The question heading the second section of this essay must at the
conclusion of the investigation be answered in the negative. The
form-relationship of the larvæ does not always coincide with that
of the imagines, or, in other words, a system based entirely on the
morphology of the larvæ does not always coincide with that founded
entirely on the morphology of the imagines.
Two kinds of incongruence here present themselves. The first arises
from the different amount of divergence between two systematic groups
in the larvæ and in the imagines, these groups being of equal extent.
The second form of incongruence consists essentially in that the
two stages form systematic groups of different extents, either the
one stage constituting a group of a higher order than the other and
therefore forming a group of unequal value, or else the two stages form
groups of equal systematic value, these groups, however, not coinciding
in extent, but the one overlapping the other.
This second form of incongruence is very frequently connected with the
first kind, and is mostly the direct consequence of the latter.
The cause of the incongruences is to be found in unequal phyletic
development, either the one stage within the same period of time having
been influenced by a greater number of transforming impulses than the
other, or else these impulses have been different in strength, _i.e._
have affected parts of greater or less physiological value, or have
influenced parts of equal value with unequal strength.
In all these cases in which there are deep-rooted form-differences,
it can be shown that these correspond exactly with inequalities in
the conditions of life, this correspondence being in two directions,
viz. in strength and in extent: the former determines the _degree_ of
form-difference, the latter its _extent_ throughout a larger or smaller
group of species.
The different forms of incongruence are manifested in the following
manner:--
(1.) Different amount of form-divergence between the larvæ on the one
side and the imagines on the other. Among the Lepidoptera this is found
most frequently in varieties and species, and there is evidence to
show that in this case the one stage has been affected by transforming
influences, either alone (varieties), or at any rate to a greater
extent (species). In the last case it can be shown in many ways that
one stage (the larva) has actually remained at an older phyletic grade
(_Deilephila_ species). Incongruences of this kind depending entirely
upon the more frequent action of transforming impulses can only become
observable in the smaller systematic groups, in the larger they elude
comparative examination. In the higher groups unequal form-divergence
may be produced by the transforming impulses affecting parts of unequal
physiological and morphological value, or by their influencing parts
of equal value in different degrees. All effects of this kind can,
however, only become manifest after a long-continued accumulation of
single changes, _i.e._ only in those systematic groups which require
a long period of time for their formation. By this means we can
completely explain why the incongruences of form-divergence continually
diminish from varieties to genera, and then increase again from genera
upwards through families, tribes, and sub-orders: the first diminishing
incongruence depends upon an _unequal number_ of transforming impulses,
the latter increasing incongruence depends upon the _unequal power_ of
these impulses.
Cases of the second kind are found among the Lepidopterous families,
and especially in the higher groups (_Rhopalocera_ and _Heterocera_),
and appear still more striking in the higher groups of the Hymenoptera
and Diptera. Thus the caterpillar shaped and maggot-formed larvæ of
the Hymenoptera differ from one another to a much greater extent
than their imagines, since the latter have experienced a complete
transformation of typical parts; whilst in the caterpillar-formed larvæ
these parts vary only within moderate limits. Similarly in the case
of the Diptera, of which the gnat-like larvæ diverge more widely from
those of the grub type than do the gnats from the true flies. On the
other hand the divergence between the imagines of the fleas and gnats
is considerably greater than that between their larvæ--indeed the larvæ
of the fleas would have to be ranked as a family of the sub-order of
the gnat-like larvæ if we wished to carry out a larval classification.
By this it is also made evident that these unequal divergences, when
they occur in the higher systematic groups, always induce at the same
time the second form of incongruence--that of the formation of unequal
systematic groups.
In general whenever such unequal divergences occur in the higher groups
they run parallel with a strong deviation in the conditions of life.
If these differ more strongly on the side of the larvæ, we find that
the structure of the latter likewise diverges the more widely, and that
their form-relationship is in consequence made more remote (saw-flies
and ichneumons, gnats and flies); if, on the other hand, the difference
in the conditions of life is greater on the side of the imagines,
we find among the latter the greater morphological divergence
(butterflies and moths, gnats and fleas).
(2.) The second chief form of incongruence consists in the formation
of different systematic groups by the larvæ and the imagines, if the
latter are grouped simply according to their form-relationship without
reference to their genetic affinities. This incongruence again shows
itself in two forms--in the formation of groups of unequal value, and
the formation of groups equal in value but unequal in extent, _i.e._ of
overlapping instead of coinciding groups.
Of these two forms the first arises as the direct result of a different
amount of divergence. Thus the larvæ of the fleas, on account of their
small divergence from those of the gnats, could only lay claim to the
rank of a _family_, whilst their imagines are separated from the gnats
by such a wide form-divergence that they are correctly ranked as a
distinct _tribe_ or _sub-order_.
The inequalities in the lowest groups, varieties, can be regarded in a
precisely similar manner. If the larva of a species has become split up
into two local forms, but not the imago, each of the two larval forms
possesses only the rank of a _variety_, whilst the imaginal form has
the value of a _species_.
Less simple are the causes of the phenomenon that in the one stage the
lower groups can be combined into one of higher rank, whilst the other
stage does not attain to this high rank. Such a condition appears
especially complicated when the two stages can again be formed into
groups of a still higher rank.
This is the case in the tribe _Rhopalocera_, which is founded on the
imagines alone, the larvæ forming only families of butterflies. Both
stages can however be again combined into the highest systematic group
of the Lepidoptera.
In this case also the difference in the value of the systematic groups
formed by the two stages corresponds precisely with the difference in
the conditions of life. This appears very distinctly when there are
several sub-groups on each side, and not when, as in the fleas, only
one family is present as a tribe on the one side and on the other as
a family. Thus in the butterflies, on the one side there are numerous
families combined into the higher rank of a sub-order (imagines),
whilst on the other side (larvæ) a group of the same extent cannot
be formed. In this instance it can be distinctly shown that the
combination of the families into a group of a higher order, as is
possible on the side of the imagines, corresponds exactly with the
limits in which the conditions of life deviate from those of other
Lepidopterous families. The group of butterflies corresponds with an
equally large circle of uniform conditions of life, whilst a similar
uniformity is wanting on the side of the larvæ.
The second kind of unequal group formation arises from the circumstance
that groups of equal value can be formed from the two stages, but these
groups do not possess the same limits--they overlap, and only coincide
in part.
This is most clearly seen in the order Hymenoptera, in which both larvæ
and imagines form two well-defined morphological sub-orders, but in
such a manner that the one larval form not only prevails throughout the
whole of the one sub-order of the imagines, but also extends beyond and
spreads over a great portion of the other imaginal sub-order.
Here again the dependence of this phenomenon upon the influence
of the environment is very distinct, since it can be demonstrated
(by the embryology of bees) that the one form of larva--the
maggot-type--although the structure now diverges so widely, has
been developed from the other form, and that it must have arisen by
adaptation to certain widely divergent conditions of life.
This form of incongruence is always connected with unequal divergence
between the two stages of the one systematic group--in this
case the _Terebrantia_. The larvæ of this imaginal group partly
possess caterpillar-like (_Phytospheces_) and partly maggot-formed
(_Entomospheces_) larvæ, and differ from one another to a considerably
greater extent than the saw-flies from the ichneumons.[211] The final
cause of the incongruence lies therefore in this case also in the fact
that one stage has suffered stronger changes than the other, so that
a deeper division of the group has occurred in the former than in the
latter.
The analogous incongruences in single families of the Lepidoptera may
have arisen in a similar manner, as has already been more clearly shown
above; only in these cases we are as yet unable to prove in detail that
the larval structure has become more strongly changed through special
external conditions of life than that of the imagines.
In the smallest systematic group--varieties, it has been possible to
furnish some proof of this. The one-sided change here depends in part
upon the _direct action_ of external influences (seasonal dimorphism,
climatic variation), and it can be shown that these influences
(temperature) acted only on the one stage, and accordingly induced
change in this alone whilst the other stage remained unaltered.
It has now been shown--not indeed in every individual case, but for
each of the different kinds of incongruence of form-relationship--that
there is an exact parallelism corresponding throughout with the
incongruence in the conditions of life. Wherever the forms diverge
more widely in one stage than in the other we also find more widely
divergent conditions of life; wherever the morphological systemy of
one stage fails to coincide with that of the other--whether in the
extent or in the value of the groups--the conditions of life in that
stage also diverge, either more widely or at the same time within
other limits; whenever a morphological group can be constructed from
one stage but not from the other, we find that this stage alone is
submitted to certain common conditions of life which fail in the other
stage.
The law that the divergence in form always corresponds exactly with
the divergence in the conditions of life[212] has accordingly received
confirmation in all cases where we have been able to pronounce
judgment. Unequal form-divergences correspond precisely with unequal
divergence in the conditions of life, and community of form appears
within exactly the same limits as community in the conditions of life.
These investigations may thus be concluded with the following law:--In
types of similar origin, _i.e._ having the same blood-relationship,
the degree of morphological relationship corresponds exactly with the
degree of difference in the conditions of life in the two stages.
With respect to the question as to the final cause of transformation
this result is certainly of the greatest importance.
The interdependence of structure and function has often been insisted
upon, but so long as this has reference only to the agreement of each
particular form with some special mode of life, this harmony could
still be regarded as the result of a directive power; but when in
metamorphic forms we not only see a double agreement between structure
and function, but also that the transformation of the form occurs in
the two chief developmental stages in successive steps at unequal
rates and with unequal strength and rhythm, we must--at least so it
appears to me--abandon the idea of an inherent transforming force; and
this becomes the more necessary when, by means of the opposite and
extremely simple assumption that transformations result entirely from
the response of the organism to the actions of the environment, all the
phenomena--so far as our knowledge of facts at present extends--can be
satisfactorily explained. A power compelling transformation, _i.e._
a phyletic vital force, must be abandoned, on the double ground that
it is incapable of explaining the phenomena (incongruence and unequal
phyletic development), and further because it is superfluous.
Against the latter half of this argument there can at most be raised
but the one objection that the phenomena of transformation are not
completely represented by the cases here analysed. In so far as this
signifies that the whole organic world, animal and vegetable, has not
been comprised within the investigation this objection is quite valid.
The question may be raised as to the limit to which we may venture to
extend the results obtained from one small group of forms. I shall
return to this question in the last essay.
But if by this objection it is meant that the restricted field of
the investigation enables us to actually analyse only a portion of
the occurring transformations,[213] and indeed only those cases,
the dependence of which upon the external conditions of life would
be generally admitted, I will not let pass the opportunity of once
more pointing out at the conclusion of the present essay that the
incongruences shown to exist by no means depend only upon those more
superficial characters the remodelling of which in accordance with
the external conditions of life may be most easily discerned and
is most difficult to deny, but that in certain cases (maggot-like
Dipterous larvæ) it is precisely the “typical” parts which become
partly suppressed and partly converted into an entirely new structure.
From the ancient typical appendages there have here arisen new
structures, which again have every right to be considered as typical.
This transformation is not to be compared with that experienced
by the swimming appendages of the _Nauplius_-like ancestor of an
_Apus_ or _Branchipus_ which have become mandibulate, nor with the
transformation which the anterior limbs must have gone through in
the reptilian ancestors of birds. The changes in question (Dipterous
larvæ) go still further and are more profound. I lay great emphasis
upon this because we have here one of the few cases which show that
typical parts are quite as dependent upon the environment as untypical
structures, and that the former are not only able to become adapted to
external conditions by small modifications--as shown in a most striking
manner by the transformations of the appendages in the Crustacea and
Vertebrata--but that these parts can become modelled on an entirely
new type which, when perfected, gives no means of divining its mode
of origin. I may here repeat a former statement:--With reference to
the causes of their origination we have no grounds for drawing a
distinction between typical and untypical structures.
It may be mentioned in concluding that quite analogous although less
sharply defined results are arrived at if, instead of fixing our
attention upon the different stages of a systematic group in their
phyletic development, we only compare the different functional parts
(organs in the wide sense) of the organisms.
A complete parallel can be drawn between the two classes of
developmental phenomena. From the very different systematic values
attached by taxonomists to this or that organ in a group of animals,
it may be concluded that the individual parts of an organism are to
a certain extent independent, and that each can vary independently,
when affected either entirely alone or in a preponderating degree by
transforming impulses, without all the other parts of the organism
likewise suffering transformation, or at least without their becoming
modified in an equal degree. Did all the parts and organs in two groups
of animals diverge from each other to the same extent, the systematic
value of such parts would be perfectly equal; we should, for example,
be able to distinguish and characterize two genera of the family of
mice by their kidneys, their liver, their salivary glands, or by the
histological structure of their hair or muscles, or even by differences
in their myology, &c. equally as well as by their teeth, length of
toes, &c. It is true that such a diagnosis has yet to be attempted; but
it may safely be predicted that it would not succeed. Judging from all
the facts at present before us, the individual parts--and especially
those connected in their physiological action, _i.e._ the system of
organs--do not keep pace with reference to the modifications which the
species undergoes in the course of time; at one period one system and
at another period some other system of organs advances while the others
remain behind.
This corresponds exactly with the result already deduced from the
unparallel development of the independent ontogenetic stages. If the
inequality in the phyletic development is more sharply pronounced in
this than in the last class of cases, this can be explained by the
greater degree of correlation which exists between the individual
systems of organs in any single organism as compared with that existing
between the ontogenetic stages, which, although developed from one
another, are nevertheless almost completely independent. We should have
expected _à priori_ that a strong correlation would have here existed,
but as a matter of fact this is not the case, or is so only in a very
small degree.
Just as in the stages of metamorphosis the inequality of phyletic
development becomes the more obliterated the more distant and
comprehensive, or, in other words, the greater the period of existence
of the groups which we compare, so does the unequal divergence of the
systems of organs become obliterated as we bring into comparison larger
and larger systematic groups.
It is not inconceivable--although a clear proof of this is certainly
as yet wanting--that a variety of the ancestral species would differ
only in one single character, such as hairiness, colour, or marking,
and such instances would thus agree precisely with the foregoing cases
in which only the caterpillar or the butterfly formed a variety.
All the more profound modifications however--such for instance as
those which determine the difference between two species--are never
limited to one character, but always affect several, this being
explicable by correlation, which, as Darwin has shown in the case of
dogs, may cause modifications in the skull of those breeds having
hanging ears in consequence of this last character alone. It must be
admitted however that one organ only would be originally affected by
a modifying influence. Thus, I am acquainted with two species of a
genus of Daphniacea which are so closely allied that they can only be
distinguished from one another by a close comparison of individual
details. But whilst most of the external and internal organs are almost
identical in the two species the sperm-cells of the males differ in a
most striking manner, in one species resembling an Australian boomerang
in form and in the other being spherical! An analogous instance is
furnished by _Daphnia Pulex_ and _D. Magna_, two species which were
for a long time confounded. Nearly all the parts of the body are here
exactly alike, but the antennæ of the males differ to a remarkable
extent, as was first correctly shown by Leydig.
Similarly in the case of genera there may be observed an incongruence
of such a kind that individual parts of the body may deviate to
a greater or to a less extent than the corresponding parts in an
allied genus. If, for instance, we compare a species of the genus
of Daphniacea, _Sida_, with a species of the nearly allied genus
_Daphnella_, we find that all the external and internal organs are in
some measure dissimilar--nevertheless certain of these parts deviate to
an especially large extent, and have without question become far more
transformed than the others. This is the case, for example, with the
antennæ and the male sexual organs. The latter, in _Daphnella_, open
out at the sides of the posterior part of the body as long, boot-shaped
generative organs, and in _Sida_ as small papillæ on the ventral side
of this region of the body. If again we compare _Daphnella_ with the
nearly allied genus _Latona_, it will be found that no part in the one
is exactly similar to the corresponding part in the other genus, whilst
certain organs differ more widely than others. This is the case for
instance with the oar-like appendages which in _Latona_ are triramous,
but in _Daphnella_, as in almost all the other Daphniacea, only
biramous.
In families the estimation of the form-divergence of the systems of
organs and parts of the body becomes difficult and uncertain: still it
may safely be asserted that the two Cladocerous families _Polyphemidæ_
and _Daphniidæ_ differ much less from one another in the structure of
their oar-like appendages than in that of their other parts, such as
the head, shell, legs, or abdominal segments. In systematic groups of
a still higher order, _i.e._ in orders, and still more in classes, we
might be inclined to consider that all the organs had become modified
to an equally great extent. Nevertheless it cannot be conclusively
said that the kidneys of a bird differ from those of a mammal to the
same extent as do the feathers from mammalian hair, since we cannot
estimate the differences between quite heterogeneous things--it can
only be stated that both differ greatly. Here also the facts are not
such as would have been expected if transformation was the result of
an internal developmental force; no uniform modification of _all_
parts takes place, but first one part varies (variety) and then others
(species), and, on the whole, as the systematic divergence increases
all parts become more and more affected by the transformation and
all tend continually to appear changed to an equal extent. This is
precisely what would be expected if the transforming impulses came
from the environment. An equalization of the differences caused by
transformation must be produced in two ways; first by correlation,
since nearly every primary transformation must entail one or more
secondary changes, and secondly because, as the period of time
increases, more numerous parts of the body must become influenced by
primary transforming factors.
A tempting theme is here also offered by attempting to trace the
inequality of phyletic development to dissimilar external influences,
and by demonstrating that individual organs have as a rule become
modified in proportion to the divergence in the conditions of life by
which they have been influenced, this action, during a given period of
time, having been more frequent in the case of one organ than in that
of the others, or, in brief, by showing the connection between the
causes and effects of transformation.
It would be quite premature, however, to undertake such a labour at
present, since it will be long before physiology is able to account
for the fine distinctions shown by morphology, and further because we
have as yet no insight into those internal adjustments of the organism
which would enable us _à priori_ to deduce definite secondary changes
from a given primary transformation. But so long as this is impossible
we have no means of distinguishing correlative changes from the primary
modifications producing them, unless they happen to arise under our
observation.