but also in passively functioning parts_. Very numerous instructive
examples are to be found among the Arthropods, whose whole skeleton
belongs to this category. It has been objected that this is not wholly
passive, but that, like the bones of vertebrates, it is stimulated by
the contraction of the muscles and incited to functional reaction,
and that it thickens at places where strong muscles are inserted, and
becomes or remains thin where it is not exposed to any strain from
the muscles. But this is not the case, for the chitinous skeleton
can only offer resistance to the muscular contractions when it is no
longer soft, as it is immediately after it is secreted. As soon as it
has become hard, it can no longer be altered, and can at most be worn
away externally by long use. The proof of this lies in the necessity
for moulting, which is indispensable to all Arthropods as long as they
continue to grow, but does not occur later. Every one who has followed
the growth of an insect or a crustacean knows well that the moultings
or ecdyses are often accompanied by great changes, and hardly ever
occur without some slight changes in the form of the body, especially
of the limbs, with their teeth, bristles, spines, and so on. These
new or transformed parts are formed before the throwing-off of the
old chitinous shell, and under its protection, and they are brought
about by an elaboration or transformation of the living soft matrix
of the skeleton, the hypodermis, which consists of cells, and is the
true skin. They must thus have arisen in the ancestors of our modern
Arthropods in the same way, that is, not by a gradual modification
_during_ use, but by a slight sudden transformation before use. The
steps in the transformation may have been very small, a bristle may
have become a little longer in the second stage of life than it was
in the first, or instead of five bristles a particular spot may bear
six in the second or third stage of life; but the variations in the
phyletic development must always be caused by germ-variations which
effect from within the variation in the relevant stage of development.
But the part which has varied can only function after it has become
firm and immodifiable.
If these circumstances be kept clearly in mind, they furnish a quite
overwhelming mass of proof against the views of the Lamarckians.
Furthermore, it is not even true that the thickest parts of the
external skeleton are those at which the muscles are inserted. The
wing-covers of beetles offer the best proof to the contrary, for
there are no muscles at all in them, yet they are, in many species,
the hardest and thickest part of the whole chitinous coat of mail.
The reason is not far to seek; they protect the wings and the soft
skin of the back, which lies concealed beneath them, and the muscles
are inserted in this!--a relation which can be explained only by its
suitability to the end, and not as due to any direct effect.
When we remember the origin--which we have just described--of the
external skeleton from the soft layer of cells underneath it, the
thickness of the chitinous skeleton, which is very different at
different places in the same animal, but always adapted to its end,
furnishes a case of co-adaptation in parts which have a purely passive
function. The thickened part cannot be due to the insertion of a
muscle, but it is always there in advance, from internal causes, so
that the muscle finds sufficient resistance. Close to it there may lie,
perhaps, the edge of a segment, and at this spot the chitinous skeleton
becomes almost suddenly thinned to a joint membrane capable of being
bent or folded, not _because_ there was no pull from the muscles at
this spot, but in order that the two segments may be connected movably.
Thus, nowhere in the whole body of the Arthropod can the adaptation
of the skeleton, in regard to thickness and power of resistance, be
regulated by function itself, but only by processes of selection
which imparted to each spot the thickness it required, in order to be
effective in its function, whether that be offering resistance to the
strain of the muscles, or giving suppleness to a joint, or affording
the necessary hardness for biting the prey, or for boring into wood or
earth, or merely for protecting the animal from external injuries.
There are, however, many individual functions of the Arthropods the
exercise of which depends on the simultaneous change of several
skeletal parts; as, for instance, many of the 'singing' or vocal
apparatuses in insects. In quite recent times such vocal organs have
been discovered in ants, in which they consist of a small striated
region on the surface of the third abdominal segment, and a sharp ridge
on the segment in front; the latter is rubbed against the former by the
movements of the two segments. Quite a similar 'stridulating organ' has
long been known in the bee-ant (_Mutilla_), and the whistling sound
produced by it is easily heard by our ears; moreover August Forel has
heard it in the large wood-ant (_Camponotus ligniperdus_), and has
described it as an 'alarm-signal,' which the animals give each other
on the approach of danger--an observation which has recently been
confirmed by Wasmann and extended by Robert Wroughton in regard to
Indian ants. All these arrangements for producing sound depend always
on two organs, of which one resembles the bow, the other the strings
of a violin; the one is of no value without the other, and they must
therefore have developed simultaneously, yet they cannot have arisen
through use, and the inheritance of the results of use, because they
are both dead chitinous parts, which are never strengthened by rubbing
against each other with the movements of the abdomen, but are rather
worn away.
[Illustration: FIG. 91 (repeated). Hind-leg of a Grasshopper
(_Stenobothrus protorma_), after Graber. _fe_, femur. _ti_, tibia.
_ta_, tarsal joints. _schr_, the stridulating ridge.]
The same is true of the chirping organs of grasshoppers, beetles,
and crickets; in all cases they consist of two different parts,
which together produce a sound, and which therefore must have arisen
simultaneously, and the origin of which cannot be referred to the
inheritance of the results of exercise, but rather to selection. It is
thus possible that co-adaptation of at least two parts may take place
even when the hypothetical Lamarckian principle is altogether excluded.
When I say that we have here a case of two parts adapted to each
other, that is, strictly speaking, understating the case, for, in the
crickets and locusts, for instance, there is a whole series of peg-like
chitinous papillæ (Fig. 86), the so-called 'bridge,' each of which
must have arisen by itself through variation of the corresponding
spot of skin. At least I can see no ground for the assumption that
the chitinous surfaces on which the 'bridge' is now placed would
necessarily, from internal reasons, have varied precisely in the line
of the bridge as it has done.
[Illustration: FIG. 102. Brush and comb on the leg of a Bee (_Nomada_).
_tib_, end of the tibia. _t^1_, first tarsal joint with the brush and
its comb (_tak_). Between these and the tibial spine (_tisp_) with its
lappet (_L_) the cross-section of an antenna (_At_) is indicated. Drawn
from a preparation by Dr. Petrunkewitsch.]
Instructive examples of the co-adaptation of several parts to a common
action in organs which are not subject to the Lamarckian principle
are afforded by the diverse arrangements for cleaning the antennæ the
bearers of the smelling-organ which are so important to the life of
insects (Fig. 102). Here even the adaptation of an indented area on
the tibia of the anterior leg to the cylindrical form of the antenna
which passes through it, is sometimes so striking (Fig. 102, _tak_)
that it might be thought that it must have arisen through a gradual
wearing out; yet this is impossible, since we have to do with hard
dead chitinous surfaces, and moreover not with a solid mass, like a
hone, which is worn down by the knife, but with a hollow, thin-walled
tube. In ants, bees, and ichneumon-flies this minute, semi-circular
indentation contains small, pointed, triangular saw-teeth, closely
set like those of a comb (_tak_), and the apparatus is made usable
by the fact that a firm spine (_tisp_), fused to the end of the
tibia, overhangs the notch and presses the antenna towards it. In
many species this spine is double, or it is furnished with a thin
comb or lappet (Fig. 102, _L_), or with rows of teeth, or with short
bristles; in short, it may be equipped in the most different ways. Not
infrequently, as in wasps of the genera _Sphex_, _Scolia_, _Ammophila_,
the spine itself is also bent in a semicircle on the surface directed
towards the notch, and this may be effected in very different ways,
either by a bending of the whole thickness of the spine, or by the
presence of a comb which is concave on its inner surface. I should
never come to an end if I were to enumerate all the remarkable details
which may be found in the two main parts of this apparatus, and which
show very clearly how essential a co-operation of the two is in
fulfilling the function of cleaning the antennæ. This fitting together
of the two main parts cannot have been brought about in accordance with
the Lamarckian principle; the adaptation must therefore have come about
in some other way.
The same thing is shown by the legs and other appendages of insects and
crustaceans, which are adapted for the most diverse functions, and the
individual sections of which must be correlated if the function is to
be possible. Let us consider only the claw structures in crustaceans
and scorpions. Here, too, it seems as if the outgrowth of the last
joint of the leg, which functions as the arm of the claw, must have
arisen as a direct effect of use, through the pressure of an object
held fast by the last joint, the movable half of the claw. Frequently,
moreover, tooth-like protuberances occur on the fixed blade of the
claw (Fig. 103). But how could these have arisen as a direct effect
of pressure, since they are always preformed during the soft state of
the appendage _before use_, and are only made use of after it is fully
hardened. The soft crustaceans, the so-called 'butter-crabs' which
have just cast their shells, creep carefully away and avoid using
their limbs until they have become hard again. Here, too, we have the
co-adaptation of two parts which vary independently, and which cannot
be affected by the Lamarckian principle.
[Illustration: FIG. 103. Claw (_Sch_) on the leg of a 'Beach-fly,' an
Amphipod Crustacean (_Orchestia_). _I_, _II_, the two first joints.
_uA_, the lower blade of the claw, a non-mobile prolongation of the
penultimate joint. _oA_, the upper blade of the claw, the movable
last joint; the tubercles and indentations of the two blades fit one
another. After F. Müller.]
But the appendages furnish more complex examples of mutual adaptation.
Thus the individual sections of the anterior leg of the mole-cricket
(_Gryllotalpa_) have varied greatly, yet quite differently, and the
whole together forms a most effective digging-tool. With it the animal
digs out the earth before it to right and to left, and to do this
it makes with both legs simultaneous outward movements, which are
otherwise quite unusual among insects, and does so with such strength
that Rösel von Rosenhof saw two bodies each weighing three pounds
pushed away in this manner. In this case four chief parts of the leg
(Fig. 104), the coxa (_cox_), the femur (_fe_), the tibia (_tib_),
and the tarsi (_tars_) are so adapted to each other in form, joints,
thickness of skeleton, and size, that they cannot have varied otherwise
than in relation to each other, but each piece has done so in an
individual manner. Most remarkable of all is the short broad tibia,
equipped with four large, hard teeth, which has to perform the digging
in the ground after the manner of a spade, while the disproportionately
thin and weak tarsal joints, the last of which bears two perfectly
straight spines instead of claws, are directed upwards, and do not
touch the ground, being no longer used for walking. Rösel supposed,
probably correctly, that they are used for cleaning the spade when it
becomes clogged up with earth, since the animal cannot clean it with
its mouth. These quite unusually formed parts of the limb cannot have
become what they are as the direct results of use, because, for one
thing, it would have been not their broad surfaces, but their narrow
edges, which would most easily cut through the earth, that would have
been directed outwards. The peculiar curving, first concave, then
convex, of the outer surface of the digging foot is exactly what is
best adapted for cutting into the earth and for the pushing aside which
follows, but it is not what it would have become if the chitin-wall had
yielded to the pressure of the earth and adapted itself to it. But,
as we are again dealing with the chitinous skeleton, there can be no
question of the direct effect of use, and, it seems to me, it must be
admitted that here we have a case of co-adaptation of at least seven
different parts, which have varied independently of each other, without
any assistance from the Lamarckian principle.
[Illustration: FIG. 104. Digging leg of the Mole-cricket
(_Gryllotalpa_). _cox_, coxa attaching the limb to the thorax. _fe_,
the short broad femur. _tib_, the tibia forming a broad spade with six
large sharp teeth. _tars_, the tarsal joints, which are turned upwards
and cannot be used in locomotion. After Rösel.]
But much more complicated cases than this might be cited, if we were in
a position to estimate exactly the functional value of the individual
parts of the wing-venation in the different insects, for it is well
known that this venation serves the systematist as a basis for the
definition of genera, especially in Lepidoptera and Hymenoptera. That
is to say, it varies from genus to genus in a characteristic manner,
obviously corresponding to the differences in the wing-form, and in
the flight itself. But, unfortunately, we are still far from being
able to make more than quite general hypotheses as to the meaning of
the lengthening and strengthening, or conversely, the degeneration or
elimination, of this or that vein. From extreme cases, however, as for
instance the rich venation in good fliers with large wings, and the
scanty venation in poor fliers with small wings, we learn at least so
much, that the degree and even the manner of venation bears a definite
relation to the function of the wing, and this we might have assumed.
But these wing-veins, in as far as they serve as a support for the
weak wing-membrane, are purely chitinous structures, skeletal parts
which are not even renewed from time to time like the skeletal parts
of the leg and many other parts of the insect. As they are laid down
at first in the pupa as soft strings of cells, so they remain, and
they only begin to be used when they are completely hardened. _They
can therefore never have been caused to vary through use in the course
of the phyletic development of species and genera_, and the Lamarckian
principle can have no part in their transformations. But if they follow
the most subtle changes, which we cannot precisely demonstrate, of the
whole wing-surface and in the mode of flight, as a man is followed by
his shadow, there must be some other principle which adapts the organ
to its function, and which is able continually to adapt the large
number of individual wing-veins in the manner most advantageous for the
general function. Here, therefore, we have a state of matters exactly
corresponding to that obtaining in the transformation of actively
functioning parts which form a system with common co-operative action,
as, for instance, in the case we first discussed, that of the stag's
antlers.
Other even more complicated examples of harmonious adaptation of
passively functioning parts are afforded by the markings of animals,
such as those of the butterfly's wing. The colours have only a passive
rôle, whether they be due to pigments alone, or to structure, or to
both combined. When the coloration of a surface undergoes adaptive
variation, this cannot be due to any action of the colour, but must
depend on adaptation through selection. Yet it is well known that there
are many butterfly-wings whose surfaces exhibit different colours and
different shades of colour on their different parts, and that in such
a way that together they form a picture, that of a leaf, a piece of
bark, a stone overgrown with lichen, an eye, and so on. In such a case
the individual colour-spots stand in a particular, indirect relation
to each other; although they are independent of each other in their
variation, they are not indifferent and due to chance, for together
they produce a common picture; this is harmonious adaptation of many
parts, where the Lamarckian principle is absolutely excluded.
It may, perhaps, be objected that this mimetic picture does not
arise all at once, but very slowly in the course of long series of
generations, and, indeed, of species. This must of course be so; the
simple beginnings are complicated and perfected through the course
of long ages. This is implied in the principle of selection as we
understand it. But does any one suppose that the gigantic antlers of
the giant-stag were developed in a few generations? In this case, too,
must not numerous races have succeeded each other before the primitive
antlers attained this enormous size? If this must be assumed there was
abundance of time for the adaptation, through _germinal_ variations, of
the secondarily varying parts, the muscles, tendons, nerves, and bones,
for all these parts function actively, and can without difficulty meet,
in the individual life, the increased claims made upon them by a slight
increase in the size of the antlers. For the certain and indubitable
consequence of exercise, of increased use, is the strengthening of the
functioning parts.
Thus the appropriate germinal variation of the secondarily varying
parts may be delayed for a little without the individual being any
the less effective, or being obliged to succumb in the struggle for
existence. I do not, however, assert for a moment that the whole
explanation of the phenomena of co-adaptation is included in this;
on the contrary, I hope soon to be able to show that we may in such
cases assume a preponderance of variational tendencies in a favourable
direction, and that there is thus an indirect connexion between
the utility of a variation and its actual occurrence. In the first
place, however, I must refer to the other group of facts which I have
indicated, which show, likewise, that the simultaneous co-adaptation
of different parts may arise in certain circumstances, although the
Lamarckian principle be excluded. These are the facts presented to us
by the sterile forms of those insects, which, like bees, termites, and
ants, live together in large societies.
Ants and bees are of special interest to us in this connexion, because
they have long been carefully watched by a number of distinguished
naturalists, and most of their vital functions have been precisely
studied. Ever since the days of 'Old Peter Huber' in Geneva there have
again and again been excellent observers who have devoted almost the
whole of their life-work and talents to the more complete study of
these wonderful animals. These insects are of interest to us here,
because, in the course of the social life, a type of individual has
arisen which diverges in structure in many parts of the body from
both the male and the female, although it is sterile and does not
reproduce, or does so in so few instances that the fact is of no moment
in considering the origin of the present bodily structure. As is well
known, these neuters, or better, workers, are, among ants and bees,
females which differ from the true females not only in their smaller
size and their infertility, but in many other points as well. Among
ants, for instance, they are absolutely wingless, and at the same time
they have a much smaller and differently formed thorax and a larger
head. But the most striking point is the difference in their instincts,
for while the females, concerned only with reproduction, pair and
lay eggs, it is the workers who feed and clean the helpless emerging
larvæ, and put them in places of safety, who carry the pupæ into the
warm sunshine, and afterwards back again to the sheltered nest, who
make this nest itself, and keep it in order, after having collected or
prepared the material for it; it is they alone who defend the colony
against the attacks of enemies, who undertake predatory expeditions,
attacking the nests of other ants, and engaging in obstinate combats
with them.
How can all these peculiarities have arisen, since the workers do not
reproduce, or do so only exceptionally, and, in any case, are incapable
of pairing, and therefore--among bees at least--only produce male
offspring? Obviously it cannot have been through the transmission of
the effects of use and disuse, since they leave no offspring to which
anything could be transmitted.
Herbert Spencer has attempted to maintain the position that the
characters of the workers of to-day already existed in the pre-social
state, that is, before the ants began to form colonies, and that,
therefore, they have not been newly evolved but only preserved. But,
even if this be conceded in regard to the care of the brood and the
building instinct, so much remains that could not have existed at that
stage, that the problem of the origin of these new characters remains
unsolved. The wings, for instance, among ants, can only have been lost
when females appeared which did not reproduce, for the pairing of ants
is associated with a nuptial flight high in the air. The wings are not
merely absent in the workers, they do not even develop in the pupæ;
they are, as Dewitz showed, present even now in the larva in the form
of imaginal disks, but from the pupa-stage onwards they degenerate, and
the segments of the thorax to which they are attached likewise appear
small and modified. A variation of the germ-plasm must therefore have
taken place, and to this is due the fact that the wing-primordia no
longer develop, and that the thorax has a different development from
what it had at the time when the animals were still fertile.
It has indeed been said that there is no need for assuming a variation
of the germ-plasm, since the degeneration of the wing might be produced
by inferior nourishment. This opinion is based on the fact that, among
bees, the workers do actually arise from female larvæ which have
received a meagre diet poor in nitrogenous elements, while the same
female larvæ supplied with an abundant diet rich in nitrogen develop
into queens.
But even though we may assume that there is a similar difference in the
mode of feeding among most ants, because the workers are considerably
smaller than the fertile females, it would be quite erroneous to
conclude that the difference between the two types rests solely on the
effect of differences in diet. The elimination of an individual organ
has never yet been determined by bad and scanty nourishment; it is the
whole animal with all its parts that degenerates and becomes small and
weakly. Often as caterpillars of different species have been placed
on starvation diet, whether for experimental purposes or to procure
very small butterflies, it has never yet happened that a single organ,
such as antenna, leg, or wing, has thereby been eliminated or caused
to degenerate. I have myself instituted many such experiments with the
maggots of the blue-bottle fly, by supplying them from their earliest
youth with just as little food as possible without actually starving
them to death, yet never have these larvæ given rise to flies in which
the wings were absent or rudimentary.
Nor did these starved flies ever exhibit degenerate ovaries; they
were always completely developed and equipped with the full number
of ovarian-tubes. It was to decide this particular point that
these experiments were instituted, for my opponents maintained
that degeneration of the ovaries was a direct result of inferior
nourishment. But that is not the case. Special investigations in regard
to ants, undertaken at my request by Miss Elizabeth Bickford, showed
that the anatomical results reached by earlier investigators, like
Adlerz and Lespès, in regard to the degeneration of the ovaries in
workers, were absolutely correct, and that the 'degeneration' consists
not merely in the fact that the ovarian-tubes and ovum-primordia remain
small, but also in a diminution of the _number_ of ovarian-tubes (Fig.
105); the workers have always fewer ovarian-tubes than the females of
the same species, and--what is of especial importance--the reduction
in the number of ovarian-tubes has been effected to a different extent
in different species of ants. In the red wood-ant (_Formica rufa_) the
workers still possess from twelve to sixteen ovarian-tubes; in the
meadow-ant (_Formica pratensis_) only eight, six, or four; in _Lasius
fuliginosus_ there are usually only two (one on either side); and in
the little turf-ant (_Tetramorium cæspitum_) there are none at all. We
have here, therefore, a phylogenetic process of degeneration, which
has reached different degrees in the different species, and has only
been completed in one (_Tetramorium_). The case stands as I previously
stated it: 'The elimination of a typical organ is not an ontogenetic
process, but a phylogenetic one,' it depends not upon 'the mere
influences of nutrition which affect the development of the individual,
but always on variations in the germ-plasm, which, to all appearance,
can only come about in the course of a long series of generations'[14].
[14] _Aeussere Einflüsse als Entwickelungsreize_, Jena, 1894.