywhere; the upland tracts are often undulating and broad, and the
buttresses which slope towards the sea are sometimes expansive and
irregular: yet upon the whole the above description is correct, and we
may accept it in a generic sense. Now we may premise that, even to
this day, it is an island of floods; therefore, how much more must it
have been so when its primaeval forests, in all their splendour, caused
an amount of exhalation and moisture of which at present we can have
but a remote conception! Hence, it is hardly to be imagined, that
(however limited may have been the naturally acquired areas of those
of its inmates which are most sluggish and sedentary) a fusion would
not have taken place, in the course of ages, so as to render its
modern fauna, in a large measure, homogeneous throughout. Yet, in
spite of this esoteric tendency, it is surprising how little
amalgamation has been effected amongst the tenants of its several
districts. Scarcely a gorge or woodland serra exists within its bounds
which does not harbour some species essentially its own; and in many
instances the ranges of these creatures are so local or confined, that
they might be easily overlooked even in their respective
neighbourhoods. It is certain, however, that the floods (which happen
periodically) have done considerable work in naturalizing many of the
subalpine forms, which could adapt themselves to the climatal change,
in altitudes below their normal ones: and, in the north of the island,
where the temperature is cooler than on the opposite side, and where
the lofty defiles terminate, even at their lowest outlets, in abrupt
precipices along the coast, so that the _rejectamenta_ during the
annual rains are brought into direct contact with the shore, this
gradual process of deportation is particularly evident,--a
circumstance to which I have already alluded elsewhere[76].
But, after making due allowance for these powerful means of
dissemination (which, in the common order of things, must necessarily
obtain in _mountain islands_, as it were, _par excellence_), the fact
still remains, that in the Madeiran Group the acquired areas, even up
to the present date, of a vast proportion of the insect inhabitants,
are wonderfully circumscribed. The real state of the case, however,
would appear to be simply this: that the floods, although they may
have tended to diffuse the members of a comparatively uniform alpine
fauna in the various clefts or gorges beneath, can have had no power
to combine the aborigines of the several gorges themselves; and, since
a large proportion of the endemic species of those islands are (as I
have previously stated) apterous, the perpendicular edges of the
ravines, which in many instances rise to an elevation of 2000 feet,
have acted (and ever _will_ act) as impassable barriers to vast
numbers of the insect tribes.
With this single example (by way of illustration), which the Madeiras
have supplied, I will take my leave of the question of _natural
barriers, as tending to regulate the topographical diffusion of the
Annulosa_,--feeling that I have already devoted too much time and
space to this portion of the subject (if such indeed it be) which I
had proposed in the present treatise to discuss. Other barriers might
have been adverted to,--such as large rivers, extensive deserts, and
thickly set forests (especially of pine-trees, which frequently offer
a very decided impediment to insect progress),--but they are of
secondary importance, when compared with marine and alpine ones; and
their consequences may be, to a certain extent, deduced from the
considerations which I have just entered into. My main object has been
to draw attention to the fact, that the great obstacles which Nature
has placed against the too rapid dispersion of animal life should be
more strictly taken into account (as a matter of positive reality)
than it is, during our investigations into entomological geography. To
be aware that these barriers exist, and yet to feel surprised,
especially in a country where the species are principally wingless,
that we do not discover indications of a general uniformity in its
fauna, involves an absurdity,--unless the doctrine of specific centres
of creation be a mere coinage of the brain. But, if we believe in that
theory (which, until it can be shown to be impossible, I hold that we
are _a priori_ bound to do), we must at least act consistently with
ourselves, and not anticipate phaenomena where we have neither reason
nor right to look for them.
We are too apt to draw a line of imaginary demarcation between the
sciences, as though each had its own propositions to establish, and
nothing more: indeed, some of us would appear to assume (though
perhaps tacitly), that what is proved to be true in one department may
be, at least, rendered inconsistent (if not actually negatived) in
another. But surely this requires no argument to refute,--since a
_principle_ which is _true_, is true under every circumstance and
condition; for otherwise, it could be both true and false. We need not
therefore be afraid of comparing truth with truth, under whatever
shape it may arrive, as though it were possible that either of its
phases could ever suffer from the ordeal of a close contact; since, if
they be really true, and free from deception, they must needs go hand
in hand, and _may_ become (however opposite they be in their subjects)
directly explanatory of each other. The astronomer who is not
intimately acquainted with pure mathematical analysis, in its various
aspects and bearings, is in fact no astronomer at all. The geologist
who would interpret the grand phaenomena of the earth's crust apart
from statical and dynamical knowledge, and without the help which the
chemist, mineralogist, anatomist, zoologist, and botanist can afford
him, stands a fair chance of leaving his problems unsolved; whilst the
students of zoology and botany who would endeavour to understand, and
account for, what they see in the animal and vegetable worlds around
them, without calling in geology to their aid, must assuredly be
prepared to fail signally in their attempts. All indeed must work in
concert, if the whole is to be advanced,--and not only in concert, but
as mutually assisting each other. "By the help of truths already
known, more may be discovered; for those inferences which arise from
the application of general truths to the particular things and cases
contained under them, must be just.[77]"
FOOTNOTES:
[60] "When we consider indeed the apterous nature of _Deucalion_, its
subconnate elytra, and its attachment (at any rate in the larva state)
to the interior of the stems of particular, local plants, or its
retiring propensities within the crevices of rocks; we are at once
struck with the conviction, that, during the enormous interval of time
which has elapsed since the mighty convulsions which rent asunder
these regions terminated, it has probably never removed many yards
from the weather-beaten ledges which it now inhabits."
[61] Since the above was published, I have succeeded in detecting one
more example,--namely (in June 1855) on the summit of the Ilheo Bugio,
or Southern Dezerta, within a few yards of the self-same spot where it
was found by the Rev. R. T. Lowe in May 1850. Although I searched
diligently on the Dezerta Grande, during my late campaign in the
Madeira Islands, I was not able (so great is its rarity) to discover
farther traces of it on that rock.
[62] Insecta Maderensia, p. 435.
[63] It would seem, when viewed on a broad scale, as if particular
districts throughout the world had been made as it were the special
fields for the exercise of the creative force,--or that, _generic
areas of radiation_ were part of the elementary design. Thus,
Professor E. Forbes records his belief that most, if not indeed _all_,
of the terrestrial animals and plants now inhabiting Britain are
members of specific centres beyond bounds,--they having migrated to it
over a continuous land, before, during, or after the glacial epoch.
Hence, since the greater number of them are supposed to have come from
the central Germanic plains, we may assume that those plains were one
of the primary areas of diffusion for a large mass of created beings.
There is good cause for suspecting that the Pyrenean region may have
been another; and certainly all evidence would tend to prove that this
vast Atlantic province was, also, well stocked with aboriginal forms.
[64] Assuming the _Helix Lowei_ and _Bowdichiana_ to be gigantic
phases of the _H. Portosanctana_ and _punctulata_, respectively; four
only, namely _H. fluctuosa_ and _lapicida_, _Achatina Eulina_, and
_Cyclostoma lucidum_ (the first three of which are extinct throughout
the entire group), seem to have altogether disappeared. Nevertheless,
the gradual dying-out, as it were, of species, both here and in
Madeira proper, is singularly evident. Thus, in the latter, the
Canical beds show the _H. tiarella_ to have been once most abundant
(it literally teems in those calcareous formations). Yet so rare is it
in a recent state, that, until the summer of 1855, when it was
detected by myself and the Rev. R. T. Lowe in two remote spots along
the perpendicular cliffs of the northern coast, it was supposed to
have been lost for ages. And the same may be said of its counterpart,
the _H. coronata_, in Porto Santo,--which, likewise, swarms in every
fossil-bed of that island; but which was, also, until I met with it,
on the 15th of December 1848, adhering to slabs of stone at a
considerable depth beneath the ground, on the extreme eastern peak
(opposite to the Ilheo de Cima), imagined to have long passed away.
And so, reasoning from analogy, I think it far from improbable that
the third representative of this little geographical assemblage,--the
_H. coronula_ of the Bugio (which has hitherto only occurred in the
mud deposits on the summit of that rock),--may be still alive, though
perhaps in very small numbers, on some of the inaccessible ridges of
those dangerous heights.
[65] Origin of the Fauna and Flora of the British Isles (in Mem. of
the Geol. Survey of Great Britain, vol. i. p. 336, A.D. 1846).
[66] "My own belief," says Professor Forbes, "is, that the great belt
of gulf-weed, ranging between the 15th and 45th degrees of north
latitude, and constant in its place, marks the position of the
coast-line of that ancient land."
[67] Although, for want of a better name, it may be admissible, when
speaking either figuratively or poetically, to allude to this former
region (as I have done in the above quotation) under the title of
"Atlantis;" yet it seems incredible that certain writers (assuming its
_quondam_ existence) should have recently referred to it seriously as
the possible "Atlantis _of the ancients_!" Considering that there is
good reason to believe that all these islands _were islands in a
miocene sea_, and that, if (through a general elevation) they were
subsequently connected, the land of passage was broken up long
anterior to the appearance of man upon the earth, "the ancients" must
have assuredly merited their appellation, if they could have thrown
any light on a problem which belongs to an epoch thus remote. Whether
the "Atlantis" had any being at all except in the imagination of the
Latin poets, or whether (as Lord Bacon has suggested) it was the New
World, will probably never now be known; yet the fact that the _Insulae
Fortunatae_ of Juba are almost universally identified with the present
Canarian Group (as indeed the accurate description of Pliny well nigh
demonstrates), and the _Purpurariae_ with the Madeiras, ought at once,
apart from geological evidence, to point out the absurdity of the
hypothesis, that an Atlantic continent, _in the very position which
those islands occupy_, could have been acknowledged to have any
existence by the literature of either Rome or Greece.
[68] Insecta Maderensia, p. 214.
[69] Journal of Researches, pp. 326, 327.
[70] Many of the _Calosomata_ would appear to possess this power of
crossing, either by flight or by abandoning themselves to the waves
(though more probably by the assistance of both), even marine barriers
with impunity. Numerous instances are on record to this effect; and I
am informed by Mr. Darwin that a _Calosoma_ flew on board the
'Beagle,' off the Bay of San Blas, in South America, whilst they were
ten miles from shore. It seems likely, therefore, that the occasional
occurrence of the _C. Syncophanta_ in our own country, along the
southern and eastern coasts, is due to this generic capability,--and
consequently (as indeed it is usually acknowledged to be), the result
of accident.
[71] Introduction to Entomology, ii. p. 13.
[72] Principles of Geology, 9th ed. p. 657.
[73] Although this is true on a broad scale, a reference to my
observations in a preceding chapter will show, that in some countries,
especially islands, the reverse will frequently be found to obtain.
[74] Principles of Geology, p. 656.
[75] Journal of Researches, p. 159.
[76] Insecta Maderensia, p. 81.
[77] Religion of Nature Delineated, pp. 73, 74.