+-------------------+-----------------------------------------------+
| | Number of Spines on the head and segments |
| | of the larva. |
| +-----+-----+-----+-----+-----+-----+-----+-----+
| |Head.|Segm.|Segm.|Segm.|Segm.|Segm.|Segm.|Segm.|
| | | I. | II. | III.| IV. | V. |VI.- | XII.|
| | | | | | | | XI. | |
| +-----+-----+-----+-----+-----+-----+-----+-----+
|V. Io | 0 | 0 | 2 | 2 | 4 | 6 | 6 | 4 |
|V. Antiopa | 0 | 0 | 4 | 4 | 6 | 6 | 7 | 4 |
|V. Urticæ | 0 | 0 | 4 | 4 | 7 | 7 | 7 | 4 |
|V. Polychloros | 0 | 0 | 4 | 4 | 7 | 7 | 7 | 4 |
|V. Ichnusa | 0 | 0 | 4 | 4 | 7 | 7 | 7 | 4 |
|V. Atalanta | 0 | 0 | 4 | 4 | 7 | 7 | 7 | 4 |
|V. C.-album | 2 | 0 | 4 | 4 | 7 | 7 | 7 | 4 |
|V. Interrogationis | 2 | 0 | 4 | 4 | 7 | 7 | 7 | 4 |
|V. Levana | 2 | 0 | 4 | 4 | 7 | 7 | 7 | 4 |
+-------------------+-----+-----+-----+-----+-----+-----+-----+-----+
This character of the number of spines will not be considered as too
unimportant when we observe how perfectly constant it remains in the
nearly allied species. This is the case in the three consecutive
forms, _Urticæ_, _Polychloros_, and _Ichnusa_. Now when we see
that two species which differ in their imaginal characters present
correspondingly small differences in their larvæ, this exact systematic
congruence indicates a completely parallel phyletic development.
Exceptions are, however, to be met with here. Thus, Hübner has united
one group of the species of _Vanessa_ into the genus _Pyrameis_ just
mentioned, on account of certain characteristic distinctions of the
butterflies. I do not know, however, how this genus admits of being
grounded on the structure of the larvæ; the latter, as appears from the
above table, agree exactly in the number and position of the spines
with the caterpillars of _Vanessa_ (_sensû strictiori_), nor can any
common form of marking be detected which would enable them to be
separated from _Vanessa_.
Still more striking is the incongruence in the genus _Araschnia_,
Hübn. (_A. Prorsa-Levana_), which, like the genus _Pyrameis_, is
entirely based on imaginal characters. This is distinguished from all
the other sub-genera of the old genus _Vanessa_ by a small difference
in the venation of the wings (the discoidal cell of the hind-wings is
open instead of closed). Now it is well-known that in butterflies the
wing-venation, as most correctly shown by Herrich-Schäffer, is the
safest criterion of “relationship.” It thus happens that this genus,
typified by the common _Levana_, is in Kirby’s Catalogue separated
from _Vanessa_ by two genera, and according to Herrich-Schäffer[188]
by forty genera! Nevertheless, the larvæ agree so exactly in their
spinal formula with _Grapta_ that we should have no hesitation in
regarding them as a species of this sub-genus. It appears to me very
probable that in this case the form-relationship of the caterpillar
gives more correct information as to the blood-relationship of the
species than that of the imago--in any case the larvæ show a different
form-relationship to the imagines.
Just as in the case of butterflies there are many genera of _Sphingidæ_
which can be based on the structure of the larvæ, and which agree with
those founded on the imagines.
Thus, the genus _Macroglossa_ is characterized by a straight anal horn,
a spherical head, and by a marking composed of longitudinal stripes,
these characters not occurring elsewhere in this combination. The
nearly allied genus _Pterogon_, on the other hand, cannot be based
on the larvæ only, since not only is the marking of the adult larva
very distinct in the different species, but the anal horn is present
in two species, whilst in a third (_P. Œnotheræ_) it is replaced by
a knob-like eye-spot. The genus _Sphinx_ (_sensû strictiori_) is
distinguished by the simple, curved caudal horn, the smooth, egg-shaped
head and smooth skin, and by a marking mainly composed of seven oblique
stripes. The genus _Deilephila_ is distinguished from the preceding by
a dorsal plate, situated on the prothorax and interrupting the marking,
as well as by the pattern, which here consists of a subdorsal line
with ring-spots more or less numerous and developed; the skin also
is rough, “shagreened,” although it must be admitted that there are
exceptions (_Vespertilio_). The genus _Chærocampa_ admits also of being
based on the form-relationship of its caterpillars, although this is
certainly only possible by disregarding the marking and taking alone
into consideration the peculiar pig-like form of the larvæ. The genus
_Acherontia_, so nearly related to _Sphinx_, possesses in the doubly
curved caudal horn a character common to the genus (three species
known[189]). Finally may be mentioned the genus _Smerinthus_, of which
the larvæ, by their anteriorly tapering form, their shagreened skin and
almost triangular head with the apex upwards, their simply curved anal
horn, and by their seven oblique stripes on each side, constitute a
genus as sharply defined as that formed by the moths.
Although in all the systematic divisions hitherto treated of there are
cases where the form-relationship of the larva does not completely
coincide with that of the imago, such incongruences are of far more
frequent occurrence in the smallest systematic group, viz. species.
The larvæ of two species have very frequently a much nearer
form-relationship than their imagines. Thus, the caterpillars of
_Smerinthus Ocellatus_ and _S. Populi_ are closely allied in
structure, marking, and colouring, whilst the moths in these two last
characters and in the form of the wings are widely separated.[190]
Judging from the larvæ we should expect to obtain two very similar
moths, but in fact both _Populi_ and _Ocellatus_ have many near allies,
and these closely related species sometimes possess larvæ which differ
more widely than those of more distantly related species of imagines.
Thus, in Amur-land and North America there occur species of
_Smerinthus_ which closely resemble our _Ocellatus_ in colour, marking,
and form of wing, and which possess the characteristic large blue
ocellus on the hind-wings. _S. Excæcatus_ is quite correctly regarded
as the representative American form of our _Ocellatus_, but its
caterpillar, instead of being leaf-green, is of a chrome-yellow, and
possesses dark green instead of white oblique stripes, and has moreover
a number of red spots, and a red band on the head--in brief, in the
very characters (colour and certain of the markings) in which the
imagines completely agree it is widely different from _Ocellatus_. It
appears also to be covered with short bristles, judging from Abbot and
Smith’s figure.[191]
Just in the same way that the species having the nearest conceivable
form-relationship to _Ocellatus_ possesses a relatively strongly
diverging larva, so does the nearest form-relation of _Populi_ (imago)
offer a parallel case. This species, which is also North American,
lives on _Juglans Alba_. The imago of _Smerinthus Juglandis_ differs
considerably from _S. Populi_ in the form of the wings, but it
resembles the European species so closely in marking and colouring that
no doubt can exist as to the near relationship of the two forms. The
caterpillar of _S. Juglandis_,[192] however, differs to a great extent
from that of _Populi_ in colour--it is not possible to confound these
two larvæ; but those of _Populi_ and _Ocellatus_ are not only easily
mistaken for one another, but are distinguished with difficulty even by
experts.
In this same family of the _Sphingidæ_ cases are not wanting in which,
on the other hand, the moths are far more closely allied than the
larvæ. This is especially striking in the genus _Deilephila_, eight
species of which are allied in the imaginal state in a remarkable
degree, whilst the larvæ differ greatly from one another in colour,
and to as great an extent in marking. These eight species are _D.
Nicæa_, _Euphorbiæ_, _Dahlii_, _Galii_, _Livornica_, _Lineata_,
_Zygophylli_, and _Hippophaës_. Of these, _Nicæa_, _Euphorbiæ_,
_Dahlii_, _Zygophylli_, and _Hippophaës_ are so much alike in their
whole structure, in the form of the wings, and in marking, that
few entomologists can correctly identify them off-hand without
comparison. The larvæ of these four species, however, are of very
different appearances. Those of _Euphorbiæ_ and _Dahlii_ are most
alike, both being distinguished by the possession of a double row of
large ring-spots. _Zygophylli_ (see Fig. 50, Pl. VI.) possesses only
faint indications of ring-spots on a white subdorsal line; and in
_Hippophaës_ there is only an orange-red spot on the eleventh segment,
the entire marking consisting of a subdorsal line on which, in some
individuals, there are situated more or less developed ring-spots (see
Figs. 59 and 60, Pl. VII.). If we only compare the larvæ and imagines
of _D. Euphorbiæ_ and _Hippophaës_, we cannot but be struck with
astonishment at the great difference of form-relationship in the two
stages of development.
In the case of _D. Euphorbiæ_ and _Nicæa_ this difference is almost
greater. Whilst these larvæ show great differences in colour, marking,
and in the roughness or smoothness of the skin (compare Fig. 51, Pl.
VI. with Figs. 43 and 44, Pl. V.), the moths cannot be distinguished
with certainty. As has already been stated, the imago of the rare
_D. Nicæa_ is for this reason wanting in most collections; it cannot
be detected whether a specimen is genuine, _i.e._ whether it may not
perhaps be a somewhat large example of _D. Euphorbiæ_.
An especially striking instance of incongruence is offered by the
two species of _Chærocampa_ most common with us, viz., _Elpenor_ and
_Porcellus_, the large and small Elephant Hawk-moths. The larvæ are
so similar, even in the smallest details of marking, that they could
scarcely be identified with certainty were it not that one species
(_Elpenor_) is considerably larger and possesses a less curved caudal
horn than the other. The moths of these two species much resemble
one another in their dull green and red colours, but differ in the
arrangement of these colours, _i.e._ in marking, and also in the form
of their wings, to such an extent that _Porcellus_ has been referred to
the genus _Pergesa_[193] of Walker. If systemy, as is admitted on many
sides, has only to indicate the morphological relationship, this author
is not to blame--but in this case a special larval classification must
likewise be admitted, in a manner somewhat similar to that at present
adopted provisionally in text-books of zoology for the Hydroid Polypes
and inferior Medusæ. This case of _Porcellus_, however, shows that
those are correct who maintain that systemy claims to express, although
incompletely, the blood-relationship, and that systematists have always
unconsciously formed their groups as though they intended to express
the genetic connection of the forms. Only on this supposition can it
appear incorrect to us to thus separate two species of which the larvæ
agree so completely.
I cannot conclude this review of the various systematic groups without
taking a glance at the groups comprised within species, viz. varieties.
Whilst in species incongruence is of frequent occurrence, in varieties
this is the rule, for which reason it admits in this case of being more
sharply defined, since we are not concerned with a double difference
but only with the question whether in the one stage a difference or an
absolute similarity is observable. By far the majority of varieties are
either simply imaginal or merely larval varieties--only the one stage
diverges, the other is quite constant.
Thus, as has already been shown, in all the seasonally dimorphic
butterflies known to me the caterpillars of the two generations of
imagines, which are often so widely different, are exactly alike; and
the same obtains for the majority of purely climatic varieties of
butterflies. Unfortunately there are as yet no connected observations
on this point. The only certain instance that I can here mention is
that of the Alpine and Polar form of _Pteris Napi_. This variety,
_Bryoniæ_, the female of which differs so greatly in marking and
colouring, possesses larvæ which cannot be distinguished from those of
the ordinary form of _Napi_.(See part I. appendix I. p. 124.)
That caterpillars can also vary locally without thereby affecting the
imagines is shown by the frequently mentioned and closely investigated
cases of di- and polymorphism in the larvæ of a number of _Sphingidæ_
(_M. Stellatarum_, _A. Atropos_, _S. Convolvuli_, _C. Elpenor_, and
_Porcellus_, &c.). The same thing is still more clearly shown by those
instances in which there are not several but only one distinct larval
form occurring in each of two different localities.
To this class belongs the above-mentioned case of _Chærocampa Celerio_
(p. 197), supposing our information concerning this species to be
correct; likewise the recently-mentioned case of the Ligurian variety
of the caterpillar of _Saturnia Carpini_; and finally the case of
_Eriogaster Lanestris_, so well known to lepidopterists. This insect
inhabits the plains of Germany, and in the Alps extends to an elevation
of 7000 feet, where it possesses a larva differently marked and
coloured (_E. Arbusculæ_) to those of the lowlands whilst the moths are
smaller, but do not differ in other respects from those of the plains.
Among the Alpine species many other such cases may occur, but these
could only be discovered by making investigations having special
reference to this point. Of the Alpine butterflies, for example,
not a single species can have been reared from the caterpillar;
for this reason but few observations have on the whole been given
by entomologists respecting the Alpine larvæ, which are not known
sufficiently well to enable such a question to be decided.
The investigation of the form-relationships existing between larvæ on
the one hand and imagines on the other has thus led to the following
results:--
We learn on comparison that incongruences or inequalities of
form-relationship occur in all systematic groups from varieties to
families. These incongruences are of two kinds, in some cases being
disclosed by the fact that the larvæ of two systematic groups, _e.g._
two species, are more closely related in form than their imagines (or
inversely), whilst in other cases the larvæ form different systematic
groups to those formed by the imagines.
The results of the investigation into the occurrence of incongruences
among the various systematic groups may be thus briefly summarised:--
Incongruences appear to occur most frequently among varieties, since
it very frequently happens that it is only the larva or only the
imago which has diverged into a variety, the other stage remaining
monomorphic. The systematic division of varieties is thus very often
one-sided.
Among species also incongruences are of frequent occurrence. Sometimes
the imagines are much more nearly related in form than the larvæ, and
at others the reverse happens; whilst again the case appears also
to occur in which only the one stage (larva) diverges to the extent
of specific difference, the other stage remaining monomorphic (_D.
Euphorbiæ_ and _Nicæa_).
The agreement in form-relationship appears to be most complete in
genera. In the greater number of cases the larval and imaginal genera
coincide, not only in the sharpness of their limits, but also--as far
as one can judge--in the weight of their distinctive characters, and
therefore in the amount of their divergence. Of all the systematic
groups, genera show the greatest congruence.
In families there is again an increase of irregularity. Although larval
and imaginal families generally agree, there are so many exceptions
that the groups would be smaller if they were based exclusively
on the larval structure than if founded entirely on the imagines
(_Nymphalidæ_, _Bombycidæ_).
If we turn to the groups of families we find a considerably increased
incongruence; complete agreement is here again rather the exception,
and it further happens in these cases that it is always the larvæ
which, to a certain extent, remain at a lower grade, and which form
well defined families; but these can seldom be associated into groups
of a higher order having a common character, as in the case of the
imagines (_Rhopalocera_).
After having thus collected (so far as I am able) the facts, we have
now to attempt their interpretation, and from the observed congruence
and incongruence of form-relationship of the two stages to endeavour to
draw a conclusion as to the underlying causes of the transformations.
It is clear at starting that all cases of incongruence can only be the
expression or the consequence of a phyletic development which has not
been exactly parallel in the two stages of larva and imago--that one
stage must have changed either more rapidly or more slowly than the
other. An “unequal phyletic development” is thus the immediate cause of
incongruence.
Thus, the occurrence of different larvæ in species of which the
imagines have remained alike may be simply understood as cases in
which the imago only has experienced a change--has taken a forward
step in phyletic development, whilst the larvæ have remained behind.
If we conceive this one-sided development to be repeated several
times, there would arise two larval forms as widely different as those
of _Deilephila Nicæa_, and _Euphorbiæ_, whilst the imagines, as is
actually the case in these species, would remain the same.
The more commonly occurring case in which one stage has a greater
form-divergence than the other, is explicable by the one stage having
changed more frequently or more strongly than the other.
The explanation of the phenomena thus far lies on the surface, and it
is scarcely possible to advance any other; but why should one stage
become changed more frequently or to a greater extent than the other?
why should one portion be induced to change more frequently or more
strongly than another? whence come these inducements to change? These
questions bring us to the main point of inquiry:--Are the causes which
give rise to these changes internal or external? Are the latter the
result of a phyletic vital force, or are they only due to the action of
the external conditions of life?
Although an answer to this question will be found in the preceding
essay, I will not support myself on the results there obtained, but
will endeavour to give another solution of the problem on fresh
grounds. The answer will indeed be the same as before:--A phyletic
force must be discountenanced, since in the first place it does not
explain the phenomena, and in the second place the phenomena can be
well explained without its assumption.
The admission of a phyletic vital force does not explain the phenomena.
The assumption that there is a transforming power innate in the
organism indeed agrees quite well with the phenomenon of congruence,
but not with that of incongruence. Since a large number of cases of
the latter depend upon the fact that the larvæ are more frequently
influenced by causes of change than their imagines, or _vice versâ_,
how can this be reconciled with such an internal force? On this
assumption would not each stage of a species be compelled to change, if
not contemporaneously at least successively, with the same frequency
and intensity, by the action of an innate force? and how by means of
the latter can there ever result a greater form-divergence in the larvæ
than in the imagines?
It is delusive to believe that these unequal deviations can be
explained by assuming that the phyletic force acts periodically.
Granting that it does so, and that the internal power successively
compels the imago, pupa, and finally the larva to change, there would
then pass a kind of wave of transformation over the different stages
of the species, as was actually shown above to be the case in the
single larval stages. The only possible way of explaining the unequal
distances between larvæ and imagines would therefore be to assume that
two allied groups, _e.g._ species, were not contemporaneously affected
by the wave, so that at a certain period of time the imago alone of one
species had become changed, whilst in the other species the wave of
transformation had also reached the larva. In this case the imagines of
the two species would thus appear to be more nearly related than their
larvæ.
Now this strained explanation is eminently inapplicable to varieties,
still less to species, and least of all to higher systematic groups,
for the simple reason that every wave of transformation may be
assumed to be at the most of such strength as to produce a deviation
of form equal to that of a variety. Were the change resulting from
a single disturbance greater, we should not only find one-sided
varieties, _i.e._ those belonging to _one_ stage, but we should
also meet as frequently with one-sided species. If, however, a wave
of transformation can only produce a variety even in the case of
greatest form-divergence, the above hypothetical uncontemporaneous
action of such a wave in two species could only give rise to such
small differences in the two stages that we could but designate them
as varieties. An accumulation of the results of the action of several
successive waves passing over the same species could not happen,
because the distance from a neighbouring species would always become
the same in two stages as soon as _one_ wave had ended its course. In
this manner there could therefore only arise divergences of the value
of varieties, and incongruences in systematic groups of a higher rank
could not thus be explained.
All explanations of the second form of incongruence from the point of
view of a phyletic force can also be shown to be absurd. How can the
fact be explained that larval and imaginal families by no means always
coincide; or that the larvæ can only be formed into families whilst
the imagines partly form sharply defined groups of a higher order? How
can an internal directive force within the same organism urge in two
quite distinct directions? If the evolution of a definite system were
designed, and the admission of such a continually acting power rendered
necessary, why such an incomplete, uncertain, and confused performance?
I must leave others to answer these questions; to me a vital force
appears to be inadmissible, not only because we cannot understand the
phenomena by its aid, but above all because it is superfluous for their
explanation. In accordance with general principles the assumption of an
unknown force can, however, only be made when it is indispensable to
the comprehension of the phenomena.
I believe that the phenomena can be quite well understood without any
such assumption--both the phenomena of congruence and incongruence, in
their two forms of unequal divergence and unequal group-formation.
Let us in the first place admit that there is no directive force in
the organism inciting periodic change, but that every change is always
the consequence of external conditions, being ultimately nothing but
the reaction--the response of the organism to some of the influences
proceeding from the environment; every living form would in this case
remain constant so long as it was not compelled to change by inciting
causes. Such transforming factors can act directly or indirectly,
_i.e._ they can produce new changes immediately, or can bring about
a remodelling by the combination, accumulation, or suppression
of individual variations already present (adaptation by natural
selection). Both forms of this action of external influences have long
been shown to be in actual operation, so that no new assumption will be
made, but only an attempt to explain the phenomena in question by the
sole action of these known factors of species formation.
If, in the first instance, we fix our attention upon that form of
incongruence which manifests itself through unequal divergence of
form-relationship, it will appear prominently that this bears precise
relations to the different systematic groups. This form of incongruence
constitutes the rule in varieties of the order Lepidoptera, it is of
very frequent occurrence in species, but disappears almost completely
in genera, and entirely in the case of families and the higher groups.
On the whole, therefore, as we turn to more and more comprehensive
groups, the incongruence diminishes whilst the congruence increases,
until finally the latter becomes the rule.
Now if congruence presupposes an equal number of transforming
impulses, we perceive that the number of the impulses which have
affected larvæ and imagines agree with one another the more closely the
larger the systematic groups which are compared together. How can this
be otherwise? The larger the systematic group the longer the period of
time which must have been necessary for its formation, and the more
numerous the transforming impulses which must have acted upon it before
its formation was completed.
But if the supposition that the impulse to change always comes from
the environment in no way favours the idea that such impulses always
affect both stages contemporaneously, and are equal in number during
the same period of time, there is not, on the other hand, the least
ground for assuming that throughout long periods the larvæ or the
imagines only would have been affected by such transforming influences.
This could have been inferred from the fact that varieties frequently
depend only upon one stage, whilst specific differences in larvæ only
also occur occasionally, the imagines remaining alike; but no single
genus is known of which all the species possess similar larvæ. Within
the period of time during which genera can be formed the transforming
impulses therefore never actually affect the one stage only, but always
influence both.
But if this is the case--if within the period of time which is
sufficient for the production of species, the one stage only is but
seldom and quite exceptionally influenced by transforming impulses,
whilst both stages are as a rule affected, although not with the same
frequency, it must necessarily follow that on the whole, as the period
of time increases, the difference in the number of these impulses which
affect the larva and of those which affect the imago must continually
decrease, and with this difference the magnitude of the morphological
differences resulting from the transforming influences must at the same
time also diminish. With the number of the successively increasing
changes the difference in the magnitude of the change in the two stages
would always relatively diminish until it had quite vanished from our
perception; just in the same manner as we can distinguish a group of
three grains of corn from one composed of six, but not a heap of 103
grains from one containing 106 grains.
That the small systematic groups must have required a short period and
the large groups a long period of time for their formation requires no
special proof, but results immediately from the theory of descent.
All the foregoing considerations would, however, only hold good if
the transforming impulses were equal in strength, or, not to speak
figuratively, if the changes only occurred in equivalent portions of
the body, _i.e._ in such portions as those in which the changes are
of the same physiological and morphological importance to the whole
organism.
Now in the lower systematic groups this is always the case. Varieties,
species, and genera are always distinguished by only relatively small
differences; deep-seated distinctions do not here occur, as is implied
in the conception of these categories. The true cause of this is, I
believe, to be found in the circumstance that all changes take place
only by the smallest steps, so that greater differences can only arise
in the course of longer periods of time, within which a great number of
types (species) can, however, come into existence, and these would be
related by blood and in form in different degrees, and would therefore
form a systematic group of a higher rank.
The short periods necessary for the production of inferior groups, such
as genera, would not result in incongruences if only _un_typical parts
of the larvæ, such as marking or spines, underwent change, whilst in
the imagines typical parts--wings and legs--became transformed. The
changes which could have occurred in the wings, &c., during this period
of time would have been much too small to produce any considerable
influence on the other parts of the body by correlation; and two
species of which the larvæ and imagines, had changed with the same
frequency, would show a similar amount of divergence between the larvæ
and between the imagines, although on the one side only _un_typical
parts--_i.e._ those of no importance to the whole organization--and
on the other side typical parts, were affected. The _number_ of the
changes would here alone determine whether congruence or incongruence
occurred between the two stages.
The case would be quite different if, throughout a long period of time,
in the one stage only typical and in the other only untypical parts
were subjected to change. In the first case a complete transformation
of the whole structure would occur, since not only would the typical
parts, such as the wings, undergo a much further and increasing
transformation in the same direction, but these changes would also lead
to secondary alterations.
In this manner, I believe, must be explained the fact that in the
higher groups still greater form-divergences of the two stages occur;
and if this explanation is correct, the cause of this striking
phenomenon, viz., that incongruence diminishes from varieties to
genera, in which latter it occurs but exceptionally, whilst in families
and in the higher groups it again continually increases, is likewise
revealed. Up to genera the incongruence depends entirely upon the one
stage having become changed _more frequently_ than the other; but
in families and groups of families, and in the orders Diptera and
Hymenoptera, as will be shown subsequently, in sub-orders and tribes,
it depends upon the _importance of the part of the body_ affected by
the predominant change. In the latter case the number of changes is
of no importance, because these are so numerous that the difference
vanishes from our perception; but an equal number of changes, even
when very great, may now produce a much greater or a much smaller
transformation in the entire bodily structure according as they affect
typical or untypical portions, or according as they keep in the same
direction throughout a long period of time, or change their direction
frequently.
Those unequal form-divergences which occur in the higher systematic
groups a re always associated with a different formation of groups--the
larvæ form different systematic groups to the imagines, so that one
of these stages constitutes a higher or a lower group; or else the
groups are of equal importance in the two stages, but are of unequal
magnitude--they do not coincide, but the one overlaps the other.
Incongruences of this last kind appear in certain cases within families
(_Nymphalidæ_), but I will not now subject these to closer analysis,
because their causes will appear more clearly when subsequently
considering the orders Hymenoptera and Diptera. Incongruences of the
first kind, however, admit of a clear explanation in the case of
butterflies. They appear most distinctly in the groups composed of
families.
Nobody has as yet been able to establish the group _Rhopalocera_ by
means of any single character common to the larvæ; nevertheless, this
group in the imagines is the sharpest and best defined of the whole
order. If we inform the merest tyro that clubbed antennæ are the chief
character of the butterflies, he will never hesitate in assigning
one of these insects to its correct group. Such a typical character,
common to all families, is, however, absent in the larvæ; and it might
be correctly said that there were no Rhopalocerous larvæ, or rather
that there were only larvæ of _Equites_, _Nymphales_, and _Heliconii_.
The larvæ of the various families can be readily separated by means
of characteristic distinctions, and it would not be difficult for an
adept to distinguish to this extent in single cases a Rhopalocerous
caterpillar as such; but these larvæ possess only _family_ characters,
and not those of a higher order.
This incongruence partly depends upon the circumstance that the
form-divergence between a Rhopalocerous and a Heterocerous family is
much greater on the side of the imagines than on that of the larvæ.
Were there but a single family of butterflies in existence, such as
the _Equites_, we should be obliged to elevate this to the rank of
a sub-order on the side of the imagines, but not on that of the
larvæ. Such cases actually occur, and an instance of this kind will be
mentioned later in connection with the Diptera. But this alone does not
explain why, on the side of the imagines, a whole series of families
show the same amount of morphological divergence from the families
of other groups. There are two things, therefore, which must here be
explained:--First, why is the form-divergence between the imagines of
the Rhopalocera and Heterocera greater than that between their larvæ?
and, secondly, why can the imagines of the Rhopalocera be formed into
one large group by means of common characters whilst the larvæ cannot?
The answers to both these questions can easily be given from our
present standpoint. As far as the first question is concerned, this
finds its solution in the fact that the form-divergence always
corresponds exactly with the divergence of function, _i.e._ with the
divergence in the mode of life.
If we compare a butterfly with a moth there can be no doubt that the
difference in the conditions of life is far greater on the side of the
imagines than on that of the larvæ. The differences in the mode of life
of the larvæ are on the whole but very small. They are all vegetable
feeders, requiring large quantities of food, and can only cease
feeding during a short time, for which reason they never leave their
food-plants for long, and it is of more importance for them to remain
firmly attached than to be able to run rapidly. It is unnecessary for
them to seek long for their food, as they generally find themselves
amidst an abundance, and upon this depends the small development of
their eyes and other organs of sense. On the whole caterpillars live
under very uniform conditions, although these may vary in manifold
details.
The greatest difference in the mode of life which occurs amongst
Lepidopterous larvæ is shown by wood feeders. But even these, which
by their constant exclusion from light, the hardness of their food,
their confinement within narrow hard-walled galleries, and by the
peculiar kind of movement necessitated by these galleries, are so
differently situated in many particulars to those larvæ which live
openly on plants, have not experienced any general change in the
typical conformation of the body by adaptation to these conditions of
life. These larvæ, which, as has already been mentioned, belong to
the most diverse families, are more or less colourless and flattened,
and have very strong jaws and small feet; but in none of them do we
find a smaller number of segments, or any disappearance, or important
transformation of the typical limbs; they all without exception possess
sixteen legs, like the other larvæ excepting the _Geometræ_.
Now if even under the most widely diverging conditions of life
adaptation of form is produced by relatively small, and to a
certain extent superficial, changes, we should expect less typical
transformations in the great majority of caterpillars which live
on the exterior of plants or in their softer parts (most of the
Micro-lepidoptera). The great diversity in the forms of caterpillars
depends essentially upon a different formation of the skin and its
underlying portions. The skin is sometimes naked, and can then acquire
the most diverse colours, either protective or conspicuous, or it may
develop offensive or defensive markings; in other cases it may be
covered with hairs which sting, or with spines which prick; certain
of its glands may develop to an enormous size, and acquire brilliant
colours and the power of emitting stinking secretions (the tentacles of
the _Papilionidæ_ and Cuspidate larvæ); by the development of warts,
angles, humps, &c., any species of caterpillar may be invested with the
most grotesque shape, the significance of which with respect to the
life of the insect is as yet in most cases by no means clear: _typical
portions_ are not, however, essentially influenced by these manifold
variations. At most only the form of the individual segments of the
body, and with these the shape of the whole insect, become changed
(onisciform larvæ of _Lycænidæ_), but a segment is never suppressed,
and even any considerable lengthening of the legs occurs but very
seldom (_Stauropus Fagi_).[194]
We may therefore fairly assert that the structure of larvæ is on the
whole remarkably uniform, in consequence of the uniformity in the
conditions of life. Notwithstanding the great variety of external
aspects, the general structure of caterpillars does not become
changed--it is only their outward garb which varies, sometimes in one
direction, and sometimes in another, and which, starting from inherited
characters, becomes adapted to the various special conditions of life
in the best possible manner.
All this is quite different in the case of the imagines, where we
meet with very important differences in the conditions of life. The
butterflies, which live under the influence of direct sunlight and a
much higher temperature, and which are on the wing for a much longer
period during the day, must evidently be differently equipped to the
moths in their motor organs (wings), degree of hairiness, and in the
development of their eyes and other organs of sense. It is true that we
are not at present in a condition to furnish special proofs that the
individual organs of butterflies are exactly adapted to a diurnal life,
but we may safely draw this general conclusion from the circumstance
that no butterfly is of nocturnal habits.[195] It cannot be stated
in objection that there are many moths which fly by day. It certainly
appears that no great structural change is necessary to confer upon a
Lepidopteron organized for nocturnal life the power of also flying by
day; but this proves nothing against the view that the structure of
the butterflies depends upon adaptation to a diurnal life. Analogous
cases are known to occur in many other groups of animals. Thus, the
decapodous Crustacea are obviously organized for an aquatic life; but
there are some crabs which take long journeys by land. Fish appear
no less to be exclusively adapted to live in water; nevertheless the
“climbing-perch” (_Anabas_) can live for hours on land.
It is not the circumstance that some of the moths fly by day which is
extraordinary and demands a special explanation, but the reverse fact
just mentioned, that no known butterfly flies by night. We may conclude
from this that the organization of the latter is not adapted to a
nocturnal life.
If we assume[196] that the Lepidopterous family adapted to a diurnal
life gives rise in the course of time to a nocturnal family, there
can be no doubt but that the transformation of structure would be far
greater on the part of the imagines than on that of the larvæ. The
latter would not remain quite unchanged--not because their imagines
had taken to a nocturnal life which for the larva would be quite
immaterial, but because this change could only occur very gradually
in the course of a large number of generations, and during this long
period the conditions of life would necessarily often change with
respect to the larvæ. It has been shown above that within the period of
time necessary for the formation of a new species impulses to change
occur on both sides; how much more numerous therefore must these be
in the case of a group of much higher rank, for the establishment of
which a considerably longer period is required. In the case assumed,
therefore, the larvæ would also change, but they would suffer much
smaller transformations than the imagines. Whilst in the latter almost
all the typical portions of the body would undergo deep changes in
consequence of the entirely different conditions of life, the larvæ
would perhaps only change in marking, hairs, bristles, or other
external characters, the typical parts experiencing only unimportant
modifications.
In this manner it can easily be understood why the larvæ of a family
of _Noctuæ_ do not differ to a greater extent from those of a family of
butterflies than do the latter from some other Rhopalocerous family, or
why the imagines of a Rhopalocerous and a Heterocerous family present
much greater form-divergences than their larvæ. At the same time is
therefore explained the unequal value that must be attributed to any
single family of butterflies in its larvæ and in its imagines. The
unequal form-divergences coincide exactly with the inequalities in the
conditions of life.
When whole families of butterflies show the same structure in their
typical parts (antennæ, wings, &c.), and, what is of more importance,
can be separated as a systematic group of a higher order (_i.e._ as a
section or sub-order) from the other Lepidoptera whilst their larval
families do not appear to be connected by any common character, the
cause of this incongruence lies simply in the circumstance that the
imagines live under some peculiar conditions which are common to them
all, but which do not recur in other Lepidopterous groups. Their larvæ
live in precisely the same manner as those of all the other families of
Lepidoptera--they do not differ in their mode of life from those of the
Heterocerous families to a greater extent than they do from one another.
We therefore see here a community of form within the same compass as
that in which there is community in the conditions of life. In all
butterflies such community is found in their diurnal habits, and in
accordance with this we find that these only, and not their larvæ, can
be formed into a group having common characters.
In the larvæ also we only find agreement in the conditions of life
within a much wider compass, viz. within the whole order. Between
the limits of the order Lepidoptera the conditions of life in the
caterpillars are, as has just been shown, on the whole very uniform,
and the structure of the larvæ accordingly agrees almost exactly in all
Lepidopterous families in every essential, _i.e._ typical, part.
In this way is explained the hitherto incomprehensible phenomenon that
the sub-ordinal group _Rhopalocera_ cannot be based on the larvæ, but
that Lepidopterous caterpillars can as a whole be associated into a
higher group (order); they constitute altogether families and an order,
but not the intermediate group of a sub-order. By this means we at the
same time reply to an objection that may be raised, viz. that larval
forms cannot be formed into high systematic groups because of their
“low and undeveloped” organization.
To this form of incongruence, viz. to the formation of systematic
groups of unequal value and magnitude, I must attach the greatest
weight with respect to theoretical considerations. I maintain that
this, as I have already briefly indicated above, is wholly incompatible
with the admission of a phyletic force. How is it conceivable that such
a power could work in the same organism in two entirely different
directions--that it should in the same species lead to the constitution
of quite different systems for the larvæ and for the imagines, or that
it should lead only to the formation of families in the larvæ and to
sub-orders in the imagines? If an internal force existed which had
a tendency to call into existence certain groups of animal forms of
such a nature that these constituted one harmonious whole of which
the components bore to one another fixed morphological relationships,
it would certainly have been an easy matter for such a power to have
given to the larvæ of butterflies some small character which would
have distinguished them as such, and which would in some measure have
impressed them with the stamp of “_Rhopalocera_.” Of such a character
we find no trace however; on the contrary, everything goes to show that
the transformations of the organic world result entirely from external
influences.