On the Origin of Species, 6th Edition
by
Charles Darwin
Part 5 out of 11
Salvin), without any great break, as far as fitness for sifting is
concerned, through the beak of the Merganetta armata, and in some respects
through that of the Aix sponsa, to the beak of the common duck. In this
latter species the lamellae are much coarser than in the shoveller, and are
firmly attached to the sides of the mandible; they are only about fifty in
number on each side, and do not project at all beneath the margin. They
are square-topped, and are edged with translucent, hardish tissue, as if
for crushing food. The edges of the lower mandible are crossed by numerous
fine ridges, which project very little. Although the beak is thus very
inferior as a sifter to that of a shoveller, yet this bird, as every one
knows, constantly uses it for this purpose. There are other species, as I
hear from Mr. Salvin, in which the lamellae are considerably less developed
than in the common duck; but I do not know whether they use their beaks for
sifting the water.
Turning to another group of the same family. In the Egyptian goose
(Chenalopex) the beak closely resembles that of the common duck; but the
lamellae are not so numerous, nor so distinct from each other, nor do they
project so much inward; yet this goose, as I am informed by Mr. E.
Bartlett, "uses its bill like a duck by throwing the water out at the
corners." Its chief food, however, is grass, which it crops like the
common goose. In this latter bird the lamellae of the upper mandible are
much coarser than in the common duck, almost confluent, about twenty-seven
in number on each side, and terminating upward in teeth-like knobs. The
palate is also covered with hard rounded knobs. The edges of the lower
mandible are serrated with teeth much more prominent, coarser and sharper
than in the duck. The common goose does not sift the water, but uses its
beak exclusively for tearing or cutting herbage, for which purpose it is so
well fitted that it can crop grass closer than almost any other animal.
There are other species of geese, as I hear from Mr. Bartlett, in which the
lamellae are less developed than in the common goose.
We thus see that a member of the duck family, with a beak constructed like
that of a common goose and adapted solely for grazing, or even a member
with a beak having less well-developed lamellae, might be converted by
small changes into a species like the Egyptian goose--this into one like
the common duck--and, lastly, into one like the shoveller, provided with a
beak almost exclusively adapted for sifting the water; for this bird could
hardly use any part of its beak, except the hooked tip, for seizing or
tearing solid food. The beak of a goose, as I may add, might also be
converted by small changes into one provided with prominent, recurved
teeth, like those of the Merganser (a member of the same family), serving
for the widely different purpose of securing live fish.
Returning to the whales. The Hyperoodon bidens is destitute of true teeth
in an efficient condition, but its palate is roughened, according to
Lacepede, with small unequal, hard points of horn. There is, therefore,
nothing improbable in supposing that some early Cetacean form was provided
with similar points of horn on the palate, but rather more regularly
placed, and which, like the knobs on the beak of the goose, aided it in
seizing or tearing its food. If so, it will hardly be denied that the
points might have been converted through variation and natural selection
into lamellae as well-developed as those of the Egyptian goose, in which
case they would have been used both for seizing objects and for sifting the
water; then into lamellae like those of the domestic duck; and so onward,
until they became as well constructed as those of the shoveller, in which
case they would have served exclusively as a sifting apparatus. From this
stage, in which the lamellae would be two-thirds of the length of the
plates of baleen in the Balaenoptera rostrata, gradations, which may be
observed in still-existing Cetaceans, lead us onward to the enormous plates
of baleen in the Greenland whale. Nor is there the least reason to doubt
that each step in this scale might have been as serviceable to certain
ancient Cetaceans, with the functions of the parts slowly changing during
the progress of development, as are the gradations in the beaks of the
different existing members of the duck-family. We should bear in mind that
each species of duck is subjected to a severe struggle for existence, and
that the structure of every part of its frame must be well adapted to its
conditions of life.
The Pleuronectidae, or Flat-fish, are remarkable for their asymmetrical
bodies. They rest on one side--in the greater number of species on the
left, but in some on the right side; and occasionally reversed adult
specimens occur. The lower, or resting-surface, resembles at first sight
the ventral surface of an ordinary fish; it is of a white colour, less
developed in many ways than the upper side, with the lateral fins often of
smaller size. But the eyes offer the most remarkable peculiarity; for they
are both placed on the upper side of the head. During early youth,
however, they stand opposite to each other, and the whole body is then
symmetrical, with both sides equally coloured. Soon the eye proper to the
lower side begins to glide slowly round the head to the upper side; but
does not pass right through the skull, as was formerly thought to be the
case. It is obvious that unless the lower eye did thus travel round, it
could not be used by the fish while lying in its habitual position on one
side. The lower eye would, also, have been liable to be abraded by the
sandy bottom. That the Pleuronectidae are admirably adapted by their
flattened and asymmetrical structure for their habits of life, is manifest
from several species, such as soles, flounders, etc., being extremely
common. The chief advantages thus gained seem to be protection from their
enemies, and facility for feeding on the ground. The different members,
however, of the family present, as Schiodte remarks, "a long series of
forms exhibiting a gradual transition from Hippoglossus pinguis, which does
not in any considerable degree alter the shape in which it leaves the ovum,
to the soles, which are entirely thrown to one side."
Mr. Mivart has taken up this case, and remarks that a sudden spontaneous
transformation in the position of the eyes is hardly conceivable, in which
I quite agree with him. He then adds: "If the transit was gradual, then
how such transit of one eye a minute fraction of the journey towards the
other side of the head could benefit the individual is, indeed, far from
clear. It seems, even, that such an incipient transformation must rather
have been injurious." But he might have found an answer to this objection
in the excellent observations published in 1867 by Malm. The
Pleuronectidae, while very young and still symmetrical, with their eyes
standing on opposite sides of the head, cannot long retain a vertical
position, owing to the excessive depth of their bodies, the small size of
their lateral fins, and to their being destitute of a swim-bladder. Hence,
soon growing tired, they fall to the bottom on one side. While thus at
rest they often twist, as Malm observed, the lower eye upward, to see above
them; and they do this so vigorously that the eye is pressed hard against
the upper part of the orbit. The forehead between the eyes consequently
becomes, as could be plainly seen, temporarily contracted in breadth. On
one occasion Malm saw a young fish raise and depress the lower eye through
an angular distance of about seventy degrees.
We should remember that the skull at this early age is cartilaginous and
flexible, so that it readily yields to muscular action. It is also known
with the higher animals, even after early youth, that the skull yields and
is altered in shape, if the skin or muscles be permanently contracted
through disease or some accident. With long-eared rabbits, if one ear
flops forward and downward, its weight drags forward all the bones of the
skull on the same side, of which I have given a figure. Malm states that
the newly-hatched young of perches, salmon, and several other symmetrical
fishes, have the habit of occasionally resting on one side at the bottom;
and he has observed that they often then strain their lower eyes so as to
look upward; and their skulls are thus rendered rather crooked. These
fishes, however, are soon able to hold themselves in a vertical position,
and no permanent effect is thus produced. With the Pleuronectidae, on the
other hand, the older they grow the more habitually they rest on one side,
owing to the increasing flatness of their bodies, and a permanent effect is
thus produced on the form of the head, and on the position of the eyes.
Judging from analogy, the tendency to distortion would no doubt be
increased through the principle of inheritance. Schiodte believes, in
opposition to some other naturalists, that the Pleuronectidae are not quite
symmetrical even in the embryo; and if this be so, we could understand how
it is that certain species, while young, habitually fall over and rest on
the left side, and other species on the right side. Malm adds, in
confirmation of the above view, that the adult Trachypterus arcticus, which
is not a member of the Pleuronectidae, rests on its left side at the
bottom, and swims diagonally through the water; and in this fish, the two
sides of the head are said to be somewhat dissimilar. Our great authority
on Fishes, Dr. Gunther, concludes his abstract of Malm's paper, by
remarking that "the author gives a very simple explanation of the abnormal
condition of the Pleuronectoids."
We thus see that the first stages of the transit of the eye from one side
of the head to the other, which Mr. Mivart considers would be injurious,
may be attributed to the habit, no doubt beneficial to the individual and
to the species, of endeavouring to look upward with both eyes, while
resting on one side at the bottom. We may also attribute to the inherited
effects of use the fact of the mouth in several kinds of flat-fish being
bent towards the lower surface, with the jaw bones stronger and more
effective on this, the eyeless side of the head, than on the other, for the
sake, as Dr. Traquair supposes, of feeding with ease on the ground.
Disuse, on the other hand, will account for the less developed condition of
the whole inferior half of the body, including the lateral fins; though
Yarrel thinks that the reduced size of these fins is advantageous to the
fish, as "there is so much less room for their action than with the larger
fins above." Perhaps the lesser number of teeth in the proportion of four
to seven in the upper halves of the two jaws of the plaice, to twenty-five
to thirty in the lower halves, may likewise be accounted for by disuse.
>From the colourless state of the ventral surface of most fishes and of many
other animals, we may reasonably suppose that the absence of colour in
flat-fish on the side, whether it be the right or left, which is under-
most, is due to the exclusion of light. But it cannot be supposed that the
peculiar speckled appearance of the upper side of the sole, so like the
sandy bed of the sea, or the power in some species, as recently shown by
Pouchet, of changing their colour in accordance with the surrounding
surface, or the presence of bony tubercles on the upper side of the turbot,
are due to the action of the light. Here natural selection has probably
come into play, as well as in adapting the general shape of the body of
these fishes, and many other peculiarities, to their habits of life. We
should keep in mind, as I have before insisted, that the inherited effects
of the increased use of parts, and perhaps of their disuse, will be
strengthened by natural selection. For all spontaneous variations in the
right direction will thus be preserved; as will those individuals which
inherit in the highest degree the effects of the increased and beneficial
use of any part. How much to attribute in each particular case to the
effects of use, and how much to natural selection, it seems impossible to
decide.
I may give another instance of a structure which apparently owes its origin
exclusively to use or habit. The extremity of the tail in some American
monkeys has been converted into a wonderfully perfect prehensile organ, and
serves as a fifth hand. A reviewer, who agrees with Mr. Mivart in every
detail, remarks on this structure: "It is impossible to believe that in
any number of ages the first slight incipient tendency to grasp could
preserve the lives of the individuals possessing it, or favour their chance
of having and of rearing offspring." But there is no necessity for any
such belief. Habit, and this almost implies that some benefit great or
small is thus derived, would in all probability suffice for the work.
Brehm saw the young of an African monkey (Cercopithecus) clinging to the
under surface of their mother by their hands, and at the same time they
hooked their little tails round that of their mother. Professor Henslow
kept in confinement some harvest mice (Mus messorius) which do not possess
a structurally prehensive tail; but he frequently observed that they curled
their tails round the branches of a bush placed in the cage, and thus aided
themselves in climbing. I have received an analogous account from Dr.
Gunther, who has seen a mouse thus suspend itself. If the harvest mouse
had been more strictly arboreal, it would perhaps have had its tail
rendered structurally prehensile, as is the case with some members of the
same order. Why Cercopithecus, considering its habits while young, has not
become thus provided, it would be difficult to say. It is, however,
possible that the long tail of this monkey may be of more service to it as
a balancing organ in making its prodigious leaps, than as a prehensile
organ.
The mammary glands are common to the whole class of mammals, and are
indispensable for their existence; they must, therefore, have been
developed at an extremely remote period, and we can know nothing positively
about their manner of development. Mr. Mivart asks: "Is it conceivable
that the young of any animal was ever saved from destruction by
accidentally sucking a drop of scarcely nutritious fluid from an
accidentally hypertrophied cutaneous gland of its mother? And even if one
was so, what chance was there of the perpetuation of such a variation?"
But the case is not here put fairly. It is admitted by most evolutionists
that mammals are descended from a marsupial form; and if so, the mammary
glands will have been at first developed within the marsupial sack. In the
case of the fish (Hippocampus) the eggs are hatched, and the young are
reared for a time, within a sack of this nature; and an American
naturalist, Mr. Lockwood, believes from what he has seen of the development
of the young, that they are nourished by a secretion from the cutaneous
glands of the sack. Now, with the early progenitors of mammals, almost
before they deserved to be thus designated, is it not at least possible
that the young might have been similarly nourished? And in this case, the
individuals which secreted a fluid, in some degree or manner the most
nutritious, so as to partake of the nature of milk, would in the long run
have reared a larger number of well-nourished offspring, than would the
individuals which secreted a poorer fluid; and thus the cutaneous glands,
which are the homologues of the mammary glands, would have been improved or
rendered more effective. It accords with the widely extended principle of
specialisation, that the glands over a certain space of the sack should
have become more highly developed than the remainder; and they would then
have formed a breast, but at first without a nipple, as we see in the
Ornithorhyncus, at the base of the mammalian series. Through what agency
the glands over a certain space became more highly specialised than the
others, I will not pretend to decide, whether in part through compensation
of growth, the effects of use, or of natural selection.
The development of the mammary glands would have been of no service, and
could not have been affected through natural selection, unless the young at
the same time were able to partake of the secretion. There is no greater
difficulty in understanding how young mammals have instinctively learned to
suck the breast, than in understanding how unhatched chickens have learned
to break the egg-shell by tapping against it with their specially adapted
beaks; or how a few hours after leaving the shell they have learned to pick
up grains of food. In such cases the most probable solution seems to be,
that the habit was at first acquired by practice at a more advanced age,
and afterwards transmitted to the offspring at an earlier age. But the
young kangaroo is said not to suck, only to cling to the nipple of its
mother, who has the power of injecting milk into the mouth of her helpless,
half-formed offspring. On this head Mr. Mivart remarks: "Did no special
provision exist, the young one must infallibly be choked by the intrusion
of the milk into the wind-pipe. But there IS a special provision. The
larynx is so elongated that it rises up into the posterior end of the nasal
passage, and is thus enabled to give free entrance to the air for the
lungs, while the milk passes harmlessly on each side of this elongated
larynx, and so safely attains the gullet behind it." Mr. Mivart then asks
how did natural selection remove in the adult kangaroo (and in most other
mammals, on the assumption that they are descended from a marsupial form),
"this at least perfectly innocent and harmless structure?" It may be
suggested in answer that the voice, which is certainly of high importance
to many animals, could hardly have been used with full force as long as the
larynx entered the nasal passage; and Professor Flower has suggested to me
that this structure would have greatly interfered with an animal swallowing
solid food.
We will now turn for a short space to the lower divisions of the animal
kingdom. The Echinodermata (star-fishes, sea-urchins, etc.) are furnished
with remarkable organs, called pedicellariae, which consist, when well
developed, of a tridactyle forceps--that is, of one formed of three
serrated arms, neatly fitting together and placed on the summit of a
flexible stem, moved by muscles. These forceps can seize firmly hold of
any object; and Alexander Agassiz has seen an Echinus or sea-urchin rapidly
passing particles of excrement from forceps to forceps down certain lines
of its body, in order that its shell should not be fouled. But there is no
doubt that besides removing dirt of all kinds, they subserve other
functions; and one of these apparently is defence.
With respect to these organs, Mr. Mivart, as on so many previous occasions,
asks: "What would be the utility of the FIRST RUDIMENTARY BEGINNINGS of
such structures, and how could such insipient buddings have ever preserved
the life of a single Echinus?" He adds, "not even the SUDDEN development
of the snapping action would have been beneficial without the freely
movable stalk, nor could the latter have been efficient without the
snapping jaws, yet no minute, nearly indefinite variations could
simultaneously evolve these complex co-ordinations of structure; to deny
this seems to do no less than to affirm a startling paradox." Paradoxical
as this may appear to Mr. Mivart, tridactyle forcepses, immovably fixed at
the base, but capable of a snapping action, certainly exist on some star-
fishes; and this is intelligible if they serve, at least in part, as a
means of defence. Mr. Agassiz, to whose great kindness I am indebted for
much information on the subject, informs me that there are other star-
fishes, in which one of the three arms of the forceps is reduced to a
support for the other two; and again, other genera in which the third arm
is completely lost. In Echinoneus, the shell is described by M. Perrier as
bearing two kinds of pedicellariae, one resembling those of Echinus, and
the other those of Spatangus; and such cases are always interesting as
affording the means of apparently sudden transitions, through the abortion
of one of the two states of an organ.
With respect to the steps by which these curious organs have been evolved,
Mr. Agassiz infers from his own researches and those of Mr. Muller, that
both in star-fishes and sea-urchins the pedicellariae must undoubtedly be
looked at as modified spines. This may be inferred from their manner of
development in the individual, as well as from a long and perfect series of
gradations in different species and genera, from simple granules to
ordinary spines, to perfect tridactyle pedicellariae. The gradation
extends even to the manner in which ordinary spines and the pedicellariae,
with their supporting calcareous rods, are articulated to the shell. In
certain genera of star-fishes, "the very combinations needed to show that
the pedicellariae are only modified branching spines" may be found. Thus
we have fixed spines, with three equi-distant, serrated, movable branches,
articulated to near their bases; and higher up, on the same spine, three
other movable branches. Now when the latter arise from the summit of a
spine they form, in fact, a rude tridactyle pedicellariae, and such may be
seen on the same spine together with the three lower branches. In this
case the identity in nature between the arms of the pedicellariae and the
movable branches of a spine, is unmistakable. It is generally admitted
that the ordinary spines serve as a protection; and if so, there can be no
reason to doubt that those furnished with serrated and movable branches
likewise serve for the same purpose; and they would thus serve still more
effectively as soon as by meeting together they acted as a prehensile or
snapping apparatus. Thus every gradation, from an ordinary fixed spine to
a fixed pedicellariae, would be of service.
In certain genera of star-fishes these organs, instead of being fixed or
borne on an immovable support, are placed on the summit of a flexible and
muscular, though short, stem; and in this case they probably subserve some
additional function besides defence. In the sea-urchins the steps can be
followed by which a fixed spine becomes articulated to the shell, and is
thus rendered movable. I wish I had space here to give a fuller abstract
of Mr. Agassiz's interesting observations on the development of the
pedicellariae. All possible gradations, as he adds, may likewise be found
between the pedicellariae of the star-fishes and the hooks of the
Ophiurians, another group of the Echinodermata; and again between the
pedicellariae of sea-urchins and the anchors of the Holothuriae, also
belonging to the same great class.
Certain compound animals, or zoophytes, as they have been termed, namely
the Polyzoa, are provided with curious organs called avicularia. These
differ much in structure in the different species. In their most perfect
condition they curiously resemble the head and beak of a vulture in
miniature, seated on a neck and capable of movement, as is likewise the
lower jaw or mandible. In one species observed by me, all the avicularia
on the same branch often moved simultaneously backwards and forwards, with
the lower jaw widely open, through an angle of about 90 degrees, in the
course of five seconds; and their movement caused the whole polyzoary to
tremble. When the jaws are touched with a needle they seize it so firmly
that the branch can thus be shaken.
Mr. Mivart adduces this case, chiefly on account of the supposed difficulty
of organs, namely the avicularia of the Polyzoa and the pedicellariae of
the Echinodermata, which he considers as "essentially similar," having been
developed through natural selection in widely distinct divisions of the
animal kingdom. But, as far as structure is concerned, I can see no
similarity between tridactyle pedicellariae and avicularia. The latter
resembles somewhat more closely the chelae or pincers of Crustaceans; and
Mr. Mivart might have adduced with equal appropriateness this resemblance
as a special difficulty, or even their resemblance to the head and beak of
a bird. The avicularia are believed by Mr. Busk, Dr. Smitt and Dr.
Nitsche--naturalists who have carefully studied this group--to be
homologous with the zooids and their cells which compose the zoophyte, the
movable lip or lid of the cell corresponding with the lower and movable
mandible of the avicularium. Mr. Busk, however, does not know of any
gradations now existing between a zooid and an avicularium. It is
therefore impossible to conjecture by what serviceable gradations the one
could have been converted into the other, but it by no means follows from
this that such gradations have not existed.
As the chelae of Crustaceans resemble in some degree the avicularia of
Polyzoa, both serving as pincers, it may be worth while to show that with
the former a long series of serviceable gradations still exists. In the
first and simplest stage, the terminal segment of a limb shuts down either
on the square summit of the broad penultimate segment, or against one whole
side, and is thus enabled to catch hold of an object, but the limb still
serves as an organ of locomotion. We next find one corner of the broad
penultimate segment slightly prominent, sometimes furnished with irregular
teeth, and against these the terminal segment shuts down. By an increase
in the size of this projection, with its shape, as well as that of the
terminal segment, slightly modified and improved, the pincers are rendered
more and more perfect, until we have at last an instrument as efficient as
the chelae of a lobster. And all these gradations can be actually traced.
Besides the avicularia, the polyzoa possess curious organs called
vibracula. These generally consist of long bristles, capable of movement
and easily excited. In one species examined by me the vibracula were
slightly curved and serrated along the outer margin, and all of them on the
same polyzoary often moved simultaneously; so that, acting like long oars,
they swept a branch rapidly across the object-glass of my microscope. When
a branch was placed on its face, the vibracula became entangled, and they
made violent efforts to free themselves. They are supposed to serve as a
defence, and may be seen, as Mr. Busk remarks, "to sweep slowly and
carefully over the surface of the polyzoary, removing what might be noxious
to the delicate inhabitants of the cells when their tentacula are
protruded." The avicularia, like the vibracula, probably serve for
defence, but they also catch and kill small living animals, which, it is
believed, are afterwards swept by the currents within reach of the
tentacula of the zooids. Some species are provided with avicularia and
vibracula, some with avicularia alone and a few with vibracula alone.
It is not easy to imagine two objects more widely different in appearance
than a bristle or vibraculum, and an avicularium like the head of a bird;
yet they are almost certainly homologous and have been developed from the
same common source, namely a zooid with its cell. Hence, we can understand
how it is that these organs graduate in some cases, as I am informed by Mr.
Busk, into each other. Thus, with the avicularia of several species of
Lepralia, the movable mandible is so much produced and is so like a bristle
that the presence of the upper or fixed beak alone serves to determine its
avicularian nature. The vibracula may have been directly developed from
the lips of the cells, without having passed through the avicularian stage;
but it seems more probable that they have passed through this stage, as
during the early stages of the transformation, the other parts of the cell,
with the included zooid, could hardly have disappeared at once. In many
cases the vibracula have a grooved support at the base, which seems to
represent the fixed beak; though this support in some species is quite
absent. This view of the development of the vibracula, if trustworthy, is
interesting; for supposing that all the species provided with avicularia
had become extinct, no one with the most vivid imagination would ever have
thought that the vibracula had originally existed as part of an organ,
resembling a bird's head, or an irregular box or hood. It is interesting
to see two such widely different organs developed from a common origin; and
as the movable lip of the cell serves as a protection to the zooid, there
is no difficulty in believing that all the gradations, by which the lip
became converted first into the lower mandible of an avicularium, and then
into an elongated bristle, likewise served as a protection in different
ways and under different circumstances.
In the vegetable kingdom Mr. Mivart only alludes to two cases, namely the
structure of the flowers of orchids, and the movements of climbing plants.
With respect to the former, he says: "The explanation of their ORIGIN is
deemed thoroughly unsatisfactory--utterly insufficient to explain the
incipient, infinitesimal beginnings of structures which are of utility only
when they are considerably developed." As I have fully treated this
subject in another work, I will here give only a few details on one alone
of the most striking peculiarities of the flowers of orchids, namely, their
pollinia. A pollinium, when highly developed, consists of a mass of
pollen-grains, affixed to an elastic foot-stalk or caudicle, and this to a
little mass of extremely viscid matter. The pollinia are by this means
transported by insects from one flower to the stigma of another. In some
orchids there is no caudicle to the pollen-masses, and the grains are
merely tied together by fine threads; but as these are not confined to
orchids, they need not here be considered; yet I may mention that at the
base of the orchidaceous series, in Cypripedium, we can see how the threads
were probably first developed. In other orchids the threads cohere at one
end of the pollen-masses; and this forms the first or nascent trace of a
caudicle. That this is the origin of the caudicle, even when of
considerable length and highly developed, we have good evidence in the
aborted pollen-grains which can sometimes be detected embedded within the
central and solid parts.
With respect to the second chief peculiarity, namely, the little mass of
viscid matter attached to the end of the caudicle, a long series of
gradations can be specified, each of plain service to the plant. In most
flowers belonging to other orders the stigma secretes a little viscid
matter. Now, in certain orchids similar viscid matter is secreted, but in
much larger quantities by one alone of the three stigmas; and this stigma,
perhaps in consequence of the copious secretion, is rendered sterile. When
an insect visits a flower of this kind, it rubs off some of the viscid
matter, and thus at the same time drags away some of the pollen-grains.
>From this simple condition, which differs but little from that of a
multitude of common flowers, there are endless gradations--to species in
which the pollen-mass terminates in a very short, free caudicle--to others
in which the caudicle becomes firmly attached to the viscid matter, with
the sterile stigma itself much modified. In this latter case we have a
pollinium in its most highly developed and perfect condition. He who will
carefully examine the flowers of orchids for himself will not deny the
existence of the above series of gradations--from a mass of pollen-grains
merely tied together by threads, with the stigma differing but little from
that of the ordinary flowers, to a highly complex pollinium, admirably
adapted for transportal by insects; nor will he deny that all the
gradations in the several species are admirably adapted in relation to the
general structure of each flower for its fertilisation by different
insects. In this, and in almost every other case, the enquiry may be
pushed further backwards; and it may be asked how did the stigma of an
ordinary flower become viscid, but as we do not know the full history of
any one group of beings, it is as useless to ask, as it is hopeless to
attempt answering, such questions.
We will now turn to climbing plants. These can be arranged in a long
series, from those which simply twine round a support, to those which I
have called leaf-climbers, and to those provided with tendrils. In these
two latter classes the stems have generally, but not always, lost the power
of twining, though they retain the power of revolving, which the tendrils
likewise possess. The gradations from leaf-climbers to tendril bearers are
wonderfully close, and certain plants may be differently placed in either
class. But in ascending the series from simple twiners to leaf-climbers,
an important quality is added, namely sensitiveness to a touch, by which
means the foot-stalks of the leaves or flowers, or these modified and
converted into tendrils, are excited to bend round and clasp the touching
object. He who will read my memoir on these plants will, I think, admit
that all the many gradations in function and structure between simple
twiners and tendril-bearers are in each case beneficial in a high degree to
the species. For instance, it is clearly a great advantage to a twining
plant to become a leaf-climber; and it is probable that every twiner which
possessed leaves with long foot-stalks would have been developed into a
leaf-climber, if the foot-stalks had possessed in any slight degree the
requisite sensitiveness to a touch.
As twining is the simplest means of ascending a support, and forms the
basis of our series, it may naturally be asked how did plants acquire this
power in an incipient degree, afterwards to be improved and increased
through natural selection. The power of twining depends, firstly, on the
stems while young being extremely flexible (but this is a character common
to many plants which are not climbers); and, secondly, on their continually
bending to all points of the compass, one after the other in succession, in
the same order. By this movement the stems are inclined to all sides, and
are made to move round and round. As soon as the lower part of a stem
strikes against any object and is stopped, the upper part still goes on
bending and revolving, and thus necessarily twines round and up the
support. The revolving movement ceases after the early growth of each
shoot. As in many widely separated families of plants, single species and
single genera possess the power of revolving, and have thus become twiners,
they must have independently acquired it, and cannot have inherited it from
a common progenitor. Hence, I was led to predict that some slight tendency
to a movement of this kind would be found to be far from uncommon with
plants which did not climb; and that this had afforded the basis for
natural selection to work on and improve. When I made this prediction, I
knew of only one imperfect case, namely, of the young flower-peduncles of a
Maurandia which revolved slightly and irregularly, like the stems of
twining plants, but without making any use of this habit. Soon afterwards
Fritz Muller discovered that the young stems of an Alisma and of a Linum--
plants which do not climb and are widely separated in the natural system--
revolved plainly, though irregularly, and he states that he has reason to
suspect that this occurs with some other plants. These slight movements
appear to be of no service to the plants in question; anyhow, they are not
of the least use in the way of climbing, which is the point that concerns
us. Nevertheless we can see that if the stems of these plants had been
flexible, and if under the conditions to which they are exposed it had
profited them to ascend to a height, then the habit of slightly and
irregularly revolving might have been increased and utilised through
natural selection, until they had become converted into well-developed
twining species.
With respect to the sensitiveness of the foot-stalks of the leaves and
flowers, and of tendrils, nearly the same remarks are applicable as in the
case of the revolving movements of twining plants. As a vast number of
species, belonging to widely distinct groups, are endowed with this kind of
sensitiveness, it ought to be found in a nascent condition in many plants
which have not become climbers. This is the case: I observed that the
young flower-peduncles of the above Maurandia curved themselves a little
towards the side which was touched. Morren found in several species of
Oxalis that the leaves and their foot-stalks moved, especially after
exposure to a hot sun, when they were gently and repeatedly touched, or
when the plant was shaken. I repeated these observations on some other
species of Oxalis with the same result; in some of them the movement was
distinct, but was best seen in the young leaves; in others it was extremely
slight. It is a more important fact that according to the high authority
of Hofmeister, the young shoots and leaves of all plants move after being
shaken; and with climbing plants it is, as we know, only during the early
stages of growth that the foot-stalks and tendrils are sensitive.
It is scarcely possible that the above slight movements, due to a touch or
shake, in the young and growing organs of plants, can be of any functional
importance to them. But plants possess, in obedience to various stimuli,
powers of movement, which are of manifest importance to them; for instance,
towards and more rarely from the light--in opposition to, and more rarely
in the direction of, the attraction of gravity. When the nerves and
muscles of an animal are excited by galvanism or by the absorption of
strychnine, the consequent movements may be called an incidental result,
for the nerves and muscles have not been rendered specially sensitive to
these stimuli. So with plants it appears that, from having the power of
movement in obedience to certain stimuli, they are excited in an incidental
manner by a touch, or by being shaken. Hence there is no great difficulty
in admitting that in the case of leaf-climbers and tendril-bearers, it is
this tendency which has been taken advantage of and increased through
natural selection. It is, however, probable, from reasons which I have
assigned in my memoir, that this will have occurred only with plants which
had already acquired the power of revolving, and had thus become twiners.
I have already endeavoured to explain how plants became twiners, namely, by
the increase of a tendency to slight and irregular revolving movements,
which were at first of no use to them; this movement, as well as that due
to a touch or shake, being the incidental result of the power of moving,
gained for other and beneficial purposes. Whether, during the gradual
development of climbing plants, natural selection has been aided by the
inherited effects of use, I will not pretend to decide; but we know that
certain periodical movements, for instance the so-called sleep of plants,
are governed by habit.
I have now considered enough, perhaps more than enough, of the cases,
selected with care by a skilful naturalist, to prove that natural selection
is incompetent to account for the incipient stages of useful structures;
and I have shown, as I hope, that there is no great difficulty on this
head. A good opportunity has thus been afforded for enlarging a little on
gradations of structure, often associated with strange functions--an
important subject, which was not treated at sufficient length in the former
editions of this work. I will now briefly recapitulate the foregoing
cases.
With the giraffe, the continued preservation of the individuals of some
extinct high-reaching ruminant, which had the longest necks, legs, etc.,
and could browse a little above the average height, and the continued
destruction of those which could not browse so high, would have sufficed
for the production of this remarkable quadruped; but the prolonged use of
all the parts, together with inheritance, will have aided in an important
manner in their co-ordination. With the many insects which imitate various
objects, there is no improbability in the belief that an accidental
resemblance to some common object was in each case the foundation for the
work of natural selection, since perfected through the occasional
preservation of slight variations which made the resemblance at all closer;
and this will have been carried on as long as the insect continued to vary,
and as long as a more and more perfect resemblance led to its escape from
sharp-sighted enemies. In certain species of whales there is a tendency to
the formation of irregular little points of horn on the palate; and it
seems to be quite within the scope of natural selection to preserve all
favourable variations, until the points were converted, first into
lamellated knobs or teeth, like those on the beak of a goose--then into
short lamellae, like those of the domestic ducks--and then into lamellae,
as perfect as those of the shoveller-duck--and finally into the gigantic
plates of baleen, as in the mouth of the Greenland whale. In the family of
the ducks, the lamellae are first used as teeth, then partly as teeth and
partly as a sifting apparatus, and at last almost exclusively for this
latter purpose.
With such structures as the above lamellae of horn or whalebone, habit or
use can have done little or nothing, as far as we can judge, towards their
development. On the other hand, the transportal of the lower eye of a
flat-fish to the upper side of the head, and the formation of a prehensile
tail, may be attributed almost wholly to continued use, together with
inheritance. With respect to the mammae of the higher animals, the most
probable conjecture is that primordially the cutaneous glands over the
whole surface of a marsupial sack secreted a nutritious fluid; and that
these glands were improved in function through natural selection, and
concentrated into a confined area, in which case they would have formed a
mamma. There is no more difficulty in understanding how the branched
spines of some ancient Echinoderm, which served as a defence, became
developed through natural selection into tridactyle pedicellariae, than in
understanding the development of the pincers of crustaceans, through
slight, serviceable modifications in the ultimate and penultimate segments
of a limb, which was at first used solely for locomotion. In the
avicularia and vibracula of the Polyzoa we have organs widely different in
appearance developed from the same source; and with the vibracula we can
understand how the successive gradations might have been of service. With
the pollinia of orchids, the threads which originally served to tie
together the pollen-grains, can be traced cohering into caudicles; and the
steps can likewise be followed by which viscid matter, such as that
secreted by the stigmas of ordinary flowers, and still subserving nearly
but not quite the same purpose, became attached to the free ends of the
caudicles--all these gradations being of manifest benefit to the plants in
question. With respect to climbing plants, I need not repeat what has been
so lately said.
It has often been asked, if natural selection be so potent, why has not
this or that structure been gained by certain species, to which it would
apparently have been advantageous? But it is unreasonable to expect a
precise answer to such questions, considering our ignorance of the past
history of each species, and of the conditions which at the present day
determine its numbers and range. In most cases only general reasons, but
in some few cases special reasons, can be assigned. Thus to adapt a
species to new habits of life, many co-ordinated modifications are almost
indispensable, and it may often have happened that the requisite parts did
not vary in the right manner or to the right degree. Many species must
have been prevented from increasing in numbers through destructive
agencies, which stood in no relation to certain structures, which we
imagine would have been gained through natural selection from appearing to
us advantageous to the species. In this case, as the struggle for life did
not depend on such structures, they could not have been acquired through
natural selection. In many cases complex and long-enduring conditions,
often of a peculiar nature, are necessary for the development of a
structure; and the requisite conditions may seldom have concurred. The
belief that any given structure, which we think, often erroneously, would
have been beneficial to a species, would have been gained under all
circumstances through natural selection, is opposed to what we can
understand of its manner of action. Mr. Mivart does not deny that natural
selection has effected something; but he considers it as "demonstrably
insufficient" to account for the phenomena which I explain by its agency.
His chief arguments have now been considered, and the others will hereafter
be considered. They seem to me to partake little of the character of
demonstration, and to have little weight in comparison with those in favour
of the power of natural selection, aided by the other agencies often
specified. I am bound to add, that some of the facts and arguments here
used by me, have been advanced for the same purpose in an able article
lately published in the "Medico-Chirurgical Review."
At the present day almost all naturalists admit evolution under some form.
Mr. Mivart believes that species change through "an internal force or
tendency," about which it is not pretended that anything is known. That
species have a capacity for change will be admitted by all evolutionists;
but there is no need, as it seems to me, to invoke any internal force
beyond the tendency to ordinary variability, which through the aid of
selection, by man has given rise to many well-adapted domestic races, and
which, through the aid of natural selection, would equally well give rise
by graduated steps to natural races or species. The final result will
generally have been, as already explained, an advance, but in some few
cases a retrogression, in organisation.
Mr. Mivart is further inclined to believe, and some naturalists agree with
him, that new species manifest themselves "with suddenness and by
modifications appearing at once." For instance, he supposes that the
differences between the extinct three-toed Hipparion and the horse arose
suddenly. He thinks it difficult to believe that the wing of a bird "was
developed in any other way than by a comparatively sudden modification of a
marked and important kind;" and apparently he would extend the same view to
the wings of bats and pterodactyles. This conclusion, which implies great
breaks or discontinuity in the series, appears to me improbable in the
highest degree.
Everyone who believes in slow and gradual evolution, will of course admit
that specific changes may have been as abrupt and as great as any single
variation which we meet with under nature, or even under domestication.
But as species are more variable when domesticated or cultivated than under
their natural conditions, it is not probable that such great and abrupt
variations have often occurred under nature, as are known occasionally to
arise under domestication. Of these latter variations several may be
attributed to reversion; and the characters which thus reappear were, it is
probable, in many cases at first gained in a gradual manner. A still
greater number must be called monstrosities, such as six-fingered men,
porcupine men, Ancon sheep, Niata cattle, etc.; and as they are widely
different in character from natural species, they throw very little light
on our subject. Excluding such cases of abrupt variations, the few which
remain would at best constitute, if found in a state of nature, doubtful
species, closely related to their parental types.
My reasons for doubting whether natural species have changed as abruptly as
have occasionally domestic races, and for entirely disbelieving that they
have changed in the wonderful manner indicated by Mr. Mivart, are as
follows. According to our experience, abrupt and strongly marked
variations occur in our domesticated productions, singly and at rather long
intervals of time. If such occurred under nature, they would be liable, as
formerly explained, to be lost by accidental causes of destruction and by
subsequent intercrossing; and so it is known to be under domestication,
unless abrupt variations of this kind are specially preserved and separated
by the care of man. Hence, in order that a new species should suddenly
appear in the manner supposed by Mr. Mivart, it is almost necessary to
believe, in opposition to all analogy, that several wonderfully changed
individuals appeared simultaneously within the same district. This
difficulty, as in the case of unconscious selection by man, is avoided on
the theory of gradual evolution, through the preservation of a large number
of individuals, which varied more or less in any favourable direction, and
of the destruction of a large number which varied in an opposite manner.
That many species have been evolved in an extremely gradual manner, there
can hardly be a doubt. The species and even the genera of many large
natural families are so closely allied together that it is difficult to
distinguish not a few of them. On every continent, in proceeding from
north to south, from lowland to upland, etc., we meet with a host of
closely related or representative species; as we likewise do on certain
distinct continents, which we have reason to believe were formerly
connected. But in making these and the following remarks, I am compelled
to allude to subjects hereafter to be discussed. Look at the many outlying
islands round a continent, and see how many of their inhabitants can be
raised only to the rank of doubtful species. So it is if we look to past
times, and compare the species which have just passed away with those still
living within the same areas; or if we compare the fossil species embedded
in the sub-stages of the same geological formation. It is indeed manifest
that multitudes of species are related in the closest manner to other
species that still exist, or have lately existed; and it will hardly be
maintained that such species have been developed in an abrupt or sudden
manner. Nor should it be forgotten, when we look to the special parts of
allied species, instead of to distinct species, that numerous and
wonderfully fine gradations can be traced, connecting together widely
different structures.
Many large groups of facts are intelligible only on the principle that
species have been evolved by very small steps. For instance, the fact that
the species included in the larger genera are more closely related to each
other, and present a greater number of varieties than do the species in the
smaller genera. The former are also grouped in little clusters, like
varieties round species; and they present other analogies with varieties,
as was shown in our second chapter. On this same principle we can
understand how it is that specific characters are more variable than
generic characters; and how the parts which are developed in an
extraordinary degree or manner are more variable than other parts of the
same species. Many analogous facts, all pointing in the same direction,
could be added.
Although very many species have almost certainly been produced by steps not
greater than those separating fine varieties; yet it may be maintained that
some have been developed in a different and abrupt manner. Such an
admission, however, ought not to be made without strong evidence being
assigned. The vague and in some respects false analogies, as they have
been shown to be by Mr. Chauncey Wright, which have been advanced in favour
of this view, such as the sudden crystallisation of inorganic substances,
or the falling of a facetted spheroid from one facet to another, hardly
deserve consideration. One class of facts, however, namely, the sudden
appearance of new and distinct forms of life in our geological formations
supports at first sight the belief in abrupt development. But the value of
this evidence depends entirely on the perfection of the geological record,
in relation to periods remote in the history of the world. If the record
is as fragmentary as many geologists strenuously assert, there is nothing
strange in new forms appearing as if suddenly developed.
Unless we admit transformations as prodigious as those advocated by Mr.
Mivart, such as the sudden development of the wings of birds or bats, or
the sudden conversion of a Hipparion into a horse, hardly any light is
thrown by the belief in abrupt modifications on the deficiency of
connecting links in our geological formations. But against the belief in
such abrupt changes, embryology enters a strong protest. It is notorious
that the wings of birds and bats, and the legs of horses or other
quadrupeds, are undistinguishable at an early embryonic period, and that
they become differentiated by insensibly fine steps. Embryological
resemblances of all kinds can be accounted for, as we shall hereafter see,
by the progenitors of our existing species having varied after early youth,
and having transmitted their newly-acquired characters to their offspring,
at a corresponding age. The embryo is thus left almost unaffected, and
serves as a record of the past condition of the species. Hence it is that
existing species during the early stages of their development so often
resemble ancient and extinct forms belonging to the same class. On this
view of the meaning of embryological resemblances, and indeed on any view,
it is incredible that an animal should have undergone such momentous and
abrupt transformations as those above indicated, and yet should not bear
even a trace in its embryonic condition of any sudden modification, every
detail in its structure being developed by insensibly fine steps.
He who believes that some ancient form was transformed suddenly through an
internal force or tendency into, for instance, one furnished with wings,
will be almost compelled to assume, in opposition to all analogy, that many
individuals varied simultaneously. It cannot be denied that such abrupt
and great changes of structure are widely different from those which most
species apparently have undergone. He will further be compelled to believe
that many structures beautifully adapted to all the other parts of the same
creature and to the surrounding conditions, have been suddenly produced;
and of such complex and wonderful co-adaptations, he will not be able to
assign a shadow of an explanation. He will be forced to admit that these
great and sudden transformations have left no trace of their action on the
embryo. To admit all this is, as it seems to me, to enter into the realms
of miracle, and to leave those of science.
CHAPTER VIII.
INSTINCT.
Instincts comparable with habits, but different in their origin --
Instincts graduated -- Aphides and ants -- Instincts variable -- Domestic
instincts, their origin -- Natural instincts of the cuckoo, molothrus,
ostrich, and parasitic bees -- Slave-making ants -- Hive-bee, its
cell-making instinct -- Changes of instinct and structure not necessarily
simultaneous -- Difficulties of the theory of the Natural Selection of
instincts -- Neuter or sterile insects -- Summary.
Many instincts are so wonderful that their development will probably appear
to the reader a difficulty sufficient to overthrow my whole theory. I may
here premise, that I have nothing to do with the origin of the mental
powers, any more than I have with that of life itself. We are concerned
only with the diversities of instinct and of the other mental faculties in
animals of the same class.
I will not attempt any definition of instinct. It would be easy to show
that several distinct mental actions are commonly embraced by this term;
but every one understands what is meant, when it is said that instinct
impels the cuckoo to migrate and to lay her eggs in other birds' nests. An
action, which we ourselves require experience to enable us to perform, when
performed by an animal, more especially by a very young one, without
experience, and when performed by many individuals in the same way, without
their knowing for what purpose it is performed, is usually said to be
instinctive. But I could show that none of these characters are universal.
A little dose of judgment or reason, as Pierre Huber expresses it, often
comes into play, even with animals low in the scale of nature.
Frederick Cuvier and several of the older metaphysicians have compared
instinct with habit. This comparison gives, I think, an accurate notion of
the frame of mind under which an instinctive action is performed, but not
necessarily of its origin. How unconsciously many habitual actions are
performed, indeed not rarely in direct opposition to our conscious will!
yet they may be modified by the will or reason. Habits easily become
associated with other habits, with certain periods of time and states of
the body. When once acquired, they often remain constant throughout life.
Several other points of resemblance between instincts and habits could be
pointed out. As in repeating a well-known song, so in instincts, one
action follows another by a sort of rhythm; if a person be interrupted in a
song, or in repeating anything by rote, he is generally forced to go back
to recover the habitual train of thought: so P. Huber found it was with a
caterpillar, which makes a very complicated hammock; for if he took a
caterpillar which had completed its hammock up to, say, the sixth stage of
construction, and put it into a hammock completed up only to the third
stage, the caterpillar simply re-performed the fourth, fifth, and sixth
stages of construction. If, however, a caterpillar were taken out of a
hammock made up, for instance, to the third stage, and were put into one
finished up to the sixth stage, so that much of its work was already done
for it, far from deriving any benefit from this, it was much embarrassed,
and, in order to complete its hammock, seemed forced to start from the
third stage, where it had left off, and thus tried to complete the already
finished work.
If we suppose any habitual action to become inherited--and it can be shown
that this does sometimes happen--then the resemblance between what
originally was a habit and an instinct becomes so close as not to be
distinguished. If Mozart, instead of playing the pianoforte at three years
old with wonderfully little practice, had played a tune with no practice at
all, be might truly be said to have done so instinctively. But it would be
a serious error to suppose that the greater number of instincts have been
acquired by habit in one generation, and then transmitted by inheritance to
succeeding generations. It can be clearly shown that the most wonderful
instincts with which we are acquainted, namely, those of the hive-bee and
of many ants, could not possibly have been acquired by habit.
It will be universally admitted that instincts are as important as
corporeal structures for the welfare of each species, under its present
conditions of life. Under changed conditions of life, it is at least
possible that slight modifications of instinct might be profitable to a
species; and if it can be shown that instincts do vary ever so little, then
I can see no difficulty in natural selection preserving and continually
accumulating variations of instinct to any extent that was profitable. It
is thus, as I believe, that all the most complex and wonderful instincts
have originated. As modifications of corporeal structure arise from, and
are increased by, use or habit, and are diminished or lost by disuse, so I
do not doubt it has been with instincts. But I believe that the effects of
habit are in many cases of subordinate importance to the effects of the
natural selection of what may be called spontaneous variations of
instincts;--that is of variations produced by the same unknown causes which
produce slight deviations of bodily structure.
No complex instinct can possibly be produced through natural selection,
except by the slow and gradual accumulation of numerous, slight, yet
profitable, variations. Hence, as in the case of corporeal structures, we
ought to find in nature, not the actual transitional gradations by which
each complex instinct has been acquired--for these could be found only in
the lineal ancestors of each species--but we ought to find in the
collateral lines of descent some evidence of such gradations; or we ought
at least to be able to show that gradations of some kind are possible; and
this we certainly can do. I have been surprised to find, making allowance
for the instincts of animals having been but little observed, except in
Europe and North America, and for no instinct being known among extinct
species, how very generally gradations, leading to the most complex
instincts, can be discovered. Changes of instinct may sometimes be
facilitated by the same species having different instincts at different
periods of life, or at different seasons of the year, or when placed under
different circumstances, etc.; in which case either the one or the other
instinct might be preserved by natural selection. And such instances of
diversity of instinct in the same species can be shown to occur in nature.
Again, as in the case of corporeal structure, and conformably to my theory,
the instinct of each species is good for itself, but has never, as far as
we can judge, been produced for the exclusive good of others. One of the
strongest instances of an animal apparently performing an action for the
sole good of another, with which I am acquainted, is that of aphides
voluntarily yielding, as was first observed by Huber, their sweet excretion
to ants: that they do so voluntarily, the following facts show. I removed
all the ants from a group of about a dozen aphides on a dock-plant, and
prevented their attendance during several hours. After this interval, I
felt sure that the aphides would want to excrete. I watched them for some
time through a lens, but not one excreted; I then tickled and stroked them
with a hair in the same manner, as well as I could, as the ants do with
their antennae; but not one excreted. Afterwards, I allowed an ant to
visit them, and it immediately seemed, by its eager way of running about to
be well aware what a rich flock it had discovered; it then began to play
with its antennae on the abdomen first of one aphis and then of another;
and each, as soon as it felt the antennae, immediately lifted up its
abdomen and excreted a limpid drop of sweet juice, which was eagerly
devoured by the ant. Even the quite young aphides behaved in this manner,
showing that the action was instinctive, and not the result of experience.
It is certain, from the observations of Huber, that the aphides show no
dislike to the ants: if the latter be not present they are at last
compelled to eject their excretion. But as the excretion is extremely
viscid, it is no doubt a convenience to the aphides to have it removed;
therefore probably they do not excrete solely for the good of the ants.
Although there is no evidence that any animal performs an action for the
exclusive good of another species, yet each tries to take advantage of the
instincts of others, as each takes advantage of the weaker bodily structure
of other species. So again certain instincts cannot be considered as
absolutely perfect; but as details on this and other such points are not
indispensable, they may be here passed over.
As some degree of variation in instincts under a state of nature, and the
inheritance of such variations, are indispensable for the action of natural
selection, as many instances as possible ought to be given; but want of
space prevents me. I can only assert that instincts certainly do vary--for
instance, the migratory instinct, both in extent and direction, and in its
total loss. So it is with the nests of birds, which vary partly in
dependence on the situations chosen, and on the nature and temperature of
the country inhabited, but often from causes wholly unknown to us. Audubon
has given several remarkable cases of differences in the nests of the same
species in the northern and southern United States. Why, it has been
asked, if instinct be variable, has it not granted to the bee "the ability
to use some other material when wax was deficient?" But what other natural
material could bees use? They will work, as I have seen, with wax hardened
with vermilion or softened with lard. Andrew Knight observed that his
bees, instead of laboriously collecting propolis, used a cement of wax and
turpentine, with which he had covered decorticated trees. It has lately
been shown that bees, instead of searching for pollen, will gladly use a
very different substance, namely, oatmeal. Fear of any particular enemy is
certainly an instinctive quality, as may be seen in nestling birds, though
it is strengthened by experience, and by the sight of fear of the same
enemy in other animals. The fear of man is slowly acquired, as I have
elsewhere shown, by the various animals which inhabit desert islands; and
we see an instance of this, even in England, in the greater wildness of all
our large birds in comparison with our small birds; for the large birds
have been most persecuted by man. We may safely attribute the greater
wildness of our large birds to this cause; for in uninhabited islands large
birds are not more fearful than small; and the magpie, so wary in England,
is tame in Norway, as is the hooded crow in Egypt.
That the mental qualities of animals of the same kind, born in a state of
nature, vary much, could be shown by many facts. Several cases could also
be adduced of occasional and strange habits in wild animals, which, if
advantageous to the species, might have given rise, through natural
selection, to new instincts. But I am well aware that these general
statements, without the facts in detail, can produce but a feeble effect on
the reader's mind. I can only repeat my assurance, that I do not speak
without good evidence.
INHERITED CHANGES OF HABIT OR INSTINCT IN DOMESTICATED ANIMALS.
The possibility, or even probability, of inherited variations of instinct
in a state of nature will be strengthened by briefly considering a few
cases under domestication. We shall thus be enabled to see the part which
habit and the selection of so-called spontaneous variations have played in
modifying the mental qualities of our domestic animals. It is notorious
how much domestic animals vary in their mental qualities. With cats, for
instance, one naturally takes to catching rats, and another mice, and these
tendencies are known to be inherited. One cat, according to Mr. St. John,
always brought home game birds, another hares or rabbits, and another
hunted on marshy ground and almost nightly caught woodcocks or snipes. A
number of curious and authentic instances could be given of various shades
of disposition and taste, and likewise of the oddest tricks, associated
with certain frames of mind or periods of time. But let us look to the
familiar case of the breeds of dogs: it cannot be doubted that young
pointers (I have myself seen striking instances) will sometimes point and
even back other dogs the very first time that they are taken out;
retrieving is certainly in some degree inherited by retrievers; and a
tendency to run round, instead of at, a flock of sheep, by shepherd-dogs.
I cannot see that these actions, performed without experience by the young,
and in nearly the same manner by each individual, performed with eager
delight by each breed, and without the end being known--for the young
pointer can no more know that he points to aid his master, than the white
butterfly knows why she lays her eggs on the leaf of the cabbage--I cannot
see that these actions differ essentially from true instincts. If we were
to behold one kind of wolf, when young and without any training, as soon as
it scented its prey, stand motionless like a statue, and then slowly crawl
forward with a peculiar gait; and another kind of wolf rushing round,
instead of at, a herd of deer, and driving them to a distant point, we
should assuredly call these actions instinctive. Domestic instincts, as
they may be called, are certainly far less fixed than natural instincts;
but they have been acted on by far less rigorous selection, and have been
transmitted for an incomparably shorter period, under less fixed conditions
of life.
How strongly these domestic instincts, habits, and dispositions are
inherited, and how curiously they become mingled, is well shown when
different breeds of dogs are crossed. Thus it is known that a cross with a
bull-dog has affected for many generations the courage and obstinacy of
greyhounds; and a cross with a greyhound has given to a whole family of
shepherd-dogs a tendency to hunt hares. These domestic instincts, when
thus tested by crossing, resemble natural instincts, which in a like manner
become curiously blended together, and for a long period exhibit traces of
the instincts of either parent: for example, Le Roy describes a dog, whose
great-grandfather was a wolf, and this dog showed a trace of its wild
parentage only in one way, by not coming in a straight line to his master,
when called.
Domestic instincts are sometimes spoken of as actions which have become
inherited solely from long-continued and compulsory habit, but this is not
true. No one would ever have thought of teaching, or probably could have
taught, the tumbler-pigeon to tumble--an action which, as I have witnessed,
is performed by young birds, that have never seen a pigeon tumble. We may
believe that some one pigeon showed a slight tendency to this strange
habit, and that the long-continued selection of the best individuals in
successive generations made tumblers what they now are; and near Glasgow
there are house-tumblers, as I hear from Mr. Brent, which cannot fly
eighteen inches high without going head over heels. It may be doubted
whether any one would have thought of training a dog to point, had not some
one dog naturally shown a tendency in this line; and this is known
occasionally to happen, as I once saw, in a pure terrier: the act of
pointing is probably, as many have thought, only the exaggerated pause of
an animal preparing to spring on its prey. When the first tendency to
point was once displayed, methodical selection and the inherited effects of
compulsory training in each successive generation would soon complete the
work; and unconscious selection is still in progress, as each man tries to
procure, without intending to improve the breed, dogs which stand and hunt
best. On the other hand, habit alone in some cases has sufficed; hardly
any animal is more difficult to tame than the young of the wild rabbit;
scarcely any animal is tamer than the young of the tame rabbit; but I can
hardly suppose that domestic rabbits have often been selected for tameness
alone; so that we must attribute at least the greater part of the inherited
change from extreme wildness to extreme tameness, to habit and
long-continued close confinement.
Natural instincts are lost under domestication: a remarkable instance of
this is seen in those breeds of fowls which very rarely or never become
"broody," that is, never wish to sit on their eggs. Familiarity alone
prevents our seeing how largely and how permanently the minds of our
domestic animals have been modified. It is scarcely possible to doubt that
the love of man has become instinctive in the dog. All wolves, foxes,
jackals and species of the cat genus, when kept tame, are most eager to
attack poultry, sheep and pigs; and this tendency has been found incurable
in dogs which have been brought home as puppies from countries such as
Tierra del Fuego and Australia, where the savages do not keep these
domestic animals. How rarely, on the other hand, do our civilised dogs,
even when quite young, require to be taught not to attack poultry, sheep,
and pigs! No doubt they occasionally do make an attack, and are then
beaten; and if not cured, they are destroyed; so that habit and some degree
of selection have probably concurred in civilising by inheritance our dogs.
On the other hand, young chickens have lost wholly by habit, that fear of
the dog and cat which no doubt was originally instinctive in them, for I am
informed by Captain Hutton that the young chickens of the parent stock, the
Gallus bankiva, when reared in India under a hen, are at first excessively
wild. So it is with young pheasants reared in England under a hen. It is
not that chickens have lost all fear, but fear only of dogs and cats, for
if the hen gives the danger chuckle they will run (more especially young
turkeys) from under her and conceal themselves in the surrounding grass or
thickets; and this is evidently done for the instinctive purpose of
allowing, as we see in wild ground-birds, their mother to fly away. But
this instinct retained by our chickens has become useless under
domestication, for the mother-hen has almost lost by disuse the power of
flight.
Hence, we may conclude that under domestication instincts have been
acquired and natural instincts have been lost, partly by habit and partly
by man selecting and accumulating, during successive generations, peculiar
mental habits and actions, which at first appeared from what we must in our
ignorance call an accident. In some cases compulsory habit alone has
sufficed to produce inherited mental changes; in other cases compulsory
habit has done nothing, and all has been the result of selection, pursued
both methodically and unconsciously; but in most cases habit and selection
have probably concurred.
SPECIAL INSTINCTS.
We shall, perhaps, best understand how instincts in a state of nature have
become modified by selection by considering a few cases. I will select
only three, namely, the instinct which leads the cuckoo to lay her eggs in
other birds' nests; the slave-making instinct of certain ants; and the
cell-making power of the hive-bee: these two latter instincts have
generally and justly been ranked by naturalists as the most wonderful of
all known instincts.
INSTINCTS OF THE CUCKOO.
It is supposed by some naturalists that the more immediate cause of the
instinct of the cuckoo is that she lays her eggs, not daily, but at
intervals of two or three days; so that, if she were to make her own nest
and sit on her own eggs, those first laid would have to be left for some
time unincubated or there would be eggs and young birds of different ages
in the same nest. If this were the case the process of laying and hatching
might be inconveniently long, more especially as she migrates at a very
early period; and the first hatched young would probably have to be fed by
the male alone. But the American cuckoo is in this predicament, for she
makes her own nest and has eggs and young successively hatched, all at the
same time. It has been both asserted and denied that the American cuckoo
occasionally lays her eggs in other birds' nests; but I have lately heard
from Dr. Merrill, of Iowa, that he once found in Illinois a young cuckoo,
together with a young jay in the nest of a blue jay (Garrulus cristatus);
and as both were nearly full feathered, there could be no mistake in their
identification. I could also give several instances of various birds which
have been known occasionally to lay their eggs in other birds' nests. Now
let us suppose that the ancient progenitor of our European cuckoo had the
habits of the American cuckoo, and that she occasionally laid an egg in
another bird's nest. If the old bird profited by this occasional habit
through being enabled to emigrate earlier or through any other cause; or if
the young were made more vigorous by advantage being taken of the mistaken
instinct of another species than when reared by their own mother,
encumbered as she could hardly fail to be by having eggs and young of
different ages at the same time, then the old birds or the fostered young
would gain an advantage. And analogy would lead us to believe that the
young thus reared would be apt to follow by inheritance the occasional and
aberrant habit of their mother, and in their turn would be apt to lay their
eggs in other birds' nests, and thus be more successful in rearing their
young. By a continued process of this nature, I believe that the strange
instinct of our cuckoo has been generated. It has, also recently been
ascertained on sufficient evidence, by Adolf Muller, that the cuckoo
occasionally lays her eggs on the bare ground, sits on them and feeds her
young. This rare event is probably a case of reversion to the long-lost,
aboriginal instinct of nidification.
It has been objected that I have not noticed other related instincts and
adaptations of structure in the cuckoo, which are spoken of as necessarily
co-ordinated. But in all cases, speculation on an instinct known to us
only in a single species, is useless, for we have hitherto had no facts to
guide us. Until recently the instincts of the European and of the non-
parasitic American cuckoo alone were known; now, owing to Mr. Ramsay's
observations, we have learned something about three Australian species,
which lay their eggs in other birds' nests. The chief points to be
referred to are three: first, that the common cuckoo, with rare
exceptions, lays only one egg in a nest, so that the large and voracious
young bird receives ample food. Secondly, that the eggs are remarkably
small, not exceeding those of the skylark--a bird about one-fourth as large
as the cuckoo. That the small size of the egg is a real case of adaptation
we may infer from the fact of the mon-parasitic American cuckoo laying
full-sized eggs. Thirdly, that the young cuckoo, soon after birth, has the
instinct, the strength and a properly shaped back for ejecting its foster-
brothers, which then perish from cold and hunger. This has been boldly
called a beneficent arrangement, in order that the young cuckoo may get
sufficient food, and that its foster-brothers may perish before they had
acquired much feeling!
Turning now to the Australian species: though these birds generally lay
only one egg in a nest, it is not rare to find two and even three eggs in
the same nest. In the bronze cuckoo the eggs vary greatly in size, from
eight to ten lines in length. Now, if it had been of an advantage to this
species to have laid eggs even smaller than those now laid, so as to have
deceived certain foster-parents, or, as is more probable, to have been
hatched within a shorter period (for it is asserted that there is a
relation between the size of eggs and the period of their incubation), then
there is no difficulty in believing that a race or species might have been
formed which would have laid smaller and smaller eggs; for these would have
been more safely hatched and reared. Mr. Ramsay remarks that two of the
Australian cuckoos, when they lay their eggs in an open nest, manifest a
decided preference for nests containing eggs similar in colour to their
own. The European species apparently manifests some tendency towards a
similar instinct, but not rarely departs from it, as is shown by her laying
her dull and pale-coloured eggs in the nest of the hedge-warbler with
bright greenish-blue eggs. Had our cuckoo invariably displayed the above
instinct, it would assuredly have been added to those which it is assumed
must all have been acquired together. The eggs of the Australian bronze
cuckoo vary, according to Mr. Ramsay, to an extraordinary degree in colour;
so that in this respect, as well as in size, natural selection might have
secured and fixed any advantageous variation.
In the case of the European cuckoo, the offspring of the foster-parents are
commonly ejected from the nest within three days after the cuckoo is
hatched; and as the latter at this age is in a most helpless condition, Mr.
Gould was formerly inclined to believe that the act of ejection was
performed by the foster-parents themselves. But he has now received a
trustworthy account of a young cuckoo which was actually seen, while still
blind and not able even to hold up its own head, in the act of ejecting its
foster-brothers. One of these was replaced in the nest by the observer,
and was again thrown out. With respect to the means by which this strange
and odious instinct was acquired, if it were of great importance for the
young cuckoo, as is probably the case, to receive as much food as possible
soon after birth, I can see no special difficulty in its having gradually
acquired, during successive generations, the blind desire, the strength,
and structure necessary for the work of ejection; for those cuckoos which
had such habits and structure best developed would be the most securely
reared. The first step towards the acquisition of the proper instinct
might have been mere unintentional restlessness on the part of the young
bird, when somewhat advanced in age and strength; the habit having been
afterwards improved, and transmitted to an earlier age. I can see no more
difficulty in this than in the unhatched young of other birds acquiring the
instinct to break through their own shells; or than in young snakes
acquiring in their upper jaws, as Owen has remarked, a transitory sharp
tooth for cutting through the tough egg-shell. For if each part is liable
to individual variations at all ages, and the variations tend to be
inherited at a corresponding or earlier age--propositions which cannot be
disputed--then the instincts and structure of the young could be slowly
modified as surely as those of the adult; and both cases must stand or fall
together with the whole theory of natural selection.
Some species of Molothrus, a widely distinct genus of American birds,
allied to our starlings, have parasitic habits like those of the cuckoo;
and the species present an interesting gradation in the perfection of their
instincts. The sexes of Molothrus badius are stated by an excellent
observer, Mr. Hudson, sometimes to live promiscuously together in flocks,
and sometimes to pair. They either build a nest of their own or seize on
one belonging to some other bird, occasionally throwing out the nestlings
of the stranger. They either lay their eggs in the nest thus appropriated,
or oddly enough build one for themselves on the top of it. They usually
sit on their own eggs and rear their own young; but Mr. Hudson says it is
probable that they are occasionally parasitic, for he has seen the young of
this species following old birds of a distinct kind and clamouring to be
fed by them. The parasitic habits of another species of Molothrus, the M.
bonariensis, are much more highly developed than those of the last, but are
still far from perfect. This bird, as far as it is known, invariably lays
its eggs in the nests of strangers; but it is remarkable that several
together sometimes commence to build an irregular untidy nest of their own,
placed in singular ill-adapted situations, as on the leaves of a large
thistle. They never, however, as far as Mr. Hudson has ascertained,
complete a nest for themselves. They often lay so many eggs--from fifteen
to twenty--in the same foster-nest, that few or none can possibly be
hatched. They have, moreover, the extraordinary habit of pecking holes in
the eggs, whether of their own species or of their foster parents, which
they find in the appropriated nests. They drop also many eggs on the bare
ground, which are thus wasted. A third species, the M. pecoris of North
America, has acquired instincts as perfect as those of the cuckoo, for it
never lays more than one egg in a foster-nest, so that the young bird is
securely reared. Mr. Hudson is a strong disbeliever in evolution, but he
appears to have been so much struck by the imperfect instincts of the
Molothrus bonariensis that he quotes my words, and asks, "Must we consider
these habits, not as especially endowed or created instincts, but as small
consequences of one general law, namely, transition?"
Various birds, as has already been remarked, occasionally lay their eggs in
the nests of other birds. This habit is not very uncommon with the
Gallinaceae, and throws some light on the singular instinct of the ostrich.
In this family several hen birds unite and lay first a few eggs in one nest
and then in another; and these are hatched by the males. This instinct may
probably be accounted for by the fact of the hens laying a large number of
eggs, but, as with the cuckoo, at intervals of two or three days. The
instinct, however, of the American ostrich, as in the case of the Molothrus
bonariensis, has not as yet been perfected; for a surprising number of eggs
lie strewed over the plains, so that in one day's hunting I picked up no
less than twenty lost and wasted eggs.
Many bees are parasitic, and regularly lay their eggs in the nests of other
kinds of bees. This case is more remarkable than that of the cuckoo; for
these bees have not only had their instincts but their structure modified
in accordance with their parasitic habits; for they do not possess the
pollen-collecting apparatus which would have been indispensable if they had
stored up food for their own young. Some species of Sphegidae (wasp-like
insects) are likewise parasitic; and M. Fabre has lately shown good reason
for believing that, although the Tachytes nigra generally makes its own
burrow and stores it with paralysed prey for its own larvae, yet that, when
this insect finds a burrow already made and stored by another sphex, it
takes advantage of the prize, and becomes for the occasion parasitic. In
this case, as with that of the Molothrus or cuckoo, I can see no difficulty
in natural selection making an occasional habit permanent, if of advantage
to the species, and if the insect whose nest and stored food are
feloniously appropriated, be not thus exterminated.
SLAVE-MAKING INSTINCT.
This remarkable instinct was first discovered in the Formica (Polyerges)
rufescens by Pierre Huber, a better observer even than his celebrated
father. This ant is absolutely dependent on its slaves; without their aid,
the species would certainly become extinct in a single year. The males and
fertile females do no work of any kind, and the workers or sterile females,
though most energetic and courageous in capturing slaves, do no other work.
They are incapable of making their own nests, or of feeding their own
larvae. When the old nest is found inconvenient, and they have to migrate,
it is the slaves which determine the migration, and actually carry their
masters in their jaws. So utterly helpless are the masters, that when
Huber shut up thirty of them without a slave, but with plenty of the food
which they like best, and with their larvae and pupae to stimulate them to
work, they did nothing; they could not even feed themselves, and many
perished of hunger. Huber then introduced a single slave (F. fusca), and
she instantly set to work, fed and saved the survivors; made some cells and
tended the larvae, and put all to rights. What can be more extraordinary
than these well-ascertained facts? If we had not known of any other
slave-making ant, it would have been hopeless to speculate how so wonderful
an instinct could have been perfected.
Another species, Formica sanguinea, was likewise first discovered by P.
Huber to be a slave-making ant. This species is found in the southern
parts of England, and its habits have been attended to by Mr. F. Smith, of
the British Museum, to whom I am much indebted for information on this and
other subjects. Although fully trusting to the statements of Huber and Mr.
Smith, I tried to approach the subject in a sceptical frame of mind, as any
one may well be excused for doubting the existence of so extraordinary an
instinct as that of making slaves. Hence, I will give the observations
which I made in some little detail. I opened fourteen nests of F.
sanguinea, and found a few slaves in all. Males and fertile females of the
slave-species (F. fusca) are found only in their own proper communities,
and have never been observed in the nests of F. sanguinea. The slaves are
black and not above half the size of their red masters, so that the
contrast in their appearance is great. When the nest is slightly
disturbed, the slaves occasionally come out, and like their masters are
much agitated and defend the nest: when the nest is much disturbed, and
the larvae and pupae are exposed, the slaves work energetically together
with their masters in carrying them away to a place of safety. Hence, it
is clear that the slaves feel quite at home. During the months of June and
July, on three successive years, I watched for many hours several nests in
Surrey and Sussex, and never saw a slave either leave or enter a nest. As,
during these months, the slaves are very few in number, I thought that they
might behave differently when more numerous; but Mr. Smith informs me that
he has watched the nests at various hours during May, June and August, both
in Surrey and Hampshire, and has never seen the slaves, though present in
large numbers in August, either leave or enter the nest. Hence, he
considers them as strictly household slaves. The masters, on the other
hand, may be constantly seen bringing in materials for the nest, and food
of all kinds. During the year 1860, however, in the month of July, I came
across a community with an unusually large stock of slaves, and I observed
a few slaves mingled with their masters leaving the nest, and marching
along the same road to a tall Scotch-fir tree, twenty-five yards distant,
which they ascended together, probably in search of aphides or cocci.
According to Huber, who had ample opportunities for observation, the slaves
in Switzerland habitually work with their masters in making the nest, and
they alone open and close the doors in the morning and evening; and, as
Huber expressly states, their principal office is to search for aphides.
This difference in the usual habits of the masters and slaves in the two
countries, probably depends merely on the slaves being captured in greater
numbers in Switzerland than in England.
One day I fortunately witnessed a migration of F. sanguinea from one nest
to another, and it was a most interesting spectacle to behold the masters
carefully carrying their slaves in their jaws instead of being carried by
them, as in the case of F. rufescens. Another day my attention was struck
by about a score of the slave-makers haunting the same spot, and evidently
not in search of food; they approached and were vigorously repulsed by an
independent community of the slave species (F. fusca); sometimes as many as
three of these ants clinging to the legs of the slave-making F. sanguinea.
The latter ruthlessly killed their small opponents and carried their dead
bodies as food to their nest, twenty-nine yards distant; but they were
prevented from getting any pupae to rear as slaves. I then dug up a small
parcel of the pupae of F. fusca from another nest, and put them down on a
bare spot near the place of combat; they were eagerly seized and carried
off by the tyrants, who perhaps fancied that, after all, they had been
victorious in their late combat.
At the same time I laid on the same place a small parcel of the pupae of
another species, F. flava, with a few of these little yellow ants still
clinging to the fragments of their nest. This species is sometimes, though
rarely, made into slaves, as has been described by Mr. Smith. Although so
small a species, it is very courageous, and I have seen it ferociously
attack other ants. In one instance I found to my surprise an independent
community of F. flava under a stone beneath a nest of the slave-making F.
sanguinea; and when I had accidentally disturbed both nests, the little
ants attacked their big neighbours with surprising courage. Now I was
curious to ascertain whether F. sanguinea could distinguish the pupae of F.
fusca, which they habitually make into slaves, from those of the little and
furious F. flava, which they rarely capture, and it was evident that they
did at once distinguish them; for we have seen that they eagerly and
instantly seized the pupae of F. fusca, whereas they were much terrified
when they came across the pupae, or even the earth from the nest, of F.
flava, and quickly ran away; but in about a quarter of an hour, shortly
after all the little yellow ants had crawled away, they took heart and
carried off the pupae.
One evening I visited another community of F. sanguinea, and found a number
of these ants returning home and entering their nests, carrying the dead
bodies of F. fusca (showing that it was not a migration) and numerous
pupae. I traced a long file of ants burdened with booty, for about forty
yards back, to a very thick clump of heath, whence I saw the last
individual of F. sanguinea emerge, carrying a pupa; but I was not able to
find the desolated nest in the thick heath. The nest, however, must have
been close at hand, for two or three individuals of F. fusca were rushing
about in the greatest agitation, and one was perched motionless with its
own pupa in its mouth on the top of a spray of heath, an image of despair
over its ravaged home.
Such are the facts, though they did not need confirmation by me, in regard
to the wonderful instinct of making slaves. Let it be observed what a
contrast the instinctive habits of F. sanguinea present with those of the
continental F. rufescens. The latter does not build its own nest, does not
determine its own migrations, does not collect food for itself or its
young, and cannot even feed itself: it is absolutely dependent on its
numerous slaves. Formica sanguinea, on the other hand, possesses much
fewer slaves, and in the early part of the summer extremely few. The
masters determine when and where a new nest shall be formed, and when they
migrate, the masters carry the slaves. Both in Switzerland and England the
slaves seem to have the exclusive care of the larvae, and the masters alone
go on slave-making expeditions. In Switzerland the slaves and masters work
together, making and bringing materials for the nest: both, but chiefly
the slaves, tend and milk as it may be called, their aphides; and thus both
collect food for the community. In England the masters alone usually leave
the nest to collect building materials and food for themselves, their
slaves and larvae. So that the masters in this country receive much less
service from their slaves than they do in Switzerland.
By what steps the instinct of F. sanguinea originated I will not pretend to
conjecture. But as ants which are not slave-makers, will, as I have seen,
carry off pupae of other species, if scattered near their nests, it is
possible that such pupae originally stored as food might become developed;
and the foreign ants thus unintentionally reared would then follow their
proper instincts, and do what work they could. If their presence proved
useful to the species which had seized them--if it were more advantageous
to this species, to capture workers than to procreate them--the habit of
collecting pupae, originally for food, might by natural selection be
strengthened and rendered permanent for the very different purpose of
raising slaves. When the instinct was once acquired, if carried out to a
much less extent even than in our British F. sanguinea, which, as we have
seen, is less aided by its slaves than the same species in Switzerland,
natural selection might increase and modify the instinct--always supposing
each modification to be of use to the species--until an ant was formed as
abjectly dependent on its slaves as is the Formica rufescens.
CELL-MAKING INSTINCT OF THE HIVE-BEE.
I will not here enter on minute details on this subject, but will merely
give an outline of the conclusions at which I have arrived. He must be a
dull man who can examine the exquisite structure of a comb, so beautifully
adapted to its end, without enthusiastic admiration. We hear from
mathematicians that bees have practically solved a recondite problem, and
have made their cells of the proper shape to hold the greatest possible
amount of honey, with the least possible consumption of precious wax in
their construction. It has been remarked that a skilful workman, with
fitting tools and measures, would find it very difficult to make cells of
wax of the true form, though this is effected by a crowd of bees working in
a dark hive. Granting whatever instincts you please, it seems at first
quite inconceivable how they can make all the necessary angles and planes,
or even perceive when they are correctly made. But the difficulty is not
nearly so great as at first appears: all this beautiful work can be shown,
I think, to follow from a few simple instincts.
I was led to investigate this subject by Mr. Waterhouse, who has shown that
the form of the cell stands in close relation to the presence of adjoining
cells; and the following view may, perhaps, be considered only as a
modification of his theory. Let us look to the great principle of
gradation, and see whether Nature does not reveal to us her method of work.
At one end of a short series we have humble-bees, which use their old
cocoons to hold honey, sometimes adding to them short tubes of wax, and
likewise making separate and very irregular rounded cells of wax. At the
other end of the series we have the cells of the hive-bee, placed in a
double layer: each cell, as is well known, is an hexagonal prism, with the
basal edges of its six sides bevelled so as to join an inverted pyramid, of
three rhombs. These rhombs have certain angles, and the three which form
the pyramidal base of a single cell on one side of the comb, enter into the
composition of the bases of three adjoining cells on the opposite side. In
the series between the extreme perfection of the cells of the hive-bee and
the simplicity of those of the humble-bee, we have the cells of the Mexican
Melipona domestica, carefully described and figured by Pierre Huber. The
Melipona itself is intermediate in structure between the hive and humble
bee, but more nearly related to the latter: it forms a nearly regular
waxen comb of cylindrical cells, in which the young are hatched, and, in
addition, some large cells of wax for holding honey. These latter cells
are nearly spherical and of nearly equal sizes, and are aggregated into an
irregular mass. But the important point to notice is, that these cells are
always made at that degree of nearness to each other that they would have
intersected or broken into each other if the spheres had been completed;
but this is never permitted, the bees building perfectly flat walls of wax
between the spheres which thus tend to intersect. Hence, each cell
consists of an outer spherical portion, and of two, three, or more flat
surfaces, according as the cell adjoins two, three or more other cells.
When one cell rests on three other cells, which, from the spheres being
nearly of the same size, is very frequently and necessarily the case, the
three flat surfaces are united into a pyramid; and this pyramid, as Huber
has remarked, is manifestly a gross imitation of the three-sided pyramidal
base of the cell of the hive-bee. As in the cells of the hive-bee, so
here, the three plane surfaces in any one cell necessarily enter into the
construction of three adjoining cells. It is obvious that the Melipona
saves wax, and what is more important, labour, by this manner of building;
for the flat walls between the adjoining cells are not double, but are of
the same thickness as the outer spherical portions, and yet each flat
portion forms a part of two cells.
Reflecting on this case, it occurred to me that if the Melipona had made
its spheres at some given distance from each other, and had made them of
equal sizes and had arranged them symmetrically in a double layer, the
resulting structure would have been as perfect as the comb of the hive-bee.
Accordingly I wrote to Professor Miller, of Cambridge, and this geometer
has kindly read over the following statement, drawn up from his
information, and tells me that it is strictly correct:-
If a number of equal spheres be described with their centres placed in two
parallel layers; with the centre of each sphere at the distance of radius x
sqrt(2) or radius x 1.41421 (or at some lesser distance), from the centres
of the six surrounding spheres in the same layer; and at the same distance
from the centres of the adjoining spheres in the other and parallel layer;
then, if planes of intersection between the several spheres in both layers
be formed, there will result a double layer of hexagonal prisms united
together by pyramidal bases formed of three rhombs; and the rhombs and the
sides of the hexagonal prisms will have every angle identically the same
with the best measurements which have been made of the cells of the
hive-bee. But I hear from Professor Wyman, who has made numerous careful
measurements, that the accuracy of the workmanship of the bee has been
greatly exaggerated; so much so, that whatever the typical form of the cell
may be, it is rarely, if ever, realised.
Hence we may safely conclude that, if we could slightly modify the
instincts already possessed by the Melipona, and in themselves not very
wonderful, this bee would make a structure as wonderfully perfect as that
of the hive-bee. We must suppose the Melipona to have the power of forming
her cells truly spherical, and of equal sizes; and this would not be very
surprising, seeing that she already does so to a certain extent, and seeing
what perfectly cylindrical burrows many insects make in wood, apparently by
turning round on a fixed point. We must suppose the Melipona to arrange
her cells in level layers, as she already does her cylindrical cells; and
we must further suppose, and this is the greatest difficulty, that she can
somehow judge accurately at what distance to stand from her
fellow-labourers when several are making their spheres; but she is already
so far enabled to judge of distance, that she always describes her spheres
so as to intersect to a certain extent; and then she unites the points of
intersection by perfectly flat surfaces. By such modifications of
instincts which in themselves are not very wonderful--hardly more wonderful
than those which guide a bird to make its nest--I believe that the hive-bee
has acquired, through natural selection, her inimitable architectural
powers.
But this theory can be tested by experiment. Following the example of Mr.
Tegetmeier, I separated two combs, and put between them a long, thick,
rectangular strip of wax: the bees instantly began to excavate minute
circular pits in it; and as they deepened these little pits, they made them
wider and wider until they were converted into shallow basins, appearing to
the eye perfectly true or parts of a sphere, and of about the diameter of a
cell. It was most interesting to observe that, wherever several bees had
begun to excavate these basins near together, they had begun their work at
such a distance from each other that by the time the basins had acquired
the above stated width (i.e. about the width of an ordinary cell), and were
in depth about one sixth of the diameter of the sphere of which they formed
a part, the rims of the basins intersected or broke into each other. As
soon as this occurred, the bees ceased to excavate, and began to build up
flat walls of wax on the lines of intersection between the basins, so that
each hexagonal prism was built upon the scalloped edge of a smooth basin,
instead of on the straight edges of a three-sided pyramid as in the case of
ordinary cells.
I then put into the hive, instead of a thick, rectangular piece of wax, a
thin and narrow, knife-edged ridge, coloured with vermilion. The bees
instantly began on both sides to excavate little basins near to each other,
in the same way as before; but the ridge of wax was so thin, that the
bottoms of the basins, if they had been excavated to the same depth as in
the former experiment, would have broken into each other from the opposite
sides. The bees, however, did not suffer this to happen, and they stopped
their excavations in due time; so that the basins, as soon as they had been
a little deepened, came to have flat bases; and these flat bases, formed by
thin little plates of the vermilion wax left ungnawed, were situated, as
far as the eye could judge, exactly along the planes of imaginary
intersection between the basins on the opposite side of the ridge of wax.
In some parts, only small portions, in other parts, large portions of a
rhombic plate were thus left between the opposed basins, but the work, from
the unnatural state of things, had not been neatly performed. The bees
must have worked at very nearly the same rate in circularly gnawing away
and deepening the basins on both sides of the ridge of vermilion wax, in
order to have thus succeeded in leaving flat plates between the basins, by
stopping work at the planes of intersection.
Considering how flexible thin wax is, I do not see that there is any
difficulty in the bees, whilst at work on the two sides of a strip of wax,
perceiving when they have gnawed the wax away to the proper thinness, and
then stopping their work. In ordinary combs it has appeared to me that the
bees do not always succeed in working at exactly the same rate from the
opposite sides; for I have noticed half-completed rhombs at the base of a
just-commenced cell, which were slightly concave on one side, where I
suppose that the bees had excavated too quickly, and convex on the opposed
side where the bees had worked less quickly. In one well-marked instance,
I put the comb back into the hive, and allowed the bees to go on working
for a short time, and again examined the cell, and I found that the rhombic
plate had been completed, and had become PERFECTLY FLAT: it was absolutely
impossible, from the extreme thinness of the little plate, that they could
have effected this by gnawing away the convex side; and I suspect that the
bees in such cases stand in the opposed cells and push and bend the ductile
and warm wax (which as I have tried is easily done) into its proper
intermediate plane, and thus flatten it.
>From the experiment of the ridge of vermilion wax we can see that, if the
bees were to build for themselves a thin wall of wax, they could make their
cells of the proper shape, by standing at the proper distance from each
other, by excavating at the same rate, and by endeavouring to make equal
spherical hollows, but never allowing the spheres to break into each other.
Now bees, as may be clearly seen by examining the edge of a growing comb,
do make a rough, circumferential wall or rim all round the comb; and they
gnaw this away from the opposite sides, always working circularly as they
deepen each cell. They do not make the whole three-sided pyramidal base of
any one cell at the same time, but only that one rhombic plate which stands
on the extreme growing margin, or the two plates, as the case may be; and
they never complete the upper edges of the rhombic plates, until the
hexagonal walls are commenced. Some of these statements differ from those
made by the justly celebrated elder Huber, but I am convinced of their
accuracy; and if I had space, I could show that they are conformable with
my theory.
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 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. 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, 1/352 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 1/229 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 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
it 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 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 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
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: cells constructed
like those of the bee or the wasp gain in strength, and save much in labour
and space, and in the materials of which they are constructed. With
respect to the formation 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 experimentally proved that from twelve to fifteen pounds of dry
sugar are consumed by a hive of bees for the secretion of a 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, and the
time consumed in collecting the honey, must be an important element of
success any family of bees. Of course the success of the species may be
dependent on the number of its enemies, or parasites, or on quite distinct
causes, and so be altogether 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 community 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 of 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 labour 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 labour 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 disappear 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 economising labour 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
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 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 labour and wax; that
individual swarm which thus made the best cells with least labour, and
least waste of honey in the secretion of wax, having succeeded best, and
having transmitted their newly-acquired economical instincts to new swarms,
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 without
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 nuthatch, 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
accordance 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 nuthatch. 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
instincts 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 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 progenitor, and consequently must believe that they were
independently acquired 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
profitable to the community that a number should have been annually born
capable of work, but incapable of procreation, I can see no especial
difficulty 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 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 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 unhesitatingly 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
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 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 bulls 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.
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 carefully 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 carefully 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 applied to the family, and not to the
individual, for the sake of gaining a serviceable end. Hence, we may
conclude that slight modifications of structure 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 offspring a tendency to produce sterile
members with the same modifications. This process must have been repeated
many times, 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 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 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 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
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
communities with females which produced most neuters having the
advantageous 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 carefully examined. Mr. F. Smith has shown that the neuters of
several British ants differ surprisingly from each other in size and
sometimes in colour; 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 rudimentary 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 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
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 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 survival 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 Muller, of certain
Brazilian 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 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
ants, on the same principle that the division of labour is useful to
civilised man. Ants, however, work by inherited instincts and by inherited
organs or tools, while man works by acquired knowledge and manufactured
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
insects 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 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 may be effected by the accumulation of numerous,
slight, spontaneous variations, which are in any way profitable, without
exercise or habit having been brought into play. For peculiar habits,
confined to the workers of sterile females, however long they might be
followed, could not possibly affect the males and fertile females, which
alone leave descendants. I am surprised that no one has advanced this
demonstrative case of neuter insects, against the well-known doctrine of
inherited habit, as advanced by Lamarck.
SUMMARY.
I have endeavoured in this chapter briefly 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, there is no real
difficulty, under changing conditions of life, in natural selection
accumulating to any extent slight modifications of instinct which are in
any way useful. In many cases habit or use and disuse have probably come
Back to Full Books
|