which proximity to the shore plays, in all probability, an important
part) on the external aspect of insects exists in the _Aphodius
plagiatus_, Linn.,--which in this country is generally deep black. "It
is a circumstance worth noticing," I remarked in the 'Zoologist,' in
1846, "that the form which is looked upon by the continental
naturalists _as the variety_, is in England evidently the typical
one,--for out of about sixty specimens which I captured [at Tenby in
South Wales], only _two_ possess the conspicuous red dashes on the
elytra which are considered abroad as the almost invariable
accompaniment." I have observed the same peculiarity in the flat and
damp spots between the sand-hills at Deal, where I have never detected
a single individual which is not perfectly dark; and I believe that
the greater number of the specimens which were originally taken at
Wisbeach, in Cambridgeshire, offered the same geographical
characteristics; whilst those which were found near the more inland
towns of Peterborough and Norwich present a larger proportion of the
ordinary European state. The _blood-red dashes_, however, with which
the elytra of numerous insects are adorned, I have constantly remarked
possess a singular tendency to become evanescent. It is indeed almost
diagnostic of the genus _Gymnaetron_, either that its representatives
should be thus ornamented typically, or else that those which are
normally black should, _when they vary_, keep in view, as it were,
_this principle_ for their wanderers to subscribe to. Thus, I have no
doubt that the _G. Veronicae_, Germ., is but a variety of the _G.
niger_,--an opinion which I expressed in the 'Zoologist' nine years
ago. Whilst commenting on the Coleoptera of Dorsetshire, I then
stated, that "for my own part I must confess I should have doubted the
_G. Veronicae_ being really distinct from the _G. niger_, for red
dashes on the elytra seem naturally peculiar, more or less, to the
whole genus; and I should therefore have suspected that, had
occasional aberrations from a black type existed (which is not
unlikely), those aberrations would probably assume a form which is so
common in the other species of the generic group[26]."
The _Bembidium bistriatum_, Dufts., is usually much paler when found
in saline districts (under which circumstances it was described as a
distinct species by Mr. Stephens) than when occurring in more inland
positions. The _Blemus areolatus_, Creutz., I have frequently remarked
is similarly affected in brackish places: and I think it far from
improbable that the _Stenolophus Skrimshiranus_, Steph., is but a
local modification (though not altogether, perhaps, through marine
influences) of the _S. Teutonus_, Schr. The _Dromius fasciatus_,
Gyll., not being _exclusively_ littoral, may be quoted as another case
in point,--the specimens which are collected near the coast being for
the most part singularly pale. In speaking of the _Anthicus
bimaculatus_, Illig., M. de la Ferte observes: "Il y a sculement lieu
de remarquer que les individus du bord de l'ocean sont generalement
plus pales que ceux des contrees orientales de l'Europe, et que ceux
des cotes de France et de Belgique sent entierement depourvus de tache
discoidale[27]." And bearing, in much the same manner, on the subject
of variations, the _Anthicus humilis_, Germ., "est une des especes le
plus generalement repandues en Europe; mais il lui faut le voisinage
de l'eau salee. Aussi on le rencontre non-seulement sur les rivages de
toutes les mers, meme de la Baltique, mais encore aux bords des lacs
sales, tels que celui de Mannsfeld, en Saxe. _Ceux de cette derniere
localite sont generalement noirs_; ceux que j'ai pris a Perpignan sont
d'un rouge tres-clair, ce qui me porte a croire que cette espece est
dans le meme cas que quelques autres _Anthicus_, dont les varietes les
plus foncees appartiennent au nord de l'Europe, et les plus pales au
midi[28]."
Whilst touching on this immediate question of variability _as
dependent to a great extent_, in numerous cases, _on proximity to the
sea_, we may just notice the marked tendency which even the insects
_peculiar to_ saline spots would seem in a large measure to possess,
of converging, more or less obviously, to a lurid-testaceous, or pale
brassy hue, in their colouring. True it is that we cannot (as above
suggested) deduce any evidence of direct physical modifications from
amongst species which are _strictly maritime_,--seeing that we have no
means of judging in such instances whether similar phaenomena would or
would not be produced in central districts also: nevertheless we may
perhaps detect in this general law some slight indication of the
effects which an atmosphere and soil constantly impregnated with salt
would be likely to bring about in the external aspect of those members
of the insect tribes whose range is sufficiently extensive to expose
them to its operation. The bare mention of such names as _Nebria
complanata_ and _livida_, _Calathus mollis_, _Pogonus luridipennis_,
_Trechus lapidosus_, _Aepus marinus_ and _Robinii_, _Cillenum
laterale_, _Bembidium scutellare_, _ephippium_ and _pallidipenne_,
_Ochthebius marinus_, _Psylliodes marcida_, _Phaleria cadaverina_,
_Helops testaceus_, and _Anthicus instabilis_, so eminently
characteristic as they are of briny situations, will at once appeal to
our native entomologists; whilst the acknowledgement of the same
principle is no less conspicuous in a host of other species which are
not included in the British fauna.
Hence, when we see the tendencies of coloration (not to mention other
particulars, often readily apparent) essentially the same, both in
insects which are peculiar to, and in those which have overspread
(from without) certain regions or localities, it is impossible not to
associate some inherent controlling power with the regions themselves;
and we are driven to the conclusion, that _either_ well-defined
_races_ have been gradually shaped out, by means of the physical
influences to which they have been exposed, or else that the _species
themselves_ (as witnessed by the intermediate geographical links,
which, although sometimes rare, are in all instances to be found) do
assuredly merge into each other.
In addition to those which we have been just discussing, there are
other influences (equally independent of mere heat and cold) by which
insect modifications may be brought about,--modifications moreover of
that precise character which must be referred, in general terms, to
the nature of the country and of the soil in which they severally
obtain: a very few examples, however, in illustration of their action,
must suffice for our present purpose. The _Tarus lineatus_, Schoenh.,
is slightly shorter in Madeira, as also somewhat darker on its head
and prothoracic disk (and with its elytral striae less deeply
impressed), than it is in Algeria and Spain. The Madeiran specimens of
the _Aphodius nitidulus_, Fabr., are usually a little paler, and more
distinctly punctulated, than their northern analogues; as are also, in
the latter respect, those of the _Clypeaster pusillus_, Gyll. The
_Scydmaenus Helferi_, Schaum, is permanently smaller in the Madeiran
group than it is in Sicily; and I believe that the _Achenium
Hartungii_, Heer, of those islands, is but a local state of the _A.
depressum_, Grav., of Central Europe. The _Bembidium tabellatum_ and
_Schmidtii_, Woll., may be in reality but geographical modifications
of the _B. tibiale_ and _callosum_ of higher latitudes; and the
_Malthodes Kiesenwetteri_, Woll., of the common European _M.
brevicollis_. Calcareous deposits would appear, ever and anon, to have
considerable efficacy in regulating the outward aspect of such species
as are able to adapt themselves to different geological districts; and
when in juxtaposition with the shore, their effects are often very
conspicuous. The _Dromius arenicola_, Woll., is the Portosantan
representative of the _D. obscuroguttatus_, Dufts.; and distinct as it
is in colouring from that insect (as evinced both in Madeira proper
and throughout Europe), I believe it to be in reality but a local
condition of it, occasioned by a residence through a long series of
ages on a calcareous soil. For the same reason perhaps (though
assisted, in all probability, by the qualifying power of isolation),
the _Hadrus illotus_, Woll., may be specifically identical with the
Madeiran _H. cinerascens_. In like manner, the _Bembidium Atlanticum_,
Woll., which in Madeira proper is frequently so dark that its elytral
patches are sub-obsolete, and which is but seldom brightly arrayed in
that island, assumes in Porto Santo (which is not only more calcareous
than the central mass; but is strongly impregnated, as its streams and
rills everywhere testify, with muriate of soda) a permanently paler
hue,--being at times almost testaceous. Some districts seem to be
more prolific in varieties, generally, than others. The neighbourhood
of Ipswich, in our own country, has been cited by Mr. Curtis[29] as
possessing this peculiarity; and I have remarked a similar tendency in
certain parts of Ireland. The common _Haliplus obliquus_, indeed, of
the Blackwater river, in the county of Cork, is usually so dark and
suffused in colouring, that it might be almost taken for a distinct
species,--its fasciae, especially the hinder ones, being occasionally
evanescent.
One more example must satisfy us under this section,--namely, the
_Harpalus vividus_, Dej., of the Madeiran group. So curiously is that
insect affected by the nature of the areas through which it
successively ascends, and that too irrespectively of heat and cold (as
may be gathered from the fact that its phases on the shore and upland
heights are well nigh coincident), that it may be appropriately
singled out as a concluding instance of the effects of those obscure
local influences to which we have been drawing attention. "Ranging
from the beach to the extreme summits of the loftiest mountains,
accommodating itself at one time to a low barren rock of 20 yards
circumference, at another to the deep-wooded ravines of intermediate
altitudes, around which the clouds perpetually cling, and where
vegetation and decay are ever rampant, or harbouring beneath the rough
basaltic blocks of the weather-beaten peaks (6000 feet above the sea);
we should naturally expect, _a priori_, to discover some slight
modifications of outward structure, according as the respective
localities differed in condition. And such we find to be everywhere
the case. I am satisfied, moreover, that it is only by a careful
observation on the spot that an insect like the present one can be
properly understood; for, to anybody acquainted with it practically in
all its phases, it is but too evident how many 'species' (so called)
might be established on undoubted varieties, where there exists a
desire for creating them, and where our sole knowledge is gathered
from a few stray specimens collected by another person, and
unaccompanied by local information to render the aberrations
intelligible. For it must be tracked from the shore to an elevation of
more than 6000 feet before we are enabled to discern the causes by
which its development is controlled, or even to connect by slow and
easy gradations its opposite extremes of form. And it is an
interesting fact, that the distance between its variations does not
increase in proportion to the distance between its altitudes. On the
contrary, it would seem to pass through its minimum of size and
maximum of sculpture at about the elevation of from 3000 to 4000 feet;
both above and below which,--that is to say, as it recedes from the
upper and lower limits of the sylvan districts,--it becomes gradually
modified, and almost in a similar manner. Thus, to a person who had
visited Madeira and had picked up specimens on the coast, and to
another who had perchance penetrated into the interior, as passing
visitors from the vessels are accustomed to do, and had brought away
examples from the wooded mountain-slopes, the two insects would appear
altogether distinct. For, commencing on the level of the beach, the
usual type is broad, flat, more or less opake, with the prothorax
almost impunctate, and the elytra soldered together. As we ascend
higher, the breadth invariably diminishes, the brightness, and depth
of sculpture, seem (up to a certain altitude) to increase, and the
elytra are seldom, or but very imperfectly united; until, on entering
the lower limits of the forest region, at an elevation perhaps, _ore
rotundo_, of 3000 feet, we find that it has gradually put on a very
different aspect,--being small, narrow, bright, convex, comparatively
ovate and deeply striated; the legs and antennae have become
exceedingly pale; the prothorax has altered considerably in shape,
being much narrowed behind and punctured; and the elytra are nearly
always free. In this state it continues for about 1500 feet; when
again emerging into the broad daylight of the open hills, it
recommences to mould itself as it did below; until, having reached the
summits of the loftiest peaks, more than 6000 feet above the sea, it
has almost (though not entirely) assumed the features which
characterized it on the shores beneath[30]."
Sec. IV. _Isolation; and exposure to a stormy atmosphere._
Having in the preceding pages touched upon the subject of insect
variability, as the occasional result, to a greater or less extent, of
climatal and other influences; let us now proceed to consider the
importance of a certain physical condition, which will be found, I
believe, on inquiry, to be accompanied by a more decided modifying
power than any which we have yet discussed.
Every one who has examined the natural history of islands, both in
theory and practice, must be aware of the many difficulties which have
constantly to be encountered, before the several phaenomena can be
satisfactorily explained. Laying aside those forms which are
manifestly endemic (the numerical proportion of which usually accords
with the _distance_ from the nearest mainland), again and again are we
baffled by the near resemblance of the various creatures to
continental types,--whilst the minute _differences_ which they
display, from them, are at the same time so permanently fixed, that we
are almost precluded, under the ordinary acceptation of a "species,"
from regarding the two as undoubted descendants of a common stock: and
thus it is that insular faunas have frequently been magnified, in the
novelties which they are supposed to contain, far beyond what is
right. A person however who looks to the causes of things, and is
prepared to recognize _effects_ where there are fair grounds for
anticipating them, will not be slow to perceive, that, in the small
deviations which we are so often accustomed under such circumstances
to behold, _the results of isolation itself_ (as an active controlling
principle) may be traced out; whilst geology, ever ready to lend a
helping hand when appealed to, will seldom fail to supply those
intermediate links of probability which the believer in specific
centres of creation must needs subscribe to, before he can draw any
deductions on a broad scale, or be competent to analyse even the
general bearings of a question thus necessarily comprehensive.
Having thought it desirable to defer to a subsequent chapter of this
treatise the few geological reflections which our subject may give
rise to, it will not be my aim to allude to them in the present
section more than is absolutely requisite. I propose rather to
consider some of the ordinary effects of isolation, as mere matters of
experience; and to allow geology to tell its own tale when we come to
examine the problem of _self-dispersion, as occasionally interrupted
by subsidence_.
If we except a few of the _Heteromera_ and apterous _Curculionidae_,
which appear to be influenced in a different manner, the power of
isolation over insect form is perhaps more especially to be detected
in a deterioration of stature. Whether this principally emanates from
the constant irritation of a stormy atmosphere, such as small islands
are of course exposed to, and which would seem to have stunted the
development (during a long series of ages) of the animal and vegetable
worlds, or from a diminution of area consequent on the breaking up of
a continuous land, it is difficult to pronounce: nevertheless, it is
most consistent with both reason and analogy to suppose that each of
those causes has operated to induce a similar result; and that we must
therefore view them as working in concert, if we would appreciate
their action aright.
It is a law to which a large proportion of the organic creation would
appear to be subject, that the exuberance of life (not so much,
however, as regards the number of individuals which the various
species may present, as in the grandeur of their size) has reference
to the magnitude of the spot over which it is permitted to range. The
unnatural breeding-in of a single race, which must of necessity happen
unless the intercourse with other varieties of its kind be possible,
has always been attended with effects more or less pernicious; and in
the Annulose tribes I believe that the reduction of space which
geological convulsions have at various epochs brought about, has been
commonly succeeded (_inter alia_) by a reduction of stature in those
species which have been cut off from their fellows. I do not assert
that there are no exceptions to this rule; for counter-influences may
at times prevail (as we shall shortly see), to neutralize the above
tendency. I hold it, however, as an absolute truism, in physics, that
a law without an exception is an anomaly. If, therefore, we were once
to admit the latter to negative the former, no such thing as a law
could exist. Hence it follows, as a corollary (unless, indeed, we are
prepared to endorse that conclusion), _that_ _where there is a law
there must be an exception to it_; and that, consequently, exceptional
cases, if not exceedingly numerous, should never pervert our belief
from an otherwise presumptive truth.
This dwindling-down of size has seldom failed to attract my attention,
more or less, in almost every island which I have hitherto had an
opportunity of exploring: space, however, will not permit me to dwell
upon many instances. I have already adverted to the diminished stature
of _Anthonomus ater_, Mshm, and _Ceutorhynchus contractus_, Mshm, in
Lundy Island,--the first of which scarcely ever reaches, on that rock,
more than half its natural bulk. The late Mr. Holme, of Corpus Christi
College, Oxford, in like manner, captured the common _Calathus
melanocephalus_, Linn., and _Olisthopus rotundatus_, Payk., in
Scilly,--the former of which seldom exceeded two lines, and the latter
two and a half, in length: and he also recorded, that the _Bolitochara
assimilis_, Kby, is invariably smaller in those islands than it is in
the neighbourhood of Penzance[31]. The _Vanessa Callirhoe_, Fabr. (a
geographical analogue of the Red Admiral Butterfly[32], so common in
our own country), is permanently smaller in Porto Santo than it is on
the larger, more luxuriant and varied, and therefore more protected,
island of Madeira proper. And, as regards the _Ptini_ of that group,
so completely are some of them "affected by isolation, and by exposure
to a perpetually stormy atmosphere, that they do not attain half the
bulk on many of the adjacent rocks that they do in the more sheltered
districts of the central mass; and so marvellously is this verified in
a particular instance, that I have but little doubt that five or six
_species_ (so called) might have been recorded out of one, had only a
few stray specimens been brought home for identification, without any
regard having been paid to the respective circumstances under which
they were found[33]." That "one," Protean, representative is the
_Ptinus albopictus_, Woll.; and it is so eminently a case in point,
that it may be admissible to quote, _in extenso_, a few of the
observations which I have already published concerning it:--
"The _P. albopictus_ is the commonest of the Madeiran _Ptini_, and by
far the most variable, having a separate radiating-form for almost
every island of the group,--whilst, at the same time, the whole are so
intimately connected together (and merge into each other) by
innumerable intermediate links, that it is impossible to regard them,
in spite of the opposite contour of the _extremes_, in any other light
than as different aspects of a single species, according as
circumstances may favour, retard, or otherwise regulate its
development. Instability in fact (in its broadest sense) may be
considered to be one of its most prominent characteristics, since it
appears to be more sensitive to isolation and altitude than any of the
other members of the genus with which we have here to do,--as may be
proved to a demonstration by a careful study of its habits on the
spot, where the influences of position and exposure are, in nearly all
instances, more than sufficient to account for the successive phases
assumed. Thus, commencing with _var._ alpha, which reaches its
maximum in the sheltered ravines of the central mass, the bulk is
usually large, and the tints comparatively intense. _Var._ beta.
is likewise brightly variegated, but it is smaller. Now, if our
premises be correct, that locality and the action of the external
elements have much to do with the changes in question, we might have
expected, _a priori_, that this state, from its peculiarity to the
Dezerta Grande, would not only have reduced in dimensions (which it
is), but in colour also (which it is not). Here, therefore,
observation, _in situ_, becomes extremely important; since such does
at once convince us that its almost exclusive attachment to the
interior of the stalks of the _Silybum Marianum_, Grtn. (the _Holy
Thistle_ of the ancients), with which the more protected portions of
that island everywhere abound, affords it ample conditions, even on so
bleak a rock, for its completion. Nevertheless, its _stature_ (as
already stated) is slightly diminished in spite of this: and when we
come to examine the individuals which infest the lichen of more open
situations (aberrant however on the Dezerta Grande, and answering to
the _var._ gamma. of the diagnosis), we immediately perceive that
_both_ of our required results are indicated,--the reduction not being
limited to size, but extended also to hue. In Porto Santo this
modification is the normal one,--where the insect likewise displays
the same lichenophagous tendency, and where the districts in which it
exists are equally barren. But, if its maximum be attained in Madeira
proper, and a certain number of minor deviations range throughout
Porto Santo and the Dezerta Grande, it still remains for us to show
where its _minimum_ is to be obtained:--which, true to the _modus
operandi_ by which we have conjectured its divers degrees of abortion
to have been brought about, would seem to be centred on the Northern
Dezerta, or Ilheo Chao. When we bear in mind the minute dimensions of
that flattened rock, which does not include so much as a single
valley, or depression, within its bounds, and is consequently seldom
free from the violence of the winds (which sweep across it
incessantly, from whatever quarter they may arise); it could hardly be
supposed that an insect which is so obviously subservient to
atmospheric control should not have become materially affected, in its
outward guise, through long seclusion on such a spot:--and accordingly
we are not astonished to find the race which has been thus cut
off for ages on this extraordinary little island, itself _as_
extraordinary. It is indeed very remarkable to trace out how clearly
the agencies we are discussing have here operated on the species under
consideration,--for both sexes (though especially the male) descend on
the Ilheo Chao to somewhat less than half a line in length, being
literally of scarcely greater magnitude than some of the larger
representatives of the _Ptiliadae_!"[34]
I stated above, that, although this diminution of stature is a very
general accompaniment of isolation, amongst insects which have been
_long_ cut off from the rest of their kind, there is no rule without
an exception to it; and that, therefore, we must not always anticipate
the result which has been described. We should remember that _immense_
periods of time are apparently necessary before any perceptible change
can come over creatures from the stoppage of their migratory progress,
and the unnatural in-breeding of their several tribes; so that in
islands geologically recent (which often implies, however, their
existence through epochs which would sound vast indeed to ears
unscientific) we must not invariably expect to discover evidences of
this law. On the contrary, we must first of all take into account the
age of their formation, before we can judge _a priori_ as to the
probability of its operation through a sufficient interval of time to
have become conspicuous in its effects. I say "through a sufficient
interval of time," because the process of deterioration may be
silently going on, even now, in many an island, _which has not yet
shown any matured traces of its action_, except perhaps in the case of
a few species which appear to be more particularly susceptible to
contingencies from without. We should then call to mind, that an
enormous proportion of nearly every insular fauna is composed of
accidental colonists during the last few centuries, in which
civilization and commerce have been unintentionally at work in the
cause of animal diffusion; and that, therefore, if modifications in
outward contour have not necessarily resulted during a positive
_geological_ interval, it would be absurd to look for them in the mere
settlers (as it were) of yesterday.
Thus, it will be perceived, how necessary it is to take every element
and contingency into account before we venture to pronounce
dogmatically on either the existence or non-existence of any physical
law; and how cautious we should be of denying the legitimate operation
of external influences in one region, because they would seem, _prima
facie_, to be contradicted in another. It is surely more philosophical
to endeavour to reconcile the two, by tracing out (as may frequently
be done) some opposing principle in the latter, which shall enable us
to understand the discrepancy, and to believe that the same action may
be going on in both cases, but that in one of them it is either
overruled by a greater controlling power than itself, or else has not
had sufficient time to bring its fruits to maturity. If a proposition
be true, we should recollect that it is _always_ so (under all the
circumstances and conditions to which it is applicable); for,
otherwise, it would be both true and false,--which is absurd: hence,
_if_ my premises be true, that the general tendency of isolation is to
diminish the stature of those insects which have become isolated; it
follows that that tendency must remain, so long as there are no other
special disturbing influences to absorb or neutralize it. "When any
observation," says a writer of the last century, "hath hitherto
constantly held true, or hath _most commonly_ proved to be so, it has
by this acquired an established credit: the cause may be presumed to
retain its former force; and the effect may be taken as probable, _if
in the example before us there doth not appear something
particular,--some reason for exception_[35]." Hence it is, that, even
amongst the _opposite_ phaenomena which one island may occasionally
present from those of another, I have often been able to recognize the
working of a selfsame law; and clearly to detect, that it is not from
_its failure_, in either instance, that contending results are brought
about, but simply that some counteracting agent has been exerting its
energy in the one case, so as to nullify what would have otherwise
come to pass.
The main object however of the present section being to show that a
considerable amount of power is due to isolation itself, in regulating
(after a long series of ages) the outward aspect of the insect tribes,
it is not strictly necessary that we should so rigidly insist on
deterioration of size as one of its primary consequences,--since
(whether it be so or not) we are merely concerned here to demonstrate,
that its influence, _in some shape or other_, is absolute and real.
After the above remarks, we shall not be surprised that the phaenomena
displayed in certain islands, as regards size, are sometimes (though I
believe it to be an exception to the ordinary rule) the exact opposite
of what we have been describing. Let us not however be alarmed at this
fact, on the bare statement of it,--as though the proposition which we
have been lately advancing were at once disproved; since we shall
find, on inquiry, that the case is not so desperate as might be
imagined; and that in many islands where even this principle is to be
detected, we may recognize traces of the other also. But how, it will
be asked, can this be? for, since the influences are the same,
creatures similarly exposed to them must be similarly affected. Now,
although, on a broad scale, such a notion contains much presumptive
truth; on a narrower one it does not always apply; for species are
differently constituted _ab ovo_, and will sometimes give a different
result from the operation of causes which are identical. Moreover,
there is a curious tendency which I have remarked in most islands,
that the wings (especially the metathoracic ones) of their insect
inhabitants are liable to be retarded in their development,--often
indeed to such an extent as to become actually evanescent: and I
believe it to be a law of Nature, that when any particular organ is
either stunted or taken away, the creature receives a compensation for
its loss either by the undue enlargement of some other one[36], or
else in a general increase of its bulk. If such be the case, the
presence of two apparently conflicting effects in a single island is
rendered somewhat more intelligible; nevertheless, on the above
hypothesis, the specimens which increase in dimensions should
undoubtedly have their organs of flight more or less enfeebled, whilst
those which diminish should be regularly winged. And hence we arrive
at the question, is this so? My own experience would certainly tend to
prove that it is; and I suspect that future observations will confirm
the fact. Meanwhile, I must content myself with simply advancing the
subject for consideration, and with recording such few examples, in
support of the theory, as space will permit, and which occur to me
almost spontaneously.
The Madeiras would seem to inherit, as it were, a more than usual
control over the alary system of their insect population; for, out of
about 550 species of Coleoptera which I have hitherto met with in that
group, nearly 200 are either altogether apterous, or else have their
organs of flight so imperfectly developed, that they may be
practically regarded as such; so that, if our preceding conclusions
(from the compensation-hypothesis) be correct, we should _a priori_
anticipate an increase of bulk in those islands, rather than a
decrease of it. Unfortunately the greater number of these 200
representatives are now, through the submergence of the once
surrounding continent, _endemic_, so that we have no means of judging
whether the obsoleteness of their wings is to be referred to the long
action of Madeiran influences[37], or whether they were thus created
severally in the beginning; and, for the same reason (that is to say,
having no others of their kind to compare them with), we cannot
pronounce, even if we might assume this partial organic decay to be
the consequence of their isolation on these rocks, whether their
general stature has been subsequently augmented or not. Still, there
are some few, out of the 200 just alluded to, which are of common
European distribution; and, as these would appear to have obeyed the
principles to which we have been calling attention, it is not
unreasonable to suppose, that many of the others (could we but behold
them as they formerly were,--emigrants over a vast continuous land)
would be found to have done so also.
I alluded, in a previous section, to the _Dromius obscuroguttatus_,
Dufts., as presenting permanent characteristics in Madeira,--the
combined result of latitude and isolation; and I also stated that it
was not always possible, whilst dealing with physical agents which are
necessarily obscure, to refer the respective phaenomena (whatsoever
they may be), which would seem to have departed from their types, to a
single disturbing cause. Hence, whilst I there acknowledged latitude
as in part answerable for the changes which that insect has undergone,
I may here suggest that it is, in all probability, to _isolation_ that
we must mainly look, if we would understand those changes aright. But
what _are_ the distinctive features, it may be asked, which the _D.
obscuroguttatus_ has adopted, since its first arrival from more
northern latitudes over an unbroken[38] continent? It has not altered
much, after all: it is, however, the _nature_ of the alterations, and
their constancy, which give them their real importance. In a few words
then, the insect is rather larger and more robust than its European
analogue, and (to omit other minor differences) _its wings are
evanescent_. But this, on our above hypothesis, is precisely what we
should have expected: for, since it is self-evident that the species
cannot have been naturalized accidentally on these mountains, and
since geology informs us that a _vast_ interval has elapsed since the
Madeiran islands were portions of a continuous whole, we have at once
a sufficient _time_ assured us for the modifications to be completed,
and to appear at length permanently adjusted in accordance with the
conditions and influences which locally prevail.
There are other examples which might be quoted in support of my
theory,--that isolation, when involving a sufficient period of time,
has a direct tendency either to diminish the stature of the insect
tribes, or else to neutralize their power of flight; but that, in the
latter case, the creatures, when thus despoiled of a function, do, on
the contrary (instead of deteriorating in size), often receive a
compensation for their loss by an actual _increase_ in their bulk. The
common _Bradycellus fulvus_, Mshm, is another instance in point. From
its occurrence in the almost inaccessible districts of the Madeiran
group, far removed from cultivation, I am inclined to refer its entry
into this southern region to that remote period when a connective land
offered a natural passage to wanderers from the north. Hence our first
stipulation, that of _sufficient time_, is satisfied; and what is the
result? The insect is a trifle more robust than its ordinary European
representatives, and it is _invariably apterous_. The _Calathus
fuscus_, Fabr., is also, as is clear from its special attachment to
the mountain tops, strictly indigenous in Madeira (that is to say, it
must have arrived there during the migratory epoch); and the
consequence is, that, although usually winged in our own country, it
is permanently subapterous in that island. I think it far from
unlikely that the _Dromius negrita_, Woll., may be the ultimate phasis
(from isolation) of the common _D. glabratus_, Dufts.,--from which it
may be distinguished by its somewhat larger bulk, more robust head and
prothorax, and by the obsoleteness of its wings. True it is, that the
latter species flourishes alongside it in Madeira; but, like the
_Vanessa Atalanta_ (when considered with respect to the _V.
Callirhoe_), may it not be of more recent importation from the
European continent, and as yet in a transition state?--an idea which
the _smallness_ of its wing, as compared with those of its British
analogues, would seem rather to corroborate.
But, if this slight increase of stature would appear generally to
accompany that gradual extinction of the powers of flight which
isolation is apt to induce, it follows, on the other hand (as
indeed I have lately intimated), that where wings are so essential
to the continuance of a species that they cannot, without its
positive destruction, be taken from it, the _primary_ effect of
isolation,--namely a diminution of bulk,--will for the most part
happen instead. As this fact, however, has been already commented
upon, we will not discuss it afresh.
Why it is, in the Insecta, that _islands_[39] should predispose to an
apterous state more than continents, it is not easy to speculate. Mr.
Darwin has indeed suggested, and with much apparent reason, that, were
wings fully developed, the indiscriminate use of them might lead to
unhappy results, by tempting the creatures to venture too far from
their native rocks; and that, therefore, this partial decay is, under
such circumstances, a wise provision in their favour: whilst it has
been urged, on the other hand, that since insular species are at all
times liable during heavy gales to be blown out to sea, they should
in reality be gifted with _stronger_ powers of flight (rather than
weaker ones), to fortify them against such disasters; and that,
consequently, the above phaenomena are not explicable on Mr. Darwin's
hypothesis. For my own part, I am inclined to accept that theory, in
all its fullness; and, furthermore, I do not believe that the latter
consideration (though it unquestionably contains much presumptive
truth) does at all interfere with the admission of it,--seeing that
either requirement may be fulfilled, according to the nature of the
several species which are destined to be acted upon. Thus, if _flight_
is absolutely indispensable, as in the greater number of the
Lepidoptera, and beetles of a flower-infesting tendency, we shall find
that the wings remain unaltered (if indeed they be not actually
increased in capacity, of which I am by no means certain), and that
the effect of isolation is more particularly evident in a diminution
of stature. But if, on the contrary, the creatures are less dependent
on aerial progression for their sustenance, as in the predacious
tribes generally, especially those of nocturnal habits, the reduced
area in which they are confined, in conjunction, it may be, with the
danger to which they would constantly expose themselves by the
promiscuous employment of organs which their modes of life do not
positively need, would seem to render the presence of wings
unnecessary; and they are accordingly, by degrees, removed:--in which
case, however, a compensation for the loss is not unfrequently granted
by an increase (more or less perceptible) in bulk.
In the Madeiras, this diminution and enlargement of stature,
accompanied for the most part respectively by the retention and
annihilation of the powers of flight, is singularly traceable on the
selfsame rocks, particularly the smaller ones of the group. Thus, on
the Flat Deserta, or Ilheo Chao, the _Scarites abbreviatus_, Koll.,
_Laparocerus morio_, Schoen., and the _Helops Vulcanus_, Woll., attain
a gigantic size; yet it is on that very island that the _Ptinus
albopictus_, Woll., finds its minimum of development,--scarcely
exceeding in dimensions some of the larger members of the
_Trichopterygia_. The Deserta Grande has some special modifying
capability of its own,--the _Eurygnathus Latreillei_, Lap.,
_Notiophilus geminatus_, Dej., _Zargus pellucidus_, Woll., _Calathus
complanatus_, Koll., _Olisthopus Maderensis_, Woll., _Caulotrupis
conicollis_, Woll., _Laparocerus morio_, Schoen., _Omias Waterhousei_,
Woll., _Helops Vulcanus_, Woll., and the _Ellipsodes glabratus_, Fab.,
being also larger on that rock than is typical: all of them, however,
with the exception of _Notiophilus geminatus_, are there, as
elsewhere, apterous.
Other qualifying results, from isolation, are equally apparent. Take
_colour_, for instance; and we shall perceive that in the _Dromius
sigma_, Rossi, it is sensibly affected. The normal state of that
insect "does not occur at all in Madeira proper, but only in Porto
Santo. True it is that the modifications in the several islands
present but slight differences _inter se_; nevertheless, being
constant, I would lay particular stress upon them, since they go very
materially to prove that the effects of isolation on external insect
form are even more important, if possible, than those of latitude.
That this is the case in the present instance, appears clear from
facts so minute as these. For, out of the many specimens which have
come under my observation from various countries of Europe, if there
is one point more constant than another in this otherwise variable
species, it is, I believe, to all circumstances, its immaculate
prothorax. Now, whilst this (we may almost say essential) character
obtains in Porto Santo, in Madeira it does not hold good: the
prothorax there is invariably infuscate in the centre; and on a small
adjacent rock (the Ilheo de Fora) it is entirely dark. Nor let anyone
suppose that details apparently so trivial are beneath our notice, or
the mere result of chance, since it is by the observation of such-like
points, and by marking their development according to the
circumstances of the several localities in which they obtain, that we
are alone able to appreciate their importance, and so to form, in a
wider and geographical sense, a correct estimate of their value[40]."
The _Olisthopus Maderensis_, Woll., is much paler, larger, and more
opake, on the Dezerta Grande than it is in Madeira proper. So great
indeed is the change which it has undergone through a long isolation
on that rock, "that, had the case been a solitary one, I should not
have hesitated in regarding the specimens obtained from thence as
specifically distinct; nevertheless, with the knowledge both of the
modifying effects of isolation, and also of the _kind_ of modification
essentially peculiar to that island, I am perfectly satisfied that it
is a mere local state, although a very remarkable one, and has no
claim whatsoever to be otherwise considered[41]." The _Pecteropus
Maderensis_, Woll., is of a greenish-brassy tinge in Porto Santo, and
much acuminated in front; whereas on the Dezerta Grande it is almost
invariably _coppery_, and less narrowed anteriorly. The _Caulotrupis
lucifugus_, Woll., although ranging through no very opposite phases,
either of outline or sculpture, "appears to possess a slight
modification for every island of the Madeiran Group: and hence small
shades of difference, which might otherwise be regarded as trifling,
become directly important, and cannot be ignored in a local
fauna,--even though a general collector may deem it unnecessary to
recognize them. In real fact, however, such distinctions, when viewed
geographically, are of the greatest interest, as serving to illustrate
what we have so often had occasion to comment upon, namely the
influence of isolation and other circumstances on external insect
form[42]." The _Psylliodes vehemens_, Woll., is permanently paler in
Porto Santo than it is in Madeira proper, being almost entirely
testaceous. "That the species is identical, however, with the Madeiran
one I have not the slightest doubt,--the sculpture and colour, as I
conceive, having merely undergone a change since the remote period
of its isolation on a comparatively calcareous soil[43]." The
_Scarites abbreviatus_, Koll., occupies the loftiest peaks of nearly
all the Madeiran islands, and was probably once abundant over the
entire ancient continent, whatsoever its limits may have been, of
which the present group forms but an isolated part. "There are traces
of it in the Canaries, from whence occasional specimens have been
brought, and which, from the want of local data and of sufficient
numbers to reason upon, have in their turn been severally regarded as
distinct. The fact however is, that the species in question is an
extremely variable one, assuming differences of size according to the
altitude at which it lives, and differences of sculpture according to
the circumstances of the spot on which it is isolated. That such is
actually the case, a careful observation of the many minute changes
which the insect has undergone in the various islands and altitudes of
the Madeiran group will, I think, prove to a demonstration. For it is
impossible to suppose that every rock contains its own _species_, that
is to say, has had a separate creation expressly for itself,--a
conclusion at which we must assuredly arrive, if small and even
constant differences are _of necessity_ specific. Rejecting therefore
this hypothesis as utterly untenable, and as contrary to all
experience, we are driven to acknowledge that isolation _does_, in
nearly every instance, in the course of time, affect, more or less
sensibly, external insect form;--which being admitted, we have at once
an intelligible principle whereby to account for modifications
innumerable, each of which, when viewed simply as a difference,
independently of the circumstances producing it, might have been
regarded as sufficient to erect a 'species' upon, had the desire for
multiplying them overbalanced the love of truth[44]."
* * * * *
Such are a few of the circumstances, influences, and conditions, by
which the outward aspect of the insect tribes is liable, within
definite limits, to be more or less regulated: and it is impossible to
view them with an unbiassed mind and not arrive at the conclusion,
that physical agents generally have a very decided control over the
external contour of these lower creatures. In selecting the examples
which we have lately discussed, I have avoided as much as possible
those startling instances of variation which distant quarters of the
globe will readily supply, because there are vast numbers of our
naturalists who will not acknowledge the validity of any evidence
which would tend to amalgamate, in a broad sense, the species of the
Old and New Worlds. I have therefore contented myself with such data
as must fall within our common experience, feeling satisfied that if
the principle be allowed in the one case, it cannot long be objected
to in the other. There are few entomologists who would not recognize,
in the abstract, a legitimate capacity for adaptation in every insect
with which they have to do; yet I believe there are not many, who, if
modifications were to be shown them as the fixed result of
disturbances from without, would be prepared at once practically to
accept them as such. The collectors of the present day are so prone to
regard every _permanent_ difference as a specific one, that a large
proportion of them do not sufficiently realize, that well-marked
races, or states, are no longer matters of hypothesis, but of fact;
and that, therefore, a sensible amount of aberration should not only
be _conceded_ to the action of certain physical combinations and
elements, but even anticipated and looked for. Such however ought not
to be; and earnestly therefore would I advocate a greater latitude for
geographical influences than has been hitherto admitted by many of us.
Especially would I urge the necessity for a more careful study of
_insular_ phaenomena, for I am convinced that a due allowance is
seldom, if ever, made for the qualifying power of isolation, _per
se_,--the most significant perhaps of all the conditions which we have
attempted in the preceding pages to examine.
"Felix qui potuit rerum cognoscere causas" is a motto which the
student of Nature should keep constantly in view; for it is
undoubtedly a more honourable task to discover the _reasons_ for what
we see, than the mere appearances themselves. He who has dived deeply
into the everyday circumstances around him will be reluctant to
ascribe so much as a single item of all that comes within his ken, to
chance; for to him the whole system of created things is, from first
to last, replete with design. _Natura nil agit sine causa_ is as true
now as it ever was, and it will be so to the end. Let us not therefore
be discouraged at the apparent smallness of the data from which many
of our conclusions have to be drawn, for nothing is in reality trivial
which is the effect of a wisely appointed law; and, even were such the
case, it would not be thereby proved that the investigation of the law
itself (however liable it may be to exceptions) is unimportant. Nor
ought we, on the other hand, to be discouraged if we cannot always
reconcile conflicting phaenomena, and detect in each a primary
controlling cause. We should rather bear in mind, that the elements
with which we have to deal are obscure, and subject to permutations
from which various results must of necessity arise; and that it is
only, therefore, on a broad scale that we can look for uniformity of
action, even from conditions which may appear to be identical. "Nature
is not irregular, or without method, because there are some _seeming_
deviations from the common rule. These are generally the effects of
that influence which free agents, and various circumstances, have upon
natural productions[45]."
FOOTNOTES:
[3] Religion of Nature Delineated, p. 103.
[4] Journal of Researches (London, 1852), p. 381.
[5] The great preponderance of the phytophagous over the predacious
tribes, in the hotter regions of the earth, is a remarkable fact, and
strongly suggestive of the relation which the insect and vegetable
worlds (both of which attain their maximum in those zones) bear to
each other. "The carnivorous beetles, or _Carabidae_," says Mr. Darwin,
"appear in extremely few numbers within the tropics. The
carrion-feeders and _Brachelytra_ are very uncommon; on the other
hand, the _Rhynchophora_ and _Chrysomelidae_, all of which depend on
the vegetable world for subsistence, are present in astonishing
numbers. The orders _Orthoptera_ and _Hemiptera_ are peculiarly
numerous; as is, likewise, the stinging division of the _Hymenoptera_,
the bees, perhaps, being excepted."--Journal of Researches, p. 34.
[6] Mr. Westwood states that he possesses an individual of the
_Papilio Machaon_ from the Himalayan Mountains, captured by Professor
Royle, "which scarcely exhibits the slightest differences when
compared with English specimens."--_The Butterflies of Great Britain_,
p. 4.
[7] Zoologist, xiii. p. 4655.
[8] The Butterflies of Great Britain (London, 1855), p. 17.
[9] _Id._ p. 94.
[10] Insecta Maderensia (London, 1854), pp. 7, 8, 9.
[11] Insecta Maderensia, p. 516.
[12] Insecta Maderensia, p. 17.
[13] I possess specimens of this insect captured on the summit of
Mount Olympus by my friend E. Armitage, Esq., who is also of opinion
that it may be but a mountain state of the _C. sylvatica_, Linn.
[14] Introduction to the Modern Classification of Insects (London,
1840), ii. p. 473.
[15] Id. ii. p. 158.
[16] Introduction to the Modern Classification of Insects, ii. p. 431.
[17] Journal of Researches, p. 238.
[18] That I may not be misunderstood by those of my readers who
conceive Madeira to be a kind of "arva beata," with the sky for ever
blue, and (as a consequence) an unclouded sun; I would repeat, that I
am not speaking of the vicinity of Funchal only (from which the
invalids, who resort thither for their health, almost exclusively draw
their deductions), but of _Madeira_,--and, more-over, of Madeira _as
it was_, and not of Madeira as it is. More or less of cultivation
during a period exceeding four centuries, in conjunction with the
overwhelming fire which completely devastated the entire south of the
island, immediately after its first settlers had taken possession of
it, and which is stated (in the accounts which are transmitted to us)
to have smouldered on for nearly seven years, have so altered the
features of the country, that it is only in the untouched regions of
the north (on which the woodman's axe is nevertheless encroaching,
season after season, with lamentable rapidity) that we can catch even
a glimpse of its pristine condition. The dense forests which then
everywhere abounded must have caused an amount of moisture and
exhalation of which even the northern districts as they now are
(though saturated, even yet, with dampness; and at a certain elevation
almost constantly enveloped with clouds) can give us but a faint idea.
So tremendous indeed must have been the aqueous accumulations which
then hung around the island, that even the splendour of a southern sun
cannot have penetrated the atmosphere as it does at present; and,
hence, the historical fact that Madeira proper (although separated by
a channel of only thirty miles in breadth, and _now_ usually visible
in bold relief against the sky, during a portion, at least, of every
day, from a far greater distance) was not discovered for _an entire
year_ after the colonization of Porto Santo, on account of the
thickness of the canopy which shrouded it from view, is at once
rendered intelligible. It is narrated, that, in the year 1419, Prince
Henry of Portugal organized an expedition to attempt the doubling of
Cape Bojador; but the commanders, having lost their reckoning, were
driven ashore on an island,--which they named Porto _Santo_, in
commemoration of their escape from the perils of the sea. "On their
return," says Mr. Harcourt, "Prince Henry sent out Zargo, Vaz, and
Pestrello, to plant a new colony in the island. It was not long before
a dark spot was observed on the western horizon of Porto Santo. This
was regarded by some with superstitious awe; but Zargo concluded it to
be clouds attracted by high land; and shaping his course in that
direction, in spite of the endeavours of his crew (by menaces and
supplications) to prevent him, he discovered, in the year 1420, the
island to which, from the trees that covered it, he gave the name of
_Madeira_."--_A Sketch of Madeira_, London, 1851, p. 16.
[19] Journal of Researches, pp. 209, 210.
[20] Zoologist, x. 3616.
[21] Considering that I have already detected more than one thousand
species in those islands, it may perhaps be questioned whether the
same truth _is_ to be gathered from the result of my Madeiran
researches. I would wish it therefore to be understood, first, that my
statement refers to that group _as contrasted with countries in a
similar latitude_; and, secondly, that its _real_ fauna is alone taken
into account,--the host of introductions from more northern regions, a
large proportion of which have probably taken place within a very
recent period (as may be fairly presumed from the knowledge that fresh
arrivals, an almost necessary consequence of the importation of
plants, _are_ occurring nearly every season), having been dismissed
from our present inquiry.
[22] I perceive, on reference to the original examples, still in my
collection, that this was wrongly quoted as the _Haltica rufipes_. It
is the _H. exoleta_, Fabr., and it is thus entered in Messrs. Hardy
and Bold's 'Catalogue of the Insects of Northumberland and Durham;'
where they make the observation, "variable in colour; specimens from
the sea-coast are frequently of a dark mahogany tint." I have myself
indeed, since I communicated the above remarks to the 'Zoologist,'
taken its precise counterpart, in abundance, along the Yorkshire
coast,--from Bridlington to the extremity of Flamborough Head; so that
it may perhaps be regarded as a topographical state which is more
especially peculiar to the eastern shores of England, north of the
Humber.
[23] Zoologist, iv. pp. 1283, 1284.
[24] Geodephaga Britannica (London, 1854), p. 186.
[25] Zoologist, iii. p. 900.
[26] Zoologist, v. p. 1941.
[27] Monographie des _Anthicus_ (Paris, 1848), p. 149.
[28] _Id._ pp. 127, 128.
[29] Proceedings of the Entomological Society of London (Part 3. New
Series), p. 4.
[30] Insecta Maderensia, pp. 55, 56.
[31] Trans. of the Ent. Soc. of London, ii. pp. 59, 62.
[32] Considering that the true _Vanessa Atalanta_, of more northern
latitudes, _does_ occasionally occur around Funchal, it may be
reasonably contended that the fact of its coexistence (on the same
spot) with the _V. Callirhoe_ is strong presumptive proof that the
latter is a true species, and no climatal or insular modification of
the former. And so, judging from a distance, and without local
evidence to explain this phaenomenon, I should have concluded myself:
nevertheless, recollecting how easy of transport the larvae and pupae of
Lepidoptera necessarily are (of which we have the plainest assurance
in the almost certain introduction of the _Pontia Brassicae_, _Sphinx
Convolvuli_, _Acherontia Atropos_, &c. into those islands), especially
in a region which for more than a century has been receiving a
constant supply of vegetables and ornamental plants from western
Europe; I am induced to believe that the appearance of the _Atalanta_
is a comparatively recent one, whilst that of the _Callirhoe_ (which,
unlike the typical _Red Admiral_, has naturalized itself in nearly all
portions of the group) must be referred to the remote period when
migrations over a long-lost continuous land were in regular operation.
The _slowness_ of the change, in external aspect, which the isolation
of insects from geological causes would seem to bring about (and which
follows, as a corollary, if the above conclusion be true), I propose
to discuss in a subsequent chapter of this work.
[33] Insecta Maderensia, p. 260.
[34] Insecta Maderensia, pp. 268, 269.
[35] Religion of Nature Delineated, p. 99.
[36] Although the result of a primary (or creative) adjustment to
special circumstances, rather than of a secondary adaptation, brought
about by a self-modifying capability; we may just call attention to
the fact, that most of the blind insects, whether associates within
the nests of ants, or natives of subterranean caverns, have either
their palpi _or_ antennae anomalously developed,--as though, partially
(although how, and in what degree, we cannot possibly ascertain), to
make amends for the inconvenience which a total want of sight must,
necessarily entail.
[37] This is certainly rendered _probable_, however, from the fact
that a large proportion of these apterous species are members of
_genera_ which are usually winged,--such as _Tarus_, _Loricera_,
_Calathus_, _Olisthopus_, _Argutor_, _Trechus_, _Hydrobius_,
_Ephistemus_, _Syncalypta_, _Phloe ophagus_, _Tychius_,
_Longitarsus_, _Chrysomela_, _Scymnus_, _Corylophus_, _Helops_, and
_Othius_,--whilst the knowledge that, out of twenty-nine genera which
I believe to be endemic in those islands, six only are winged (the
remaining twenty-three being apterous), will not tend to diminish the
probability that there is something peculiar in the action of Madeiran
influences generally on the alary system of the insect tribes.
[38] I do not think it necessary to apologize for the apparent
disposal of this _quaestio vexata_; because, from the wildness of the
upland ridges to which the _D. obscuroguttatus_ is in Madeira
exclusively confined, I deem it an absolute impossibility that it
could ever have been _introduced_, through any chance agencies
whatsoever. And hence, unless we reject the doctrine of specific
centres _in toto_, I contend that it must have migrated, together with
other insects similarly circumstanced, by ordinary means, and without
natural impediments, from its own area of diffusion.
[39] I am informed by Dr. Hooker, that the only two insects (belonging
respectively to the orders Coleoptera and Lepidoptera) which he
detected in Kerguelen's Land were wingless.
[40] Insecta Maderensia, p. 6.
[41] Insecta Maderensia, p. 36.
[42] _Id._ p. 310.
[43] Insecta Maderensia, p. 452.
[44] Insecta Maderensia, p. 11.
[45] Religion of Nature Delineated, p. 84.