On the Origin of Species by Means of Natural Selection
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Huber's statement that the very first cell is excavated out of a little
parallel-sided wall of wax, is not, as far as I have seen, strictly
correct; the first commencement having always been a little hood of wax;
but I will not here enter on these details. We see how important a part
excavation plays in the construction of the cells; but it would be a great
error to suppose that the bees cannot build up a rough wall of wax in the
proper position--that is, along the plane of intersection between two
adjoining spheres. I have several specimens showing clearly that they can
do this. Even in the rude circumferential rim or wall of wax round a
growing comb, flexures may sometimes be observed, corresponding in position
to the planes of the rhombic basal plates of future cells. But the rough
wall of wax has in every case to be finished off, by being largely gnawed
away on both sides. The manner in which the bees build is curious; they
always make the first rough wall from ten to twenty times thicker than the
excessively thin finished wall of the cell, which will ultimately be left.
We shall understand how they work, by supposing masons first to pile up a
broad ridge of cement, and then to begin 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, which are only about one
four-hundredth of an inch in thickness; the plates of the pyramidal basis
being about twice as thick. By this 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 working
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 I invariably found that the colour was most
delicately diffused by the bees--as delicately as a painter could have done
with his brush--by atoms of the coloured 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 trying 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 entirely pull down
and rebuild in different ways the same cell, sometimes 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 downwards 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 been
built. 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 quite 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 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. It is even conceivable that an insect
might, by fixing on a point at which to commence a cell, and then moving
outside, first to one point, and then to five other points, at the proper
relative distances from the central point and from each other, strike the
planes of intersection, and so make an isolated hexagon: but I am not
aware that any such case has been observed; nor would any good be derived
from a single hexagon being built, as in its construction more materials
would be required than for a cylinder.
As natural selection acts only by the accumulation of slight modifications
of structure or instinct, each profitable to the individual under its
conditions of life, it may reasonably be asked, how a long and graduated
succession of modified architectural instincts, all tending towards the
present perfect plan of construction, could have profited the progenitors
of the hive-bee? I think the answer is not difficult: it is known that
bees are often hard pressed to get sufficient nectar; and I am informed by
Mr. Tegetmeier that it has been experimentally found that no less than from
twelve to fifteen pounds of dry sugar are consumed by a hive of bees for
the secretion of each pound of wax; so that a 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 during the process of
secretion. A large store of honey is indispensable to support 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 must be a most important element of
success in any family of bees. Of course the success of any species of bee
may be dependent on the number of its parasites or other enemies, or on
quite distinct causes, and so be altogether independent of the quantity of
honey which the bees could collect. But let us suppose that this latter
circumstance determined, as it probably often does determine, the numbers
of a humble-bee which could exist in a country; and let us further suppose
that the community lived throughout 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 humble-bee, if a slight modification of her instinct 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 wax.
Hence it would continually be more and more advantageous to our humble-bee,
if she were to make her 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 other
cells, and much 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 disappear, and would all 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 economising 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
particular 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 economy of wax; that
individual swarm which wasted least honey in the secretion of wax, having
succeeded best, and having transmitted by inheritance its newly acquired
economical instinct to new swarms, which in their turn will have had the
best chance of succeeding in the struggle for existence.
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 possibly have originated; cases, in which no intermediate
gradations are known to exist; cases of instinct of apparently such
trifling importance, that they could hardly have been acted on 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 parent, and must therefore believe that they
have been acquired by independent acts of 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 my 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
profitable to the community that a number should have been annually born
capable of work, but incapable of procreation, I can see no very great
difficulty in this being effected by 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
structure, 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 prodigious difference in this respect between the workers
and the perfect females, would have been far better exemplified by the
hive-bee. If a working ant or other neuter insect had been an animal in
the ordinary state, I should have unhesitatingly assumed that all its
characters had been slowly acquired through natural selection; namely, by
an individual having been born with some slight profitable modification of
structure, this being inherited by its offspring, which again varied and
were again selected, and so onwards. 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 acquired modifications of
structure or instinct to its progeny. It may well be asked how is it
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 structure which have become correlated to certain ages, and
to either sex. We have differences correlated not only to one sex, but to
that short period alone 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 in other breeds, in comparison with
the horns of the bulls or cows of these same breeds. Hence I can see no
real difficulty in any character having become 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.
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. Thus, a well-flavoured vegetable is cooked, and the individual is
destroyed; but the horticulturist sows seeds of the same stock, and
confidently expects to get nearly the same variety; breeders of cattle wish
the flesh and fat to be well marbled together; the animal has been
slaughtered, but the breeder goes with confidence to the same family. I
have such faith in the powers of selection, that I do not doubt that a
breed of cattle, always yielding oxen with extraordinarily long horns,
could be slowly formed by carefully watching which individual bulls and
cows, when matched, produced oxen with the longest horns; and yet no one ox
could ever have propagated its kind. Thus I believe it has been with
social insects: a slight modification of structure, or instinct,
correlated with the sterile condition of certain members of the community,
has been advantageous to the community: consequently the fertile males and
females of the same community flourished, and transmitted to their fertile
offspring a tendency to produce sterile members having the same
modification. And I believe that this process has been repeated, until
that prodigious amount of difference between the fertile and sterile
females of the same species has been produced, which we see in many social
insects.
But we have not as yet touched on the climax 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 generally 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 different: 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 or 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 my theory. In the simpler case
of neuter insects all of one caste or of the same kind, which have been
rendered by natural selection, as I believe to be quite possible, different
from the fertile males and females,--in this case, we may safely conclude
from the analogy of ordinary variations, that each successive, slight,
profitable modification did not probably at first appear in all the
individual neuters in the same nest, but in a few alone; and that by the
long-continued selection of the fertile parents which produced most neuters
with the profitable modification, all the neuters ultimately came to have
the desired character. On this view we ought occasionally to find
neuter-insects of the same species, in the same nest, presenting gradations
of structure; and this we do find, even often, considering how few
neuter-insects out of Europe have been carefully examined. Mr. F. Smith
has shown how surprisingly the neuters of several British ants differ from
each other in size and sometimes in colour; and that the extreme forms can
sometimes be perfectly linked together by individuals taken out of the same
nest: I have myself compared perfect gradations of this kind. It often
happens that the larger or the smaller sized workers are the most numerous;
or that both large and small are numerous, with those of an intermediate
size scanty in numbers. Formica flava has larger and smaller workers, with
some 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 rudimentary in the smaller
workers than can be accounted for merely by their proportionally 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 we here 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 had
come to be in this condition; we should then have had a species of ant with
neuters very nearly in the same condition with 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 may give one other case: so confidently did I expect to find gradations
in important points of structure 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 suppose 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 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-different structure
of their jaws. I speak confidently on this latter point, as Mr. Lubbock
made drawings for me with the camera lucida of the jaws which I had
dissected from the workers of the several sizes.
With these facts before me, I believe that natural selection, by acting on
the fertile parents, could form a species which should regularly produce
neuters, either all of large size with one form of jaw, or all of small
size with jaws having a widely different structure; or lastly, and this is
our climax of 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 been first formed, as in the case of the driver
ant, and then the extreme forms, from being the most useful to the
community, having been produced in greater and greater numbers through the
natural selection of the parents which generated them; until none with an
intermediate structure were produced.
Thus, as I believe, the wonderful fact of two distinctly 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 community of insects, on the same
principle that the division of labour is useful to civilised man. As ants
work by inherited instincts and by inherited tools or weapons, and not by
acquired knowledge and manufactured instruments, a perfect division of
labour could be effected with them only by the workers being sterile; for
had they been fertile, they would have intercrossed, and their instincts
and structure would have become blended. And nature has, as I believe,
effected this admirable division of labour in the communities of ants, by
the means of natural selection. But I am bound to confess, that, with all
my faith in this principle, I should never have anticipated that natural
selection could have been efficient in so high a degree, had not the case
of these neuter insects convinced me of the fact. 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 difficulty, which my theory has
encountered. The case, also, is very interesting, as it proves that with
animals, as with plants, any amount of modification in structure can be
effected by the accumulation of numerous, slight, and as we must call them
accidental, variations, which are in any manner profitable, without
exercise or habit having come into play. For no amount of exercise, or
habit, or volition, in the utterly sterile members of a community could
possibly have affected the structure or instincts of the fertile members,
which alone leave descendants. I am surprised that no one has advanced
this demonstrative case of neuter insects, against the well-known doctrine
of Lamarck.
Summary. -- I have endeavoured briefly in this chapter to show that the
mental qualities of our domestic animals vary, and that the variations are
inherited. Still more briefly I have attempted to show that instincts vary
slightly in a state of nature. No one will dispute that instincts are of
the highest importance to each animal. Therefore I can see no difficulty,
under changing conditions of life, in natural selection accumulating slight
modifications of instinct to any extent, in any useful direction. In some
cases habit or use and disuse have probably come into play. I do not
pretend that the facts given in this chapter strengthen in any great degree
my theory; but none of the cases of difficulty, to the best of my judgment,
annihilate it. On the other hand, the fact that instincts are not always
absolutely perfect and are liable to mistakes;--that no instinct has been
produced for the exclusive good of other animals, but that each animal
takes advantage of the instincts of others;--that the canon in natural
history, of 'natura non facit saltum' is applicable to instincts as well as
to corporeal structure, and is plainly explicable on the foregoing views,
but is otherwise inexplicable,--all tend to corroborate the theory of
natural selection.
This theory is, also, strengthened by some few other facts in regard to
instincts; as by that common case of closely allied, but certainly
distinct, species, when inhabiting distant parts of the world and living
under considerably different conditions of life, yet often retaining nearly
the same instincts. For instance, we can understand on the principle of
inheritance, how it is that the thrush of South America lines its nest with
mud, in the same peculiar manner as does our British thrush: how it is
that the male wrens (Troglodytes) of North America, build 'cock-nests,' to
roost in, like the males of our distinct Kitty-wrens,--a habit wholly
unlike that of any other known bird. Finally, it may not be a logical
deduction, but to my imagination it is far more satisfactory to look at
such instincts as the young cuckoo ejecting its foster-brothers,--ants
making slaves,--the larvae of ichneumonidae feeding within the live bodies
of caterpillars,--not as specially endowed or created instincts, but as
small consequences of one general law, leading to the advancement of all
organic beings, namely, multiply, vary, let the strongest live and the
weakest die.
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