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cutting it away equally on both sides near the ground, till a smooth, very
thin wall is left in the middle; the masons always piling up the cut away
cement, and adding fresh cement on the summit of the ridge. We shall
thus have a thin wall steadily growing upward, but always crowned by a
gigantic coping. From all the cells, both those just commenced and those
completed, being thus crowned by a strong coping of wax, the bees can
cluster and crawl over the comb without injuring the delicate hexagonal
walls. These walls, as Professor Miller has kindly ascertained for me, vary
greatly in thickness; being, on an average of twelve measurements made
near the border of the comb, tI^ of an inch in thickness; whereas the
basal rhomboidal plates are thicker, nearly in the proportion of three to
two, having a mean thickness, from twenty-one measurements, of -fi? of
an inch. By the above singular manner of building, strength is continually
given to the comb, with the utmost ultimate economy of wax.

It seems at first to add to the difficulty of understanding how the cells
are made, that a multitude of bees all work together; one bee after work-
ing a short time at one cell going to another, so that, as Huber has stated, a
score of individuals work even at the commencement of the first cell. I was
able practically to show this fact, by covering the edges of the hexagonal
walls of a single cell, or the extreme margin of the circumferential rim of a
growing comb, with an extremely thin layer of melted vermilion wax; and



188 THE ORIGIN OF SPECIES

I invariably found that the color was most delicately diffused by the bees —
as delicately as a painter could have done it with his brush — ^by atoms of
the colored wax having been taken from the spot on which it had been
placed, and worked into the growing edges of the cells all round. The work
of construction seems to be a sort of balance struck between many bees, all
instinctively standing at the same relative distance from each other, all try-
ing to sweep equal spheres, and then building up, or leaving ungnawed, the
planes of intersection between these spheres. It was really curious to note in
cases of difficulty, as when two pieces of comb met at an angle, how often
the bees would pull down and rebuild in different ways the same cell, some-
times recurring to a shape which they had at first rejected.

When bees have a place on which they can stand in their proper positions
for working — for instance, on a slip of wood, placed directly under the
middle of a comb growing downward, so that the comb has to be built over
one face of the slip — in this case the bees can lay the foundations of one
wall of a new hexagon, in its strictly proper place, projecting beyond the
other completed cells. It suffices that the bees should be enabled to stand
at their proper relative distances from each other and from the walls of the
last completed cells, and then, by striking imaginary spheres, they can build
up a wall intermediate between two adjoining spheres; but as far as I have
seen, they never gnaw away and finish off the angles of a cell till a large
part both of that cell and of the adjoining cells has beenbuilt. This capacity
in bees of laying down under certain circumstances a rough wall in its
proper place between two just commenced cells, is important, as it bears
on a fact, which seems at first subversive of the foregoing theory; namely,
that the cells on the extreme margin of wasp-combs are sometimes strictly
hexagonal; but I have not space here to enter on this subject. Nor does
there seem to me any great difficulty in a single insect (as in the case of a
queen-wasp) making hexagonal cells, if she were to work alternately on
the inside and outside of two or three cells commenced at the same time,
always standing at the proper relative distance from the parts of the cells
just begun, sweeping spheres or cylinders, and building up intermediate
planes.

As natural selection acts only by the accumulation of slight modifications
of structure or instinct, each profitable to the individual under its condi-
tions of life, it may reasonably be asked, how a long and graduated succession
of modified architectural instincts, all tending toward the present perfect
plan of construction, could have profited the progenitors of the hive-
bee? I think the answer is not difficult: cells constructed like those of the
bee or the wasp gain in strength, and save much in labor and space, and
in the materials of which they are constructed. With respect to the forma-
tion of wax, it is known that bees are often hard pressed to get sufficient
nectar, and I am informed by Mr. Tegetmeier that it has been experi-
mentally proved that from twelve to fifteen pounds of dry sugar are con-
sumed by a hive of bees for the secretion of a pound of wax; so that a



INSTINCT 189

prodigious quantity of fluid nectar must be collected and consumed by the
bees in a hive for the secretion of the wax necessary for the construction of
their combs. Moreover, many bees have to remain idle for many days dur-
ing the process of secretion. A large store of honey is indispensable to sup-
port a large stock of bees during the winter; and the security of the hive is
known mainly to depend on a large number of bees being supported. Hence
the saving of wax by largely saving honey, and the time consumed in col-
lecting the honey, must be an important element of success to any family
of bees. Of course the success of the species may be dependent on the num-
ber of its enemies, or parasites, or on quite distinct causes, and so be alto-
gether independent of the quantity of honey which the bees can collect.
But let us suppose that this latter circumstance determined, as it probably
often has determined, whether a bee allied to our humble-bees could exist
in large numbers in any country; and let us further suppose that the com-
munity lived through the winter, and consequently required a store of
honey: there can in this case be no doubt that it would be an advantage to
our imaginary humble-bee if a slight modification in her instincts led her
to make her waxen cells near together, so as to intersect a little; for a wall
in common even to two adjoining cells would save some little labor and
wax. Hence, it would continually be more and more advantageous to our
humble-bees, if they were to make their cells more and more regular, nearer
together, and aggregated into a mass, like the cells of the Melipona; for in
this case a large part of the bounding surface of each cell would serve to
bound the adjoining cells, and much labor and wax would be saved. Again,
from the same cause, it would be advantageous to the Melipona, if she
were to make her cells closer together, and more regular in every way, than
at present; for then, as we have seen, the spherical surfaces would wholly dis-
appear and be replaced by plane surfaces; and the Melipona would make a
comb as perfect as that of the hive-bee. Beyond this stage of perfection in
architecture, natural selection could not lead; for the comb of the hive-bee,
as far as we can see, is absolutely perfect in economizing labor and wax.

Thus, as I believe, the most wonderful of all known instincts, that of the
hive-bee, can be explained by natural selection having taken advantage of
numerous, successive, slight modifications of simpler instincts; natural
selection having, by slow degrees, more and more perfectly led the bees to
sweep equal spheres at a given distance from each other in a double layer,
and to build up and excavate the wax along the planes of intersection; the
bees, of course, no more knowing that they swept their spheres at one par-
ticular distance from each other, than they know what are the several
angles of the hexagonal prisms and of the basal rhombic plates; the motive
power of the process of natural selection having been the construction of
cells of due strength and of the proper size and shape for the larvae, this
being effected with the greatest possible economy of labor and wax; that
individual swarm which thus made the best cells with least labor, and
least waste of honey in the secretion of wax, having succeeded best, and
having transmitted their newly-acquired economical instincts to new swarms.



190 THE ORIGIN OF SPECIES

which in their turn will have had the best chance of succeeding in the
struggle for existence.

objections to the theory of natural selection as applied to
instincts: neuter and sterile insects

It has been objected to the foregoing view of the origin of instincts, that
"the variations of structure and of instinct must have been simultaneous
and accurately adjusted to each other, as a modification in the one with-
out an immediate corresponding change in the other would have been
fatal." The force of this objection rests entirely on the assumption that the
changes in the instincts and structure are abrupt. To take as an illustration
the case of the larger titmouse (Parus major), alluded to in a previous
chapter; this bird often holds the seeds of the yew between its feet on a
branch, and hammers with its beak till it gets at the kernel. Now what
special difficulty would there be in natural selection preserving all the
slight individual variations in the shape of the beak, which were better and
better adapted to break open the seeds, until a beak was formed, as well
constructed for this purpose as that of the nut-hatch, at the same time that
habit, or compulsion, or spontaneous variations of taste, led the bird to
become more and more of a seed-eater? In this case the beak is supposed
to be slowly modified by natural selection, subsequently to, but in ac-
cordance with, slowly changing habits or taste; but let the feet of the
titmouse vary and grow larger from correlation with the beak, or from any
other unknown cause, and it is not improbable that such larger feet would
lead the bird to climb more and more until it acquired the remarkable
climbing instinct and power of the nut-hatch. In this case a gradual change
of structure is supposed to lead to changed instinctive habits. To take one
more case: few instincts are more remarkable than that which leads the
swift of the Eastern Islands to make its nest wholly of inspissated saliva.
Some birds build their nests of mud, believed to be moistened with saliva;
and one of the swifts of North America makes its nest (as I have seen) of
sticks agglutinated with saliva, and even with flakes of this substance. Is it
then very improbable that the natural selection of individual swifts, which
secreted more and more saliva, should at last produce a species with in-
stincts leading it to neglect other materials and to make its nest exclusively
of inspissated saliva? And so in other cases. It must, however, be admitted
that in many instances we cannot conjecture whether it was instinct or
structure which first varied.

No doubt many instincts of very difficult explanation could be opposed
to the theory of natural selection — cases, in which we cannot see how an
instinct could have originated; cases, in which no intermediate gradations
are known to exist; cases of instincts of such trifling importance, that they
could hardly have been acted upon by natural selection; cases of instincts
almost identically the same in animals so remote in the scale of nature that
we cannot account for their similarity by inheritance from a common pro-



■i



INSTINCT 191

genitor, and consequently must believe that they were independently ac-
quired through natural selection. I will not here enter on these several cases,
but will confine myself to one special difficulty, which at first appeared to
me insuperable and actually fatal to the whole theory. I allude to the
neuters or sterile females in insect communities; for these neuters often
differ widely in instinct and in structure from both the males and fertile
females, and yet, from being sterile, they cannot propagate their kind.

The subject well deserves to be discussed at great length, but I will here
take only a single case, that of working or sterile ants. How the workers
have been rendered sterile is a difficulty; but not much greater than that
of any other striking modification of structure ; for it can be shown that
some insects and other articulate animals in a state of nature occasionally
become sterile; and if such insects had been social, and it had been profita-
ble to the community that a number should have been annually bom
capable of work, but incapable of procreation, I can see no especial diffi-
culty in this having been effected through natural selection. But I must
pass over this preliminary difficulty. The great difficulty lies in the working
ants differing widely from both the males and the fertile females in struc-
ture, as in the shape of the thorax, and in being destitute of wings and
sometimes of eyes, and in instinct. As far as instinct alone is concerned, the
wonderful difference in this respect between the workers and the perfect
females would have been better exemplified by the hive-bee. If a working
ant or other neuter insect had been an ordinary animal, I should have un-
hesitatingly assumed that all its characters had been slowly acquired
through natural selection; namely, by individuals having been born with
slight profitable modifications, which were inherited by the offspring, and
that these again varied and again were selected, and so onward. But with
the working ant we have an insect differing greatly from its parents, yet
absolutely sterile; so that it could never have transmitted successively ac-
quired modifications of structure or instinct to its progeny. It may well be
asked how it is possible to reconcile this case with the theory of natural
selection.

First, let it be remembered that we have innumerable instances, both in
our domestic productions and in those in a state of nature, of all sorts of
differences of inherited structure which are correlated with certain ages and
with either sex. We have differences correlated not only with one sex, but
with that short period when the reproductive system is active, as in the
nuptial plumage of many birds, and in the hooked jaws of the male salmon.
We have even slight differences in the horns of different breeds of cattle in
relation to an artificially imperfect state of the male sex, for oxen of certain
breeds have longer horns than the oxen of other breeds, relatively to the
length of the horns in both the bull^ and cows of these same breeds. Hence,
I can see no great difficulty in any character becoming correlated with the
sterile condition of certain members of insect communities; the difficulty
lies in understanding how such correlated modifications of structure could
have been slowly accumulated by natural selection.



192 THE ORIGIN OF SPECIES

This difficulty, though appearing insuperable, is lessened, or, as I believe,
disappears, when it is remembered that selection may be applied to the
family, as well as to the individual, and may thus gain the desired end.
Breeders of cattle wish the flesh and fat to be well marbled together. An
animal thus characterized has been slaughtered, but the breeder has gone
with confidence to the same stock and has succeeded. Such faith may be
placed in the power of selection, that a breed of cattle always yielding oxen
with extraordinarily long horns, could, it is probable, be formed by care-
fully watching which individual bulls and cows, when matched, produced
oxen with the longest horns; and yet no one ox would ever have propagated
its kind. Here is a better and real illustration: According to M. Verlot,
some varieties of the double annual stock, from having been long and care-
fully selected to the right degree, always produce a large proportion of
seedlings bearing double and quite sterile flowers, but they likewise yield
some single and fertile plants. These latter, by which alone the variety can
be propagated, may be compared with the fertile male and female ants,
and the double sterile plants with the neuters of the same community. As
with the varieties of the stock, so with social insects, selection has been ap-
plied to the family, and not to the individual, for the sake of gaining a
serviceable end. Hence, we may conclude that slight modifications of struc-
ture or of instinct, correlated with the sterile condition of certain members
of the community, have proved advantageous; consequently the fertile
males and females have flourished, and transmitted to their fertile off"-
spring a tendency to produce sterile members with the same modifications.
This process must have been repeated many times, until that prodigious
amount of diff"erence between the fertile and sterile females of the same
species has been produced which we see in many social instincts.

But we have not as yet touched on the acme of the difficulty; namely, the
fact that the neuters of several ants differ, not only from the fertile females
and males, but from each other, sometimes to an almost incredible degree,
and are thus divided into two or even three castes. The castes, moreover,
do not commonly graduate into each other, but are perfectly well defined;
being as distinct from each other as are any two species of the same genus,
or rather as any two genera of the same family. Thus, in Eciton, there are
working and soldier neuters, with jaws and instincts extraordinarily differ-
ent: in Cryptocerus, the workers of one caste alone carry a wonderful sort
of shield on their heads, the use of which is quite unknown; in the Mexican
Myrmecocystus, the workers of one caste never leave the nest; they are fed
by the workers of another caste, and they have an enormously developed
abdomen which secretes a sort of honey, supplying the place of that excreted
by the aphides, or the domestic cattle as they may be called, which our
European ants guard and imprison.

It will indeed be thought that I have an overweening confidence in the
principle of natural selection, when I do not admit that such wonderful and
well-established facts at once annihilate the theory. In the simpler case of
neuter insects all of one caste, which, as I believe, have been rendered



INSTINCT 193

different from the fertile males and females through natural selection, we
may conclude from the analogy of ordinary variations, that the successive,
slight, profitable modifications did not first arise in all the neuters in the
same nest, but in some few alone; and that by the survival of the com-
munities with females which produced most neuters having the advantage-
ous modification, all the neuters ultimately came to be thus characterized.
According to this view we ought occasionally to find in the same nest neuter
insects, presenting gradations of structure; and this we do find, even not
rarely, considering how few neuter insects out of Europe have been care-
fully examined. Mr. F. Smith has shown that the neuters of several British
ants differ surprisingly from each other in size and sometimes in color; and
that the extreme forms can be linked together by individuals taken out of
the same nest: I have myself compared perfect gradations of this kind. It

: sometimes happens that the larger or the smaller sized workers are the
most numerous; or that both large and small are numerous, while those of
an intermediate size are scanty in numbers. Formica flava has larger and
smaller workers, with some few of intermediate size; and in this species, as
Mr. F. Smith has observed, the larger workers have simple eyes (ocelli),
which, though small, can be plainly distinguished, whereas the smaller
workers have their ocelli rudimentary. Having carefully dissected several
specimens of these workers, I can affirm that the eyes are far more rudi-
mentary in the smaller workers than can be accounted for merely by their
proportionately lesser size; and I fully believe, though I dare not assert so
positively, that the workers of intermediate size have their ocelli in an
exactly intermediate condition. So that here we have two bodies of sterile
workers in the same nest, differing not only in size, but in their organs of
vision, yet connected by some few members in an intermediate condition. I
may digress by adding, that if the smaller workers had been the most useful
to the community, and those males and females had been continually
selected, which produced more and more of the smaller workers, until all
the workers were in this condition, we should then have had a species of
ant with neuters in nearly the same condition as those of Myrmica. For the
workers of Myrmica have not even rudiments of ocelli, though the male

, and female ants of this genus have well-developed ocelli.

I I may give one other case: so confidently did I expect occasionally to
find gradations of important structures between the different castes of
neuters in the same species, that I gladly availed myself of Mr. F. Smith's
offer of numerous specimens from the same nest of the driver ant (Anomma)
of West Africa. The reader will perhaps best appreciate the amount of
difference in these workers by my giving, not the actual measurements, but
a strictly accurate illustration: the difference was the same as if we were
to see a set of workmen building a house, of whom many were five feet
four inches high, and many sixteen feet high; but we must in addition sup-
pose that the larger workmen had heads four instead of three times as big
as those of the -smaller men, and jaws nearly five times as big. The jaws,
moreover, of the working ants of the several sizes differed wonderfully in



194 THE ORIGIN OF SPECIES

shape, and in the form and number of the teeth. But the important fact
for us is that, though the workers can be grouped into castes of different
sizes, yet they graduate insensibly into each other, as does the widely dif-
ferent structure of their jaws. I speak confidently on this latter point, as
Sir J. Lubbock made drawings for me, with the camera lucida, of the jaws
which I dissected from the workers of the several sizes. Mr. Bates, in his
interesting "Naturalist on the Amazons," has described analogous cases.

With these facts before me, I believe that natural selection, by acting on
the fertile ants or parents, could form a species which should regularly
produce neuters, all of large size with one form of jaw, or all of small size
with widely different jaws; or lastly, and this is the greatest difficulty, one
set of workers of one size and structure, and simultaneously another set of
workers of a different size and structure; a graduated series having first
been formed, as in the case of the driver ant, and then the extreme forms
having been produced in greater and greater numbers, through the sur-
vival of the parents which generated them, until none with an intermediate
structure were produced.

An analogous explanation has been given by Mr. Wallace, of the equally
complex case of certain Malayan butterflies regularly appearing under two
or even three distinct female forms; and by Fritz Miiller, of certain Bra-
zilian crustaceans likewise appearing under two widely distinct male forms.
But this subject need not here be discussed.

I have now explained how, I believe, the wonderful fact of two dis-
tinctly defined castes of sterile workers existing in the same nest, both
widely different from each other and from their parents, has originated.
We can see how useful their production may have been to a social com-
munity of ants, on the same principle that the division of labor is useful
to civilized man. Ants, however, work by inherited instincts, and by in-
herited organs or tools, while man works by acquired knowledge and manu-
factured instruments. But I must confess, that, with all my faith in natural
selection, I should never have anticipated that this principle could have
been efficient in so high a degree, had not the case of these neuter instincts
led me to this conclusion. I have, therefore, discussed this case, at some
little but wholly insufficient length, in order to show the power of natural
selection, and likewise because this is by far the most serious special diffi-
culty which my theory has encountered. The case, also, is very interesting,
as it proves that with animals, as with plants, any amount of modification
may be effected by the accumulation of numerous, slight, spontaneous vari-
ations, which are in any way profitable, without exercise or habit having)
been brought into play. For peculiar habits, confined to the workers of j
sterile females, however long they might be followed, could not possiblyj



Online LibraryCharles DarwinThe origin of species → online text (page 26 of 50)