CAUSES OF VARIATION.
"It is not impossible," says a writer of the last century, "that such
laws of Nature, and such a series of causes and effects, may have been
originally designed, that not only general provisions may have been
made for the several species of beings, but that even _particular
cases_ (at least many of them) may have been provided for without
innovations in the course of Nature[3]." And let us not suppose that
this is a mere, wanton speculation, unsupported by evidence (if not
actually circumstantial, at least) strongly presumptive; since the
further we penetrate into the ramifications of the organic world, the
less are we inclined to ignore the operation of those various
modifying influences which our understanding tells us do everywhere
exist.
To investigate the causes of things, and to endeavour to trace out by
slow, inductive processes those secondary agents, by the assistance of
which a large proportion of the phaenomena around us are gradually
matured, is no insignificant task; yet how much animadversion from
without have the students in such fields of research frequently to
endure! A fact many times repeated, and which comes within our daily
experience, is too often looked upon as a matter of course, and as
therefore beneath the notice of an intelligent mind; yet the man who
regards _truth_ as valuable, for its own sake, under whatever aspect
it may come, and who can rise to the appreciation of _results_,
whether they be of rare or constant occurrence, will have learnt to
pronounce nothing as unimportant which may supply a single link in
that chain of knowledge which would be broken and imperfect without
it. A spirit of inquiry, however, is becoming, year by year, more
evident; and we may confidently anticipate the period when such
reproaches will have for ever died away. Natural history, in all its
branches, will then advance more rapidly than heretofore, and each
separate labourer, in his own peculiar province, will breathe a more
genial atmosphere; whilst observation and reason, mutually dependent
on each other, will work in concert more effectually. "Reason without
_observation_," writes the author above quoted, "wants matter to act
upon; and observations are neither to be justly made by ourselves, nor
to be rightly chosen out of those collected by others, without the
assistance of _reason_. Both together may support opinion and
practice, in the absence of knowledge and certainty."
In the last chapter we offered a few passing remarks on
insect-aberration generally, whether regarded as a _universal fact_
(which, however, even supposing such to be true, it is not the object
of the present treatise to substantiate), or as an _occasional_
one,--that is to say, as existing at all times to that extent (as an
hereditary principle), that it is _liable_ to manifest itself, or
not, according as external agencies may favour or oppose its
occurrence. In the latter case, which alone I propose to consider,
this inherent tendency may be displayed, either through the expression
of "varieties" well defined, or by a mere proneness to wander,
irregularly and at large, from an assumed diagnostic type. In the
following pages, the _former_ of these resultant conditions (namely,
that in which "varieties," technically so called, though _more or
less_ isolated in their character, are apparent) will be especially
discussed; since my principal desire is, to point out the influence of
_local disturbing causes_ in regulating, to a greater or less extent,
though of course within certain specific limits, the outward contour
of the insect tribes,--and it requires no argument to prove that,
where those local elements (whatsoever they may be) prevail, the
_same_ effects will, for the most part (in the same species), be
produced; and that, therefore, modifications which are characteristic
of countries and regions far removed from each other have an _a
priori_ claim for stability, above those which circumstances less
important than geographical ones, and which are consequently more
fluctuating in their combinations, may from time to time (as it were,
accidentally) shape out. Having then examined our premises, and
prepared ourselves, with an unbiassed mind, for the reception of
phaenomena which should be constant (and in some instances, also,
conspicuous) _in proportion as_ the conditions which unite in bringing
them about are significant; let us advert to a few of the more
prominent cases in which our instinct would seem to warrant the belief
that aberrations are to be usually anticipated. And since it will
hardly be denied that, like the representatives of other departments
of the animate world, insects _may_, in their outward configuration
and development, be in some measure under the control of the external
influences to which they are immediately exposed, we will take a rapid
glance at a few of the circumstances and conditions which are known to
have more or less of a qualifying effect on the members of large and
opposite sections of the organic creation; and then see how far we are
enabled, by means of facts, to trace out results for the Insecta,
corresponding to those which are admitted to obtain in the other
groups. And, since the existence of analogous results infers, to a
certain extent, the similarity of the agents which have brought them
about, our "causes of variation" (provided the effects can be shown)
may be in reality almost demonstrated.
Amongst the numerous influences and conditions, in obedience to which
the members of a large proportion of the animate world would appear,
at times, in their outward aspect to be modified or fashioned, the
following may be selected as perhaps of primary importance:--
1. Climatal causes _generally_ (whether dependent on latitude or upon
altitude).
2. Temporary heat or cold, of an unusual degree.
3. Nature of the country and of the soil.
4. Isolation, and exposure to a stormy atmosphere.
Sec. I. _Climatal causes generally, whether dependent on latitude or
altitude._
Perhaps, judging superficially, climatal causes generally would appear
to have more effect on insect development than any with which we are
acquainted; yet, powerful as they unquestionably are, experience
teaches us that such is not the case. In combination with other
modifying principles, hereafter to be noticed, they may be (and
probably are) exceedingly important; yet, when taken singly and alone,
we have no evidence to show that their consequences are of such
primary significance as might be anticipated. Mr. Darwin, in
describing the fauna (which includes many mundane forms) of the
Galapagos Archipelago, situated immediately under the equator,
remarks: "The birds, plants, and insects have a desert character, and
are not more brilliantly coloured than those from Patagonia; we may
therefore conclude, that the usual gaudy colouring of the
intertropical productions is not related either to the heat or light
of those zones, but to some other cause,--perhaps to the conditions of
existence being generally favourable to life[4]."
Although it is true, in a broad sense, that the nearer we approach the
Line the grander and more gorgeous are the animate beings which tenant
the surface of our earth, there are at the same time so many
exceptions to this law, that it cannot he regarded as by any means
universal; and whatever, therefore, be our ideas on a subject which
might perchance _seem_ to be self-evident, we are compelled to infer
that climatal causes, of themselves, will not suffice to account for
the numerous cases of aberration which we so constantly meet with in
representatives of the same species exposed, through a long series of
centuries, to opposite conditions of atmosphere. We need not, however,
go so far as the Galapagos to convince ourselves of this. The Madeiran
Group is placed between the 32nd and 33rd parallels of north latitude,
off the coast of Africa, and contains a Coleopterous fauna (as
hitherto ascertained) of about 550 species. Now 240 of these, at
least, occur also in Europe (many of them even in our own country);
hence, if a more southern climate may be presumed, of itself, to
exercise any very decided modifying influence on insect development,
we have an amount of material for comparison which should surely
afford us some definite and tangible result. My own experience in
those islands would tend to prove, that, amongst the many aberrations
from their northern types which are there everywhere displayed,
comparatively few of them can be referred for explanation to causes
strictly climatal. I do not say that _none_ can be thus accounted for;
yet I trust to make it obvious in the following pages that there are
even greater agencies at work than climatal ones in regulating (albeit
within prescribed limits, and by slow gradations) the outward contour
of the insect tribes.
When viewed geographically, there are two heads under which the
insects of every individual area may be classed: namely, those which
were created within its bounds, and which constitute its true
aborigines (in the strictest sense); and, secondly, those which _have
reached it_, either by ordinary migration over an intervening land, or
by accidental introduction through human or other agencies. Now it is
to the members of the _latter_ of these ideal divisions, that we
principally look for any positive evidence, whilst discussing the
causes of variation: since, by the nature of the case, we _must_ have
identical, or at any rate closely allied species to reason upon before
any sound conclusions can be drawn concerning them from the
circumstances and conditions to which they are severally exposed; and
it is clear, that the fact of creatures being specifically coincident,
and yet under influences remote, does, for the most part, actually
_imply_ a transportation of them (from their primeval centres) beyond
the limits of a naturally acquired range. Moreover, the autochthones
of the soil (if we may be excused the idiom) are in all instances
adjusted to the peculiarities of the region in which they
were formed; and, consequently, where they have not (as very
frequently happens) diffused themselves to a sufficient distance from
the birthplace of their kind to be acted upon in two opposite manners
from without, the date _they_ supply, during our inquiry into specific
modifications as dependent on external disturbing elements, cannot be
very considerable.
In spite of this severe distinction, however, which I would urge
between the insect _aborigines_ of a country and _those which_
(whether by compulsion or not) _have colonized it_, and of the
preference which (as just stated) must be given to the latter whilst
investigating the controlling principles of aberration, I would not
wish to reject _in toto_ the testimony which the former likewise may
indirectly furnish,--especially under the present section, in which
climatal causes on a large scale have to be taken into account. True
it is that we cannot hope to descry _physical results_ amongst
phaenomena which are due to the _creative_ force alone; yet we may, in
the contemplation of them, recognize such an amount of _design_, or a
primary adaptation to conditions from without, as shall afford,
through its permanence and method, fresh presumptive evidence that the
"conditions" _themselves_ may have some inherent modifying power of
their own on the aggressors from other districts, in which a contrary
influence may perchance prevail, and for the overspreading of which
they were, in the beginning, more peculiarly constituted and ordained.
It has been already mentioned (and, despite the exceptional cases
which are to be found, it is in a _general_ sense true), that the
splendour and extravagance of the insect world attain their maximum
within the tropics; and that the nearer we approach the central heat,
the more and more unmistakeable is the existence of this law. It has
been also hinted, that when viewed on a very extensive scale,
we shall not derive much _direct_ assistance (whilst examining
insect-variation, with reference to climate) from the consideration of
a fact thus seemingly important,--since there are but few species
whose range is so comprehensive as to embrace, at the same time, the
equatorial and temperate regions of the earth; and since, as lately
suggested, it is not from a comparison of the _aborigines_ of
countries far removed that we can hope to derive much positive
information during our present inquiry. It may be useful however to
speculate, why the creative energy should have been thus lavished, as
it were, in the torrid zone, whilst the fauna of the cold north is so
unpretending and sombre. I believe that in the actual _number_, both
of individuals and species, which they contain, the difference is not
so great, between the two latitudes, as might be imagined; and that,
were the minims of Scandinavia to be suddenly magnified into the
giants of Brazil, the Laplanders and Swedes might stand a fair chance
of being temporarily alarmed: nevertheless, as regards the multitude
and eccentricity of her forms, there can be no question in which field
it is that Nature has ever delighted more particularly to sport.
Laying aside, therefore, the numerical statistics from our account, is
not the exuberance of the tropics at once responsive to the conditions
imposed upon them? Do we ask why it is that the insect population is
there moulded upon a type comparatively so colossal?--let the
redundancy of the vegetation reply. Have not, also, more rapid laws of
putrefaction and decay been prescribed than in our cooler clime; and
can we imagine that it was _not_ in obedience to this decree, that
larger and more active scavengers were framed? The gaudy wings that
float idly on the breeze, and the coats of mail which glitter in the
light, have they nothing to tell of the local circumstances around
them; or, is it too much to infer, that a more glorious and
stimulating sun required creatures of superior brilliancy to bask in
its rays? A moderate degree of heat, and that only during a certain
portion of the year, may suffice in quiescent regions to keep up the
equilibrium of the organic world, the various members of which,
whether animals or plants, are ensured, in such countries, their
alternate seasons of activity and rest; but within the tropics, life,
in all its aspects, is ever vigorous; and, though the several species
may have their appointed times of partial repose, there is no such
thing as tranquillity for the mass. Hence it is, that to meet the
requirements of a Flora[5] such as there obtains, a less magnificent
Fauna would have been inadequate; and we cannot but recognize, that,
in the wonderful and almost endless modifications of the insect tribes
which people those zones, a special provision has been made to check
the overgrowth of other created things.
But how, it may be asked, does this _primary adaptation_ to external
conditions affect the question of specific development? Perhaps not
much: nevertheless, as lately urged, it is well that such adaptations
should be borne in mind, not merely that due importance may be given
to influences in conformity with which the creative act was at the
first expressly regulated; but also that we may be prepared, if any
qualifying power be admitted to reside in those influences themselves,
for the _kind_ of aberration which reason and experience would seem
alike to imply that we should, in the various instances, anticipate.
We have already stated, that climate, when taken alone, does not
appear to produce any very decided modifying effect on insect form,
seeing that there are vast numbers of species of a wide geographical
range which do not display, on their northern and southern limits,
differences sufficiently constant to be regarded as purely climatal
ones; and it is clear that, if climatal causes of themselves were of
real primary significance, we should probably seldom fail to trace
out, from their long-continued operation, some steady and positive
result. Yet when combined with other principles, there is evidence
that a considerable amount of influence must be conceded to the action
of mere heat and cold, working permanently and according to fixed
laws, on the members of the insect world. Such being the case, it is
perhaps not surprising that a slight difficulty should arise, through
our employment of separate sections under which to examine the causes
of variation; for, since it is ordinarily by the union of several
disturbing influences that aberrations are brought about, it is for
the most part impossible, to refer the results, however conspicuous
they may be, to a solitary controlling element. And hence, though we
may be able at times to point out perchance the _single_ reason for
certain phaenomena with comparative precision, it will generally happen
that two or three agents must be appealed to before we can arrive at a
conclusion by any means satisfactory. I would desire, therefore, that
the examples hereafter to be noticed may be judged of in the mass; and
may not be considered as severally assigned, of necessity, to an
isolated deranging cause, through the fact of their being placed, for
the sake of convenience, and because of the _predominance_ which
special controlling principles have had in maturing them, under
sections, both, as it were, exclusive and particular.
That climate of itself possesses but a limited modifying power on
insect development, is evident from the consideration (just alluded
to), that numerous species of comparatively wide distribution are
totally unaffected by it. Thus, for instance, the _Pissodes notatus_,
Fab., a weevil which occurs in pine forests from Lapland to Barbary,
and which has been naturalized even in the Madeira Islands, passes
through the alternations to which it is specifically subject,
irrespective of country. In like manner, the _Lixus angustatus_, Fab.,
so abundant in Central and Southern Europe, the north of Africa,
Malta, Madeira, and the Canaries, and which has been detected in
Persia, would seem to be perfectly free from atmospheric control. The
_Coccinella 7-punctata_, Linn., which exists in nearly every portion
of the Old World, is apparently unacted upon geographically.
Numberless beetles which follow in the track of man, or at any rate
are liable to do so, almost everywhere (such as _Carpophilus
hemipterus_, Linn., _Trogosita mauritanica_, Linn., _Laemophloe us
pusillus_, Schoenh., _Dermestes vulpinus_, Fab., _Anobium striatum_,
Oliv., _Rhizopertha pusilla_, Fab., _Sitophilus granarius_ and
_Oryzae_, Linn., and _Tribolium ferrugineum_, Fab.), show little or no
tendency to variation. Nor is this independence of climate to be
observed less frequently in the aquatic forms, than in the terrestrial
ones: the _Agabus bipustulatus_, Linn., common in the streams and
pools of the whole of Europe, the north of Africa, and in Madeira,
although naturally somewhat inconstant, offers no aberration, _the
result of latitude_; as is equally the case with the _Hydroporus
confluens_, Fab., which is found from Sweden to the Canaries, and the
_Eunectes sticticus_, Linn.,--an insect literally cosmopolitan. The
Swallow-Tail Butterfly (_Papilio Machaon_, Linn.), the Clouded Yellow
(_Colias Edusa_, Fab.) and the Painted Lady (_Cynthia Cardui_,
Linn.),--the first and second of which occur throughout Europe, in
Siberia, Syria, Egypt, Barbary, Nepaul, and Cashmere; whilst the third
(so general in our own country) has been recorded from India, North
America, the Brazils, Africa, Java, and New South Wales,--however
irregular they may be, afford no indications[6] of undoubted
geographical instability.
We need not however multiply examples, since our space will scarcely
admit of it, and numbers of them will be at once suggested to the
entomologist: what it mainly concerns us here to corroborate, is the
thesis, _that climatal operation_, although by no means invested with
a universal qualifying power, _has an amount of influence on certain
species, even whilst unconnected with other elements,--and therefore_,
a fortiori, _when in combination with them_.
The two principal conditions on which climatal causes generally may be
said to rest, are latitude and altitude. As regards the former of
these, however, whilst the equatorial and arctic regions of the earth
will of course give us the extremes of heat and cold, we shall often
perceive differences of temperature (the result perhaps of local
circumstances) in areas but slightly removed from each other,
sufficient to affect very materially, though by what means it is
difficult to understand, the outward contour of the insect tribes.
Thus, to go no further than Ireland, we find that the specimens of
_Silpha atrata_, Linn., so abundant throughout England and the whole
of Europe, have put on (it may be from the moisture of the atmosphere,
or from some other obscure influence) the appearance of a distinct
race,--so distinct indeed as to have long received another name, _S.
subrotundata_, from British naturalists. I think it far from
improbable that the _Tachyporus nitidicollis_, Steph., an insect
eminently characteristic of that country (and one on which I have
lately offered some remarks[7]), is but a darker climatal modification
of the common _T. obtusus_: and it is well known that the examples of
_Pelophila borealis_, Payk., from Killarney and Loch Neagh are
permanently larger, and much more metallic, than those from the
Orkneys. The _Nebria complanata_, Linn., assumes a more pallid hue in
the neighbourhood of Bordeaux than it does on the sandy coasts of
Devonshire and Wales: and I have but little doubt that the _Omaseus
nigerrimus_, Dej., of Spain, the north of Africa, and Madeira, is a
geographical state of the _O. aterrimus_ of Central Europe. The
_Sitona gressoria_, Illig., so universal throughout the Mediterranean
districts, Madeira and the Canaries, may be but the subaustral form of
_S. grisea_. The _Bembidium obtusum_, Sturm, is shorter and less
parallel in our own latitude than it is in the Madeiran group and
along the Mediterranean shores: whilst the _Holoparamecus niger_,
Aube, of Madeira and Sardinia is very much paler than the same beetle
when taken in Sicily. Specimens of _Pieris Brassicae_, Linn. (the White
Cabbage-Butterfly,--an insect of widely acquired range), from Nepaul
and Japan, are recorded[8] to have differed so strongly from the
ordinary European type as to have been referred, by Boisduval, in
doubt to that species. Mr. Westwood has received the _Vanessa
Atalanta_, Linn., from North America, receding slightly from its
British analogue; but which he, nevertheless, does not regard as
specifically distinct: and such also (he adds) was the opinion of Mr.
Kirby, who has described his American examples under that name. The
common _Hipparchia_ of Madeira I believe to be a fixed geographical
modification of the _H. Semele_, Linn., of our own country,--in which
the paler bars of the upper surface are evanescent;--there are,
however, I imagine, but few entomologists who would concur with me in
this hypothesis. The Madeiran specimens of _Lycaena Phloeas_, Linn.
(the Small Copper Butterfly), are invariably darker, and more
suffused, than the English ones: and Mr. Westwood remarks that he
possesses examples from North America which "differ in the decided
black spotting of the under side of the hind wings, in the bright red
streak near their hind margin, and in wanting the minute spot on the
costa of the fore wings; but that these characters can scarcely be
held to constitute a distinct species[9]."
Few observers can have failed to remark, that increased _altitude_
frequently corresponds, both in its fauna and flora, to a higher
_latitude_; and that, consequently, if we ascend the mountains of a
southern land, we shall be struck, at times, by the presence of a host
of species which obtain at a lower level in more temperate zones. This
is peculiarly traceable in the Madeira Islands,--which, from their
subaustral position, and height (the loftiest peak of the central mass
exceeding 6000 feet above the sea), afford a rich field to the student
of zoological geography. Yet, though the degrees of mere heat and cold
are such as to allow, in the two cases, species positively identical
to flourish; we should surely anticipate some slight change from the
different atmospheric conditions (especially when in union with other
circumstances) to which they have been, through a lapse of ages,
respectively exposed: it may be well therefore to inquire, whether
experience does at all tend to strengthen what our reason has an _a
priori_ inclination to endorse. It must be recollected however that,
in the instances to which we would draw attention, _small_ aberrations
are all that can be usually looked for, since climate _of itself_ does
not appear to be very potent in its action. We should remember, also,
that the boundaries of insect instability are restricted; and,
although we would advocate freedom of development within limits which
are more or less comprehensive according to the species, to pass
beyond them would be confusion, and such as could result from a
_lapsus Naturae_ only, rather than from a power of legitimate
variation.
In exact conformity with what the above remarks will have prepared us
for, we find that the _Dromius obscuroguttatus_, Dufts., of Central
Europe, has undergone on the mountain summits of Madeira changes
precisely to that extent which we should have calculated upon; and
although they would seem in reality to be referable to climate _and
isolation_ combined, yet, since it is not always possible (as lately
stated) to treat the elements of disturbance separately, and it is my
object in this short treatise to bring forward a few prominent
examples in support of the considerations proposed, rather than to
accumulate a mass of material for the registry of which my space would
be inadequate, I will quote _in extenso_ the reflections which, during
the compilation of the 'Insecta Maderensia,' suggested themselves to
me. "The _Dromius obscuroguttatus_ is a common European insect, and
the Madeiran specimens recede from the ordinary ones in being slightly
larger, and in having their elytra more obscurely striated, with the
humeral patch less distinct: their entire surface, moreover, is of a
deeper black, a difference which is especially perceptible on the
legs. It occurs in the greatest profusion in Madeira proper, though
only from about 5000 to 6000 feet above the sea. Although so common
throughout Europe, it is perhaps, when geographically considered, one
of the most interesting of the Madeiran Coleoptera, as affording a
striking example, not only of the modification of form in a normally
northern insect when on its southern limit, but as showing likewise
how a species, abundant on the low sandy shores and sheltered
sea-cliffs of more temperate regions, finds its position here only on
the summits of the loftiest mountains. It is true that the aberration
from the typical state is not in the present instance very
considerable; yet when the circumstances producing it are taken into
account, I am persuaded that the difference is exactly of that nature
on which too great stress cannot possibly be placed, when discussing
the general question of geographical distribution as having a
tendency, more or less directly, to affect both colour and form. It is
well known to naturalists that a multitude of insects from the New
World, receding from their European analogues merely in certain
excessively minute characters, have usually been pronounced at once as
new to science, first because those differences are constant, and
secondly because the specimens have been received from the other side
of the Atlantic. And yet in instances like the present one,--in an
island which, while it belongs artificially to Europe, is yet
naturally sufficiently distinct from it as to form at any rate a
stepping-stone to the coast of Africa and the mountains of
Barbary,--species similarly circumstanced are not necessarily received
as new (and rightly so, I apprehend), though in every respect
affording differences not only _analogous_ to those already mentioned,
but in many instances positively identical with them. If, however, a
specific line of demarcation does of necessity exist between the
creatures of the Old and New Worlds, the problem yet remains unsolved,
so long as intermediate islands present parallel modifications, where
that line is to be drawn. Meanwhile, how far geographical varieties of
this kind, concerning the non-specific claims of which confessedly but
little doubt can exist, may lead to the explanation of the
Transatlantic ones just referred to, I will not venture to suggest.
Yet certain it is, that the one case bears directly on the other; and
that, if we can prove that common European insects, when isolated in
the ocean, become in nearly all cases more or less modified externally
in form, there is at least presumptive evidence that the law will hold
good on a wider scale, and may be extended, not only to the Atlantic
itself, but even to countries beyond. The differences of the present
_Dromius_ from its more northern representatives are, as just stated,
small; nevertheless, since they are _fixed_, those naturalists who do
not believe in geographical influence might choose to consider them of
sufficient importance to erect a new species upon. But after a careful
comparison of this with other insects similarly circumstanced, I am
convinced that the modifications in question are merely local ones,
and such as may be reasonably accounted for by the combined agencies
of latitude and isolation, and the consequently altered habits of the
creature, which is thus compelled to seek alpine localities in lieu of
its natural ones[10]."
In like manner the _Calathus fuscus_, Fab., the _Anchomenus
marginatus_, Linn., and the _Anthicus fenestratus_, Schmidt, which
occur almost exclusively in the _lower_ regions of northern
latitudes, are found in Madeira on the mountain tops; each, moreover,
possessing characters which are just sufficient (although slight) to
distinguish them from their European representatives.
And if we inquire, on the other hand, into the _aboriginal_ species of
those islands,--or, at any rate, into such of them whose naturally
acquired range embraces the opposite extremes of atmosphere,--we shall
detect no less surely (albeit within a narrower space) the result of
climatal action on insect form. The _Helops confertus_, Woll., "varies
according to the altitude at which it is found; being usually deeply
striated and rugose on its lower, but subpicescent and much more
lightly sculptured on its upper limits. I have taken specimens indeed
on Pico Ruivo, and on the mountain-plain of the Fateiras, which are so
far diminished in roughness as almost to resemble, at first sight, the
_H. Pluto_[11]." The _Pecteropus Maderensis_, Woll., which ranges from
about 2500 feet above the sea to the summits of the loftiest hills,
although usually with pale legs, is distinguished by having its femora
almost invariably dusky when on its highest elevation; and, following
out the analogy with that beetle, the _Trechus alticola_, Woll.,
should perhaps be regarded as an alpine state of the _T. custos_. The
_Calathus complanatus_, Koll., assumes along the upland heights a very
different aspect to what it does in the regions below, being generally
more piceous and convex, altogether broader (in proportion) and
shorter, and with _both_ sexes (though, of course, especially the
male) shining.
Nor is this principle of topographical variability (the result of
climate) less apparent in other countries also. The _Notiophili_, for
instance, "are extremely unstable, both in their sculpture and hue,
being subject to considerable local modifications, though more
particularly affected, it would appear, by altitude. Thus, in our own
country, the _N. semipunctatus_, Fab., one of the common
representatives of the plains, is found likewise on the summits of the
mountains; but at that elevation it becomes liable to great
alternations of colour, ranging from pale brassy-brown, with the apex
testaceous, into deep black. The sculpture, however, perhaps is nearly
as much dependent on other circumstances for its modification as upon
altitude, since it seems tolerably clear that proximity to the
sea-shore, especially where the localities are saline, will frequently
produce a more faintly impressed surface[12]." It has indeed been
lately suggested, that the _Helobia nivalis_, Payk., may be perhaps,
after all, but a mountain variety of the _H. brevicollis_; the
_Leistus montanus_, Steph., of the _L. fulvibarbis_, and the _Patrobus
septentrionis_, Dej., of the _P. excavatus_; but of this I think
further proof is needed, seeing that certain species do appear to
exist which are _strictly_ alpine (that is to say, which have not
been, severally, detected in the lower regions of more northern
zones); and, in _most_ instances, where aberrations are to be met
with from the effect of _altitude_, we have a right to inquire
(provided the types from which they are supposed to have originally
sprung obtain in the less-elevated portions of the same country),
_where are the intermediate links_? Now I am not aware that any such
links have, in the examples above cited, ever been observed; whilst I
can vouch that in at any rate many districts where the _quasi_ variety
is found, the descendants of its assumed progenitor _do_ occur in the
plains beneath. I have remarked that the _Cicindelidae_ often become
inconstant in colouring as they approach their maximum of
height above the sea; and I have but little doubt that the _C.
fasciatopunctata_[13], Germ., from Asia Minor and Turkey, is the _C.
sylvatica_ modified by a long residence in elevated regions. And so it
is with the _Chrysomelae_, many of which become, in the loftiest
altitudes to which they ascend (as I have noticed at the head of the
St. Gotthard Pass of the Swiss Alps), subject to unusual changes, both
in lustre and hue.
The above examples, although few and indiscriminately selected, will
serve to illustrate the principle which we have been contending
for,--that climatal influences generally, may (and in most instances
do) tend to affect, more or less directly, the outward contour of the
insect tribes. It will be remarked that, in the cases hitherto cited
no great disturbing power has been made evident,--the aberrations to
which we have appealed being, most of them, comparatively minute.
This, however, is simply in harmony with the belief which we have
already expressed, that climatal causes, when taken singly and alone,
are not of primary importance whilst discussing the question of
specific modification. It remains for us, in the following sections,
to inquire, whether there are any other elements at work from which
greater results are to be expected. Meanwhile, let us not forget that
differences _may_ be, in the strictest sense, significant, even whilst
small; and that it is their _constancy_, rather than their magnitude,
which more particularly concerns us in the present treatise, seeing
that it is with reference to those distinctions which are less
conspicuous that the greatest amount of misunderstanding (through the
fact of their being _fixed_) usually prevails; whilst it is our main
object to show that dissimilarities do not _necessarily_ imply the
specific isolation of the creatures which display them, merely because
they are, in their several localities, _permanent_.
Sec. II. _Temporary heat or cold, of an unusual degree._
It is perhaps unnecessary that the action of temporary heat and cold,
of an unusual degree, should be considered under a separate head from
that of climatal causes generally; nevertheless, since the latter are,
in a certain sense, permanent in their operation, it may be thought
desirable that I should offer a few words on the effect of sudden
exceptions to the ordinary routine of things, such as, for instance,
seasons of peculiar intensity. It does not however appear that any
very important modifications do often occur from conditions thus
abnormal, and as it were _accidentally_ brought about: on the
contrary, indeed, it is a well-known fact, that the members of the
insect world are singularly independent of such contingencies; and
that, in the same manner as their times of maturation are neither
hastened nor retarded by them, their external development is for the
most part free from their control. Yet, in spite of this, specific
results _are_ wont to happen, ever and anon, from such circumstances,
as though it were a fundamental axiom, that every agent which Nature
can press (regularly or irregularly) into her service should have,
though it may not always exercise its privilege, some qualifying
voice.
I believe that almost the only deviation from the typical state, in
insect form, which has been observed to originate, _par excellence_,
from the occasional continuance of undue heat or cold, is curiously
enough an organic one,--having reference to the enlargement of the
wings. Every entomologist must be aware that a vast proportion of the
Coleoptera (especially the _Carabidae_) are subject to great
inconstancy in their metathoracic organs of flight. Many species, as
the common _Calathus mollis_ of our own country (to which my attention
has been more particularly drawn by the Rev. J. F. Dawson), have the
hind wings at one time ample, at another rudimentary, and at a third
nearly obsolete. Now, although other causes, hereafter to be noticed,
would seem to have far greater power than climatal ones in
_permanently_ regulating the size and capacity of these appendages; I
think it will be found on examination (and I may add that Mr. Westwood
is of the same opinion[14]), that the greater or less development of
them may be frequently explained by the unusual severity of the
seasons. My own researches would certainly tend to prove, that _heat_
does (in the main) favour, and _cold_ retard, their presence.
Exceptions (often rendered intelligible from the evident working of
counter influences) will of course arise in abundance to this
hypothesis; yet my impression is that, upon a broad scale, it will
stand the ordeal of a rigid inquiry.
Speaking of certain representatives of the Hymenoptera (_Chalcididae_),
Mr. Westwood observes: "A curious peculiarity exists in one at least
of these apterous species, which has been noticed by no previous
author, namely, _Choreius ineptus_, Westw., which, although ordinarily
found in an apterous state, was discovered by me in considerable
numbers during the hot summer of 1835, with wings[15]". And, touching
the irregularity of the alary organs in the Homopterous _Fulgoridae_,
he remarks: "Other instances, in which the wings undergo a deficiency
of development, occur in the genus _Delphax_, the majority of which,
in our English species, have the upper wings not covering more than
one half of the abdomen,--the terminal membrane being deficient, _as
well as the hind wings_. In certain seasons, however, especially hot
ones, the wings are fully developed[16]". Mr. Curtis has indeed formed
the undeveloped specimens into a different genus, _Criomorphus_.
Although the result of a more stimulating sun may be often neutralized
by that of _isolation_ (which, as we shall hereafter see, is a
resistless agent, amongst a host of species, in weakening, and
frequently rendering abortive, the powers of flight); yet _heat_, when
freed from counter influences, may be traced in its _permanent_ effect
on the alary system of insects, no less than when temporarily applied.
The consideration of this, however, belongs strictly to the preceding
pages, and we will not therefore discuss it here. The common Bed-bug
(_Cimex lectularius_, Linn.) is almost invariably apterous, or with
very short rudimental hemelytra; yet Scopoli (_Ent. Carn._ p. 354)
mentions its occurrence with perfect wings. Fallen, also, and
Latreille, state that it has been found winged; whilst Westwood
remarks that it has been reported as occasionally winged in the East
Indies; and it would seem extremely probable that, in these examples,
as in numerous others which are on record, we may detect the
consequences of heat; either as temporarily applied (in an unusual
degree), or through the accidental transportation of the insect into a
naturally warmer atmosphere.
Sec. III. _Nature of the country and of the soil._
Before we proceed to inquire to what extent the outward aspect of
insects is liable to be controlled by the physical state of the areas
in which they severally obtain, it may not be altogether out of place
to offer a few reflections on the superiority which some regions
possess intrinsically over others, both for the _increase_ and
_diffusion_ of the animal tribes. To suppose that all countries within
the same parallels of latitude are equally favourable for the
development of life (not to mention the after-dispersion of it), is
contrary to experience; for although (as we have already pointed out)
the organic world does certainly, when viewed in the mass, approach
its maximum as we near the tropics, there are at the same time so many
violations of this law, that we cannot admit its operation except in a
broad and general sense.
In a former section of this chapter, I drew attention to the fact,
that certain islands, equatorial and subaustral, are anything but
suggestive of their actual positions with respect to the line of
central heat on the surface of the earth. It was with regard to
_climate alone_, however, that I wished them to be understood: and it
is not until now that I have ventured to urge the necessity of taking
other influences into account also, if we would desire to recognize
anything like design and adaptation (I will hardly call it cause and
effect) between the continent and the thing contained. It is almost
needless to add, that there are _many_ elements to be considered, such
as local atmospheric conditions, excess or deficiency of electricity,
superabundant moisture, diminished light, and the geological
composition of the soil, before we can hope either to appreciate
zoological phaenomena as a whole, or to reconcile the apparent
inconsistencies which they are accustomed to display.
Mr. Darwin, to whom we are indebted for so much valuable information
concerning the natural history of various portions of the world, in
his notes on Tierra del Fuego, observes: "Beetles occur in very small
numbers; it was long before I could believe that a country as large as
Scotland, covered with vegetable productions and with a variety of
stations, could be so unproductive. The few which I found were alpine
species of _Harpalidae_ and _Heteromera_, living beneath stones. The
vegetable-feeding _Chrysomelidae_, so eminently characteristic of the
tropics, are here almost entirely absent. I saw very few flies,
butterflies, or bees, and no crickets or Orthoptera. In the pools of
water I found but few aquatic beetles. I have already contrasted the
climate as well as the general appearance of Tierra del Fuego with
that of Patagonia; and the difference is strongly exemplified in the
entomology. I do not believe they have one species in common;
certainly the general character of the insects is widely
dissimilar[17]." Now, it is impossible to read this account without
being at once struck with two primary considerations: first, that
there must exist some great peculiarity (apart from climate) in a
region the fauna of which is thus singularly constituted; and,
secondly, that latitude (however important it may be in a
comprehensive point of view) must exercise in this case a very
secondary influence, to allow of localities separated only by the
Straits of Magellan to present differences thus extraordinary.
Although so dissimilar in many respects, Madeira and Tierra del Fuego
have evidently much in common as regards the conditions which they
afford for the increase of organic life. Mr. Darwin describes the
latter as "a mountainous region, partly submerged in the sea." So is
Madeira. He also adds, that it is "covered to the water's edge with
one dense, gloomy forest;" that "to find an acre of level land in any
part of the country is most rare;" and that "within the forest, the
ground is concealed by a mass of slowly putrefying vegetable matter,
which, from being soaked with water, yields to the foot." Such _was_
Madeira, in its normal state[18]; and such it still is throughout a
large district towards the northern coast. I cannot indeed refrain
from quoting the following, since it portrays the characteristic
features of Madeira so vividly, as to be, literally, as suggestive of
that island as it doubtless is of Tierra del Fuego. "Finding it nearly
hopeless," says Darwin, "to push my way through the wood, I followed
the course of a mountain-torrent. At first, from the waterfalls and
number of dead trees, I could hardly crawl along; but the bed of the
stream soon became a little more open, from the floods having swept
the sides. I continued slowly to advance for an hour along the broken
and rocky banks, and was amply repaid by the grandeur of the scene.
The gloomy depth of the ravine well accorded with the universal signs
of violence. On every side were lying irregular masses of rock and
torn-up trees; other trees, though still erect, were decayed to the
heart and ready to fall. The entangled mass of the thriving and the
fallen reminded me of the forests within the tropics; yet there was a
difference,--for in these still solitudes, Death, instead of Life,
seemed the predominant spirit[19]."
As regards the paucity of species in Tierra del Fuego, there are many
instances on record of other countries, and in various latitudes, in
which the same anomaly (though perhaps in a less degree) prevails. I
have myself observed, in Madeira, large forest tracts, at a
considerable elevation above the sea, and which are so densely clothed
with wood as to be scarcely penetrable, almost destitute of insect
life. Around such altitudes however the clouds perpetually cling, and
the rain is well nigh incessant; and it would seem as if the very
dampness which causes the vegetation (especially the ferns) to
flourish in such rank luxuriance, and the timber to rot with such
rapidity that the gigantic trunks are washed, reeking with moisture,
down the mountain-slopes, was too extreme for animal existence.
Now, it will be remembered that the Madeiran group is situated at a
corresponding distance from the Equator as Morocco, Algeria, the lower
limits of Syria, Texas, and Upper Florida are,--all of which literally
teem with life; and that Tierra del Fuego lies between the same
parallels of south latitude as Durham and Central Russia do in the
northern hemisphere. From which it is evident, that the equal removal
of countries from the earth's greatest heat does not necessarily imply
an equal _exuberance_ in their Faunas,--seeing that in both the
regions just appealed to, we not only perceive a vast difference in
the _numbers_ of the insects which they respectively contain, from
those in other districts which have a similar divergence from the
tropics; but we are even able to recognize a certain _resemblance of
physical conditions_ (and, therefore, of the creatures which have been
either adapted to, or modified by, them) in lands so far asunder, not
merely with respect to latitude, but longitude also, as Madeira and
Tierra del Fuego.
Other instances might be cited, in support of the immediate principle
for which we are now contending,--namely, that many areas have (from
local circumstances) a natural superiority over others for the
increase of the animal tribes, even _apart from the direct action of
heat and cold_:--but space will only permit me to glance at a very few
of them. We may detect evidences of this fact, in Ireland; which, in
spite of the narrowness of the straits which separate it from our own
country, and of its independent commerce with all parts of the
civilized world, has an insect fauna curiously limited. From what
cause this may arise,--whether from some obscure physical influences
peculiar to the soil, or (as Professor E. Forbes has suggested) from
the sudden impediment which the establishment of St. George's Channel
presented to the westward progress of the various species from the
Germanic plains,--it is difficult to speculate: yet the _fact_ of its
poverty remains, and we must explain it as best we are able. There
can be no question, that, from more frequent communication with
England, its entomological fauna has of late years been considerably
increased; and it is equally easy to detect, through an examination of
its less inhabited provinces, that at a period geologically recent its
insect population must have been singularly scanty. I know of few
regions (not even excepting the uplands of Madeira) which are more
deficient in insect life than the mountains of Kerry. Although
abounding, throughout extensive districts, with wood and water, and
presenting every apparent requisite for its full development; the
naturalist will often be disappointed by finding that a hard day's
work has not ensured him the same amount of success as he would have
reaped in less than half an hour in many an English meadow. Do we ask,
why this is so?--it is impossible to reply, except on the supposition
that there are real physical agents, independently of heat and cold,
which are unfavourable in Ireland to the existence of these lower
creatures. We may perhaps be told, by the advocates of Professor
Forbes's theory, that it is the result of isolation,--the quondam land
of passage having been broken up before the proper complement of
species had reached this large portion of their western destination.
But even this, although I believe it to contain much presumptive
truth, will not altogether suffice to account for the phaenomena which
we see; for Ireland is not only remarkable for the paucity of its
_species_, but also for the paucity of its _individuals_,--and
the latter fact cannot be explained by any stretch of the
migration-hypothesis. We are compelled therefore to conclude, that
Ireland, like the other countries to which we have already alluded,
presents conditions (altogether irrespective of _latitude_) which must
be regarded as adverse to the general prosperity of the insect races.
And so it is with _localities_ (no less than with larger
countries),--many of which are eminently unproductive, when compared
with others situated at but a short distance from them. Thus, the
south-western corner of England is by far the most unprofitable
portion of our island, unless indeed I am much mistaken, for insect
ascendency. I have made some remarks on this subject in the
'Zoologist,'--from which I extract the following: "Unlike the easy
collecting to which we are accustomed in the more favoured East, miles
of unprofitable country have often to be gone over, be it swampy
moorland or iron-bound coast, where scarcely an insect is to be seen;
or, at any rate, where the few which exist are so ordinary, and so
sparingly dispersed, as to be scarcely worth the labour of obtaining
them,--more especially since the identical species are many of them to
be met with in the utmost profusion in more central, or eastern
districts. Whether it be the moisture of the climate, or the violence
of the south-west winds, which (continually sweeping, as they do, over
the high central mass of Devonshire and the bleak, barren downs of
Cornwall) present as great an obstacle to the development of animal,
as they clearly do of vegetable life, I will not venture to suggest;
yet certain it is, from observation, that insects not only become
fewer in number in proportion as they are exposed to these external
agencies of wind and water; but likewise, in many instances, diminish
so considerably in stature as to be scarcely reconcileable with their
normal types[20]."
There can be no doubt that islands are, for the most part, more
unproductive (even in proportion) than continents; and that, the
smaller the area, the less favourable will it be for the development
of insect life. Mr. Darwin has noticed this fact in the Galapagos
(which he remarks are only equalled by Tierra del Fuego, in
barrenness), on Keeling Island (in the Indian Ocean), where he
succeeded in detecting but thirteen species, in St. Helena, and at
Ascension; and I have added fresh evidence to the same in the various
portions of the Madeiran Group[21]. It is however to geological causes
that we must mainly look for the explanation of this phaenomenon; and,
therefore, since I propose to examine that branch of our subject in a
future chapter of this treatise, we will not discuss it now. It will
also be better perhaps to defer for the present the general question
of self-_diffusion_, which, at the opening of this section, we
proposed to consider, along with that of insect _productiveness_ (as
dependent on other local influences, besides climatal ones),--it being
scarcely possible to render the problem of dispersion in any degree
intelligible without calling in geology to our aid.
Having then disposed of this preliminary appendage to our inquiry, by
expressing our belief (which I am satisfied that observation will tend
more and more to corroborate) _that certain countries and spots are by
constitution more favourable than others for the increase_ (apart from
the after dissemination) _of the insect tribes_,--and that too through
local influences amongst which mere heat and cold are but secondary in
importance; let us proceed to consider, how far the _nature of the
several districts_ may assist us in accounting for some of those
numerous aberrations from the typical state which various insects are
accustomed to display, and on which it has too often happened that
"species" (so called) have been attempted to be established. I may
premise however, that, whilst (as already urged) I would regard
climate _per se_ as subsidiary to many other agents, I would not wish
to ignore its action altogether even under the present section, since
in combination with peculiar circumstances and conditions it may have
(and probably has) considerable controlling power: nevertheless I
would desire it to be looked upon here as, at any rate, an inferior
element, and as working in conjunction with physical influences of
greater significance than itself. If therefore under the preceding
heads it has been treated (so far at least as the exceptions would
permit) as a great geographical principle, possessing a certain
modifying quality on a large scale, let us now merely recognize it to
the extent in which we are actually compelled to do, when dealing with
areas of smaller magnitude,--namely as a _topo_graphical one.
From amongst the many results which I have been long accustomed to
associate (whether rightly so, or not, I leave it for others to
decide) with certain special situations, I would draw attention to the
singular inconstancy which numerous insects are liable to when
existing on the coast,--and which frequently causes them to assume an
aspect so permanently different from their inland types, that, without
local knowledge to guide us, they might be supposed at first sight to
be specifically distinct. Ten years ago I offered a few comments on
this fact in the pages of the 'Zoologist'; which, as I have seen no
reason subsequently to modify them, I will transcribe at length:--
"The extraordinary changes which many insects are subject to when
occurring near the sea, is a fact worthy of notice, and one which I do
not remember to have seen recorded. The strictly maritime species must
be left out of the question; for although many of them are exceedingly
variable both in size and colour, still we have no means of
ascertaining whether that variation is referable to the locality in
which they are placed,--for, never being found inland, nobody can have
an opportunity of asserting that the same changes would not take
place, were they to occur in positions far removed from the influence
of the sea. When we find, however, the same insects in profusion both
inland and on the coast, and observe also numerous and marked
deviations from the typical forms peculiar to the latter situation;
then, _a priori_, we have strong presumptive evidence that the
changes in question are the result of local circumstances, and not
referable to chance. The alteration in size I have almost always
observed to be from large to small, and scarcely ever the reverse;
whereas in colour the change takes place very nearly as much from
light to dark as it does from dark to light: nevertheless the majority
of instances I possess come under the latter department. It has been
remarked that all the specimens of _Mesites Tardii_, which I captured
in Devonshire, were much smaller than the original series taken by Mr.
Tardy at Powerscourt Waterfall, in the county of Wicklow; and so
decided was the difference, that many of my friends, at first sight,
concluded the two to be distinct species. This, however, I consider
entirely owing to their locality, for my specimens were found only on
the coast, and Mr. Tardy's at a considerable distance inland. And,
inasmuch as neither of these instances rested on mere individual
examples, but on long and conspicuous series, the certainty of the
change from large to small was the more apparent. Mr. Holme of Oxford
mentions having taken _Olisthopus rotundatus_ in the Scilly Islands,
in great profusion, none of the specimens of which exceeded two lines
and a half in length. At Whitsand Bay in Cornwall I have captured
_Gymnaetron Campanulae_, none of which exceeded three-quarters of a
line,--the usual length being from a line to a line and
three-quarters. _Anthonomus ater_, the average length of which is two
lines, I have taken a series of in Lundy Island, none of which
exceeded one. In the same locality, also, the common _Ceutorhynchus
contractus_ scarcely ever reaches its natural size; and is, moreover,
so variable in colour, that I was long before I could persuade myself
that the species was not distinct. Instead of the bluish-black elytra
which I had always considered invariable, they all possess a yellowish
or brassy tinge; and the legs, instead of being black, are in most
instances entirely of a light yellow,--and in all, more or less
inclined to that colour. I have received from Mr. Hardy, of Gateshead,
specimens of _Haltica rufipes_[22], captured by him on the coast, in
which the entire insect is of a uniform brownish-red hue. Of the rare
_Mantura Chrysanthemi_ I have taken beautiful varieties at Mount
Edgcumbe and in Lundy Island,--many of which inclined to a rich
metallic-yellow, instead of the brassy-brown of the ordinary
specimens: also, in the latter locality, particularly dark specimens
of _Telephorus testaceus_. In like manner, I might enumerate other
species equally remarkable; but I trust that those already mentioned
are sufficient to verify my observations, of the extreme liability to
change which, more or less, most insects possess when placed within
the immediate influence of the sea. How to account for it, I know not.
I mention it as a mere fact, and leave it for others to assign a
reason for its existence[23]."
Apparently dependent, in a large measure, on the same circumstance
(namely proximity to the coast), the _Bembidium saxatile_, Gyll., so
common at the edges of the mountain streams in the north of England,
in Scotland, and throughout a portion of Ireland, presents itself
along our southern shores in the form of a permanent variety; being,
as the Rev. J. F. Dawson remarks, "more depressed, never narrower in
front (the sides therefore more parallel), whilst the colour is always
much paler and the spots larger,--that before the apex being round and
very conspicuous, and the anterior one occasionally expanding over the
surface very considerably[24]." I have taken it in profusion on the
coasts of the Isle of Wight, Dorsetshire, and Devon. And so with the
_Cistela sulphurea_, Linn., which in certain maritime localities (as I
have particularly noticed on the sand-hills at Deal) is liable to
become so dark in colouring, that, without the intermediate shades to
judge from (which however may usually be obtained _in situ_), it might
stand a fair chance, occasionally, of being mistaken for a separate
species. A _Psylliodes_ in Lundy Island, allied to (if not identical
with) the _chrysocephala_, Linn., found in abundance on a _Brassica_
along the ascent from the eastern landing-place, varies "in every
consecutive shade between the limits of light yellow and dark
metallic-green[25]," the former of which states (the normal one on
that rock) might have been fairly set down as specifically distinct
from the latter, did not observation on the spot decide the question
for us without doubt.