It has been shown in the previous section that the three elements
composing the markings of the Sphinx-larvæ originally possessed a
distinct significance with respect to the life of the species, and
that they were by this means called into existence. It has likewise
been shown, that in most of the species which possess these characters
at the present time they still have a decided, although sometimes a
different use, for their possessors, so that from this point of view no
objection can be raised to their being considered as having arisen by
natural selection.
On looking at the phenomenon as a whole, however, certain instances
occur which appear quite irreconcilable with this view.
The most formidable objection is offered by the genus _Deilephila_.
The row of ring-spots which nearly all the existing species have more
or less developed, has arisen from a simple subdorsal line. It would
not, therefore, be surprising if a species were discovered which
possessed this line without any ring-spots as its only marking. If
_D. Hippophaës_ were thus marked, there would be no objection to the
theoretical assumption that this[155] was the ancestor of the other
species. It would then be said that ring-spots were first developed
in a later species by natural selection, and that they had been
transmitted to all succeeding and younger species.
Certain individuals of _D. Hippophaës_, however, possess small
ring-spots, some of which are well developed on several segments.
In this species the row of ring-spots is therefore comprised in the
development. The remaining species, which are much younger phyletically
than _Hippophaës_, could not have inherited their ring-spots from the
latter, since this species itself only possesses them occasionally,
and, so to speak, in a tentative manner. The spots would therefore
appear to have arisen spontaneously in this species, and independently
of those in the other species. But if this were the case, how should
we be able to prove that in the other species also the ring-spots did
not arise independently; and if, moreover, a large number of species
showed the same character without its being referable to inheritance
from a common ancestor, how could this be otherwise explained than as
the result of a force innate in these species and producing similar
variations? But this is nothing but Askenasy’s “fixed direction of
variation”--_i.e._, a phyletic vital force.
The only escape from this difficulty is perhaps to be found in proving
that _D. Hippophaës_ formerly possessed ring-spots, and that these
have been subsequently either partially or completely lost, so that
their occasional appearance in this species would therefore depend upon
reversion. The ontogeny, however, teaches us that this is not the case,
since the young caterpillar does not possess a greater number of more
distinct ring-spots, but wants them altogether with the exception of a
red spot on the eleventh segment, which is, however, much fainter than
in the last stage.
This last-mentioned fact contains the solution of the problem. The
premises from which this reasoning set out were all incorrect--the one
red spot on the eleventh segment is likewise a ring-spot, and indeed
the most important one of all, being primary, or the first to come
into existence. Now all specimens, without exception, possess this
first ring-spot, which is useful, and has therefore been called forth
by natural selection; it is not inherited, but newly acquired by this
species; at least, if the explanation of these spots which I have
previously offered is correct.
The primary pair of spots may have been transferred from this to later
species by heredity; and since, in all segmented animals there is a
tendency for the peculiarities of one segment to be repeated on the
others, this repetition must have occurred with greater frequency and
more completely in the later species--the more so if the process were
favoured by natural selection, _i.e._ if the row of ring-spots which
originated in this manner could in any way be turned to the use of the
species.
In _Hippophaës_ itself there must also be a tendency to the formation
of secondary ring-spots, and indeed in a number of specimens we
actually see series of such ring-spots, the latter being present in
varying numbers, and in very different states of development. The fact
that the ring-spots have not become a constant and well-developed
character, is simply explained by the circumstance that as such they
would have endangered the existence of the species.
In this case there is therefore no necessity for assuming a phyletic
vital force. The ring-spots of the genus _Deilephila_ rather furnish
us with an excellent explanation of a fact which might otherwise have
been adduced in support of a phyletic vital force, viz., the strict
uniformity in the development of larval markings.
Before I had been led to the discovery, by the study of the marking and
development of _Hippophaës_, that the spots of the genus _Deilephila_
originated on one segment only, from which they were transferred
secondarily to the others, this astonishing regularity appeared to me
an incomprehensible problem, which could only be solved by assuming
a phyletic vital force. If it be attempted, for the ten species here
considered, to construct a genealogical tree based on the supposition
that it is the _rows of spots_ which have been inherited in cases
where they occur, and not the _mere tendency to their production_
by the transference of the one originally inherited primary spot to
the remaining segments, the attempt will fail. The greater number of
the species would have to be arranged in one row, since one species
always bears a perfected form of marking, which appears in the young
stages of the following species. But it is very improbable that nine
different species, derived directly the one from the other, would
contemporaneously survive.[156] One species, _D. Vespertilio_, could
not be inserted at all in the genealogical tree, since it wants one
character which occurs in all the other species, viz., the caudal horn,
which is absent even in the third stage, and must therefore have been
lost at a very early period of the phyletic development, so that we
may consider it to be on this account genetically allied to the oldest
known form. But the markings of this larva pass through precisely the
same stages of development as do those of the other species. Now if
the ring-spots were inherited as such, the existence of a hornless
species with ring-spots would be an insoluble riddle, and would favour
the admission of parallel developmental series, which again could be
scarcely otherwise explained than by a “fixed direction of variation.”
We have here one of that class of cases which the supporters of a
phyletic vital force have already so often made use of in support of
their view.
The explanation of such a case--_i.e._ its reference to known causes of
species transformation--is never easy, and is indeed impossible without
a precise knowledge of the ontogeny of many species, as well as of the
original significance of the characters in question. In the case of the
_Deilephila_ larvæ, however, such knowledge is still wanting. It is
true that they present us with parallel developmental series, but these
do not depend on an unknown phyletic force--the parallelism can be
referred to the action of the imperfectly known laws of growth innate
in segmented organisms. Because the characters of one segment have
a tendency to repeat themselves on the others, from one parent-form
possessing ring-spots on one segment only, there may have proceeded
several developmental series, all of which developed rows of such spots
independently of each other.
From these considerations we may venture to construct the following
genealogical tree:--
[Illustration: POSSIBLE GENEALOGICAL TREE OF THE GENUS DEILEPHILA
The circles indicate the phyletic stages IV.-VIII.; the eighth is only
reached by _Nicæa_, and is distinguished from the seventh chiefly by
the ontogeny, in the third stage of which the seventh phyletic stage is
reached, whilst in _Euphorbiæ_ and _Dahlii_ this stage is reached in
the fourth ontogenetic stage. The phyletic stages indicated by queries
are extinct, and only known through the ontogeny of existing species.
It must be understood that this pedigree expresses only the ideal and
not the actual relations of the species to one another. Thus, it is
possible that _Hippophaës_ is not the parent-form, but an unknown or
extinct species, which must, however, have possessed the same marking,
and so on.]
Four parallel series here proceed from the parent-form _Hippophaës_;
there may have been five, or possibly only three, but the incomplete
state of our knowledge of the ontogeny does not permit of any certain
conclusion. For the point under consideration this is, however, quite
immaterial. The distance from the central point (the parent-form)
indicates the grade of phyletic development which the respective
species have at present reached.
There is another case which is no less instructive, because it reveals,
although in a somewhat different manner, the action of a law of growth
innate in the organism itself, but which can nevertheless by no means
be regarded as equivalent to a phyletic vital force. I refer to the
coloured edges of the oblique stripes which occur in most of the
species of the genus _Sphinx_. It has already been insisted upon in
a previous section, that the mode in which this character originates
negatives the assumption of a phyletic force, because these coloured
edges are gradually built up out of irregularly scattered spots. There
is no occasion for a “developmental force” to grope in the dark; if
such a power exists, we should expect that it would add new characters
to old ones with the precision of a master workman.
If, however, the coloured edges certainly depend on natural selection,
this agency causing the scattered spots to coalesce and become linear,
we have here the proof that such spots first arose in a precisely
similar manner in several species, quite independently of one
another--that, in fact, a “fixed direction of variation” in a certain
sense exists.
In three species of _Smerinthus_-larvæ, red spots appear towards
the end of the ontogeny; in _S. Populi_ and _Ocellatus_ in only a
minority of individuals, and always separate (not coalescent), and in
_S. Tiliæ_ in a majority of specimens, the spots frequently becoming
fused into one large, single, longish marking. These three species
cannot have inherited the spots from a common ancestor, since they are
absent in the younger ontogenetic stages, or occur only exceptionally,
becoming larger and more numerous in the last stage; they obviously
form a character which must be considered as a case of “anticipated
development.”
How is it then that three species vary independently of each other
in an analogous manner? I know of no other answer to this question
than that similar variations must necessarily arise from similar
physical constitutions--or, otherwise expressed, the three species have
inherited from an unknown parent species, devoid of spots, not this
last character itself, but a physical constitution, having a tendency
to the formation of red spots on the skin.[157] The case offers many
analogies to that of the colour varieties of _Lacerta Muralis_,
to which Eimer[158] briefly calls attention in his interesting
communications on the blue lizard of the Faraglioni Rocks at Capri.
The South Italian lizards, although having differently formed skulls,
show the same brilliantly coloured varieties as those of North Italy;
and Eimer believes that these parallel variations in widely separated
localities, some of which have long been isolated, must be referred
to a tendency towards fixed directions of variation innate in the
constitution of the species.
I long ago insisted[159] that it should not be forgotten that natural
selection is, in the first place, dependent upon the variations which
an organism offers to this agency, and that, although the number of
possible variations may be very great for each species, yet this
number is by no means to be considered as literally infinite. For
every species there may be _impossible_ variations. For this reason I
am of opinion that the physical nature of each species is of no less
importance in the production of new characters than natural selection,
which must always, in the first place, operate upon the results of this
physical nature, _i.e._ upon the variations presented, and can thus
call new ones into existence.
It requires but a slight alteration of the definition to make out of
this “restricted” or “limited variability,” which is the necessary
consequence of the physical nature of each species, a “fixed direction
of variation” in the sense of a phyletic vital force. Instead of--the
_Smerinthus_-larvæ show a tendency to produce red spots on the skin, it
is only necessary to say--these larvæ tend to produce red borders to
the oblique stripes. The latter statement would, however, be incorrect,
since the red borders first arose by the coalescence of red spots
through the action of natural selection. It is not even correct to say
that _all_ the species of _Smerinthus_ show this tendency to produce
spots, since this character does not seem to occur either in _S.
Quercus_ or _S. Tremulæ_.
The distinction between the two modes of conception will become clear
if we ask, as an example, whether those _Chærocampa_-larvæ which do not
at present possess eye-spots will subsequently acquire these markings,
supposing that they maintain their existence on the earth for a
sufficient period?
The supporters of a “fixed direction of variation” would answer this
question in the affirmative. Ocelli constitute a character which occurs
in nearly all the species of the group--they are the goal towards
which the phyletic force is urging, and which must sooner or later
be reached by each member of the group. On the other hand, I cannot
express so decidedly my own opinion, viz., that such complicated
characters as the many-coloured oblique stripes or eye-spots are never
the results of purely internal forces, but always arise by the action
of natural selection, _i.e._ by the combination of such minute and
simple variations as may present themselves. It may be replied that the
formation of eye-spots in those species which are at present devoid of
them, cannot indeed be considered impossible, but that they would only
appear if the constitution of these species had a tendency to give rise
to the production of darker spots on the edge of the subdorsal line,
and if at the same time, the possession of eye-spots would be of use to
the caterpillar under its special conditions of life.
The condition of affairs would be quite different if we were simply
concerned with the transference of a character from one segment
on which it was already present, to the remaining segments. The
transference would, in this case, result from causes purely innate in
the organism--from the action of laws of equilibration or of growth
(correlation), and the external conditions of life would play only a
negative part, since they might prevent the complete reproduction of
a character, such, for example, as eye-spots, on all the segments,
in cases where it was disadvantageous to the species. The fact that
our species of _Chærocampa_ have only faint indications, and not a
completely-developed eye-spot, on the remaining segments, may perhaps
be explained in this manner. It is conceivable that the two pairs
of ocelli on the front segments are more effective as a means of
alarm than if the insects were provided with two long rows of such
markings; but nothing can be stated with certainty on this point until
experiments have been made with caterpillars having rows of eye-spots.
The question raised above--whether the species of _Chærocampa_ at
present devoid of eye-spots are to be expected to acquire this
character in the course of their further phyletic development--brings
with it another point, which cannot be here passed over.
If the _utility_ of the four kinds of markings in their perfected form
is demonstrated, their origination through natural selection is not,
strictly speaking, thereby proved. It must also be shown that the first
rudiments of these characters were also of use to their possessors. The
question as to the utility of the “initial stages” of useful characters
must here be set at rest.
In the case of markings such as longitudinal and oblique stripes, it is
quite evident that the initial stages of these simple characters do not
differ greatly from the perfected marking, but this is certainly not
the case with eye- and ring-spots. The most light is thrown upon this
question by the latter, because a species which has remained at the
initial stage of the formation of ring-spots here presents itself for
examination, viz. _Deilephila Hippophaës_.
I have attempted to show that the orange-red spots, which, as a rule,
adorn only the eleventh segment, enhance the adaptive colouring of this
caterpillar by their resemblance to the berries of the sea-buckthorn,
whilst the general surface resembles the leaves in colour. If this be
admitted, the origination of these spots by natural selection offers no
difficulty, since a smaller spot, or one of a fainter red, must also be
of some use to its possessor.
This case is of importance, as showing that a “change of function” may
occur in markings, just as it does in certain organs among the most
diverse species of animals, in the course of phyletic development. The
spots which in _Hippophaës_ are imitations of red berries, in species
which have further advanced phyletically play quite another part--they
serve as means of alarm, or signals of distastefulness.
It appears to me very improbable, however, that the perfect ocelli of
the _Chærocampa_-larvæ have also undergone such a “functional change”
(Dohrn). I rather believe that the first rudiments of these markings
produced the same effect as that which they now exercise, viz.,
terror. We are certainly not so favourably circumstanced in this case
in knowing a species which shows the initial steps of this character
in its last stage of life; but in the initial steps which the second
stage of certain species present, we see preserved the form under which
the eye-spots first appeared in the phylogeny, and from this we are
enabled to judge with some certainty of the effect which they must have
produced at the time.
In the ontogeny of _C. Elpenor_ and _Porcellus_ we see that a small
curvature of the subdorsal line first arises, the concavity of which
becomes filled with darker green, and soon afterwards with black; the
upwardly curved piece of the subdorsal then becomes detached and more
completely surrounded by black. The white fragment of the subdorsal
which has become separated, in the next place broadens, and a black
(dark) pupil appears in its centre.
Now the first rudiments of the eye-spot certainly appear very
insignificant in a caterpillar two centimeters long, but we must
not forget that in the ancestors of the existing _Chærocampa_-larvæ
this character appeared in the adult state. If we conceive the
curvature of the white subdorsal with the underlying dark pigment
to be correspondingly magnified, its importance as a means of alarm
can scarcely be denied, particularly when we consider that this
marking stands on the enlarged fourth segment, which alone invests
the caterpillar with a singular, and, to smaller foes, an alarming
appearance. We know that in the case of those _Chærocampa_-larvæ which
possess no eye-spots, the distension of this segment is employed
against hostile attacks. (See the illustration of _Darapsa Chærilus_,
Pl. IV., Fig. 34.) Those markings which even only remotely resembled an
eye must, in such a position, have increased the terrifying action. On
these grounds I believe that it may be safely admitted, that this kind
of marking possessed the same significance in its initial stages as
it now does when fully perfected. No functional change has here taken
place.
Among all the facts brought together in the first section I only
know of one group of phenomena which at least permit of an attempt
to refer them to a phyletic vital force. This is the occurrence of
dark ground-colours in adult larvæ which are of light colours in
their young condition. I have already attempted to show that in
the _Chærocampa_-larvæ this change of colour depends on a double
adaptation, the young caterpillars being adapted to the green
colour of the plant and the adults to the soil and dead leaves.
This interpretation appears the more correct when we find the same
process, viz. the gradual replacement of the original green by brown
colours, among species of widely different genera, which, with the
dark colouring, possess the necessarily correlated habit of hiding
themselves by day when in the adult condition. This is the case with
_Sphinx Convolvuli_, _Deilephila Vespertilio_, and _Acherontia Atropos_.
Thus far all has been easily explicable by natural selection; but
when we also see a “tendency” to acquire a dark colour in the course
of development, in those species which neither conceal themselves
nor are adaptively coloured, but are very conspicuously marked--and
if, further, it can be shown that these species, such for instance
as _Deilephila Galii_, actually possess immunity from the attacks of
foes,--how can this tendency to the formation of a dark colour be
otherwise explained than by the admission of a phyletic vital force
urging the variations in this direction?
Nevertheless I believe that also on this point an appeal to unknown
forces can be dispensed with. In the first place, dark ground-colours
can be of use to a species otherwise than as means of adaptation. In
_D. Galii_, as well as in _D. Euphorbiæ_, the light ring-spots appear
rather at their brightest on the pitchy-black ground; and if this
caterpillar must (_sit venia verbo!_) become conspicuous, this purpose
would be best attained by acquiring a dark ground-colour, such as that
of _D. Euphorbiæ_.
The tendency, apparently common to all these _Sphingidæ_, to acquire
a dark colour with increasing age, depends therefore on two quite
distinct adaptations--first, in species sought by enemies, on an
adaptation to the colour of the soil; and secondly, in species rejected
by foes, on the endeavour to produce the greatest possible contrast of
colour.
Moreover, the supposition from which this last plea for a vital
force set out is not universally correct, since there are species,
such for instance as _D. Nicæa_, which never acquire a dark colour;
and in _D. Galii_ also, although all the individuals abandon the
protective green of the young stages, they by no means all acquire a
dark hue in exchange for this colour; many individuals in their light
ochreous-yellow colouring rather strikingly resemble the snake-like
caterpillar of _D. Nicæa_.