of the Yucca-moth. _op_, its sheath. _sp_, its apex. _op_^1, the
protruded oviduct. _B_, two ovaries of the Yucca, showing the holes by
which the young moths escape, and (_r_) a caterpillar in the interior.
_C_, head of the female moth, with the sickle-shaped process (_si_) on
the maxillary palps for sweeping off the pollen and rolling it into a
ball. _mx_^1, the proboscis. _au_, eye. _p_^1 base of first leg. _D_,
longitudinal section through an ovary of the Yucca, soon after the
laying of two eggs (_ei_). _stk_, the canal made by the ovipositor.]
Here, then, we find an adaptation of certain parts of the moth's body
in relation to the fertilization of the flower, but in this case it is
as much in the interest of the moth as of the plant. By carrying the
pollen to the stigma the moths secure the development of the ovules,
which serve their offspring as food, so that we have here to do with
a peculiar form of care for offspring, which is not more remarkable
than many other kinds of brood-care in insects, such as ants, bees,
Sphex-wasps, ichneumon-flies, and gall-flies.
It might be objected that this case of the Yucca is not so much one
of effecting fertilization as of parasitism; but the eggs, which are
laid in the seed-pods, are very few, and the caterpillars which emerge
from them only devour a very small proportion of the seeds, of which
there may be about 200 (Fig. 52, _B_). Thus the plants also derive
an advantage from the moth's procedure, for quite enough seeds are
left. The form and position of the stamens and of the stigma seem to
be as exactly adapted to the visits of the moth as the moth is to the
transference of the pollen, for the Yucca can only be fertilized by
this one moth, and sets no seed if the moth be absent. For this reason
the species of Yucca cultivated in Europe remain sterile.
Thus the apparent contradiction is explained, and the facts everywhere
support the hypothesis that the adaptations between flowers and insects
depend upon processes of selection.
This origin is incontrovertibly proved, it seems to me, in another way,
namely, by the merely _relative_ perfection of the adaptations, or
rather, by their relative imperfection.
I have already pointed out that all adaptations which depend upon
natural selection can only be relatively perfect, as follows from the
nature of their efficient causes, for natural selection only operates
as long as a further increase of the character concerned would be of
advantage to the existence of the species. It cannot be operative
beyond this point, because the existence of the species cannot be more
perfectly secured in this direction, or, to speak more precisely,
because further variations in the direction hitherto followed would no
longer be improvements, even though they might appear so to us.
Thus the corolla of many flowers is suited to the thick, hairy head
and thorax of the bee, for to these only does the pollen adhere in
sufficient quantity to fertilize the next flower; yet the same flowers
are frequently visited by butterflies, and in many of them there
has been no adaptation to prevent these useless visits. Obviously
this is because preventive arrangements could only begin, according
to our theory, when they were necessary to the preservation of the
species; in this case, therefore, only when the pillaging visits
of the butterflies withdrew so many flowers from the influence of
the effective pollinating visitor, the bee, that too few seeds were
formed, and the survival of the species was threatened by the continual
dwindling of the normal number. As long as the bees visit the flowers
frequently enough to ensure the formation of the necessary number of
seeds a process of selection could not set in; but should the bees
find, for instance, that nearly all the flowers had been robbed of
their nectar, and should therefore visit them less diligently, then
every variation of the flower which made honey less accessible to the
butterflies would become the objective of a process of selection.
Everywhere we find similar imperfections of adaptation which indicate
that they must depend on processes of selection. Thus numerous flowers
are visited by insects other than those which pollinate them, and these
bring them no advantage, but merely rob them of nectar and pollen;
the most beautiful contrivances of many flowers, such as _Glycinia_,
which are directed towards cross-fertilization by bees, are rendered
of no effect because wood-bees and humble-bees bite holes into the
nectaries from the outside, and so reach the nectar by the shortest
way. I do not know whether bees in the native land of the _Glycinia_
do the same thing, but in any case they can do no sensible injury to
the species, since otherwise processes of selection would have set in
which would have prevented the damage in some way or other, whether by
the production of stinging-hairs, or hairs with a burning secretion, or
in some other way. If the actual constitution of the plant made this
impossible, the species would become less abundant and would gradually
die out.
Thus the relative imperfection of the flower-adaptations, which in
general are so worthy of admiration, affords a further indication that
their origin is due to processes of selection.
ADDITIONAL NOTE TO CHAPTER X.
It has been remarked that the chapter on the Origin of Flowers in the
German Edition contains no discussion and refutation of the objections
which have up till recently been urged against the theory of flowers
propounded by Darwin and Hermann Müller. I admit that this chapter
seemed to be so harmonious and so well rounded, and at the same time so
convincing as to the reality of the processes of selection, that the
feeble objections to it, and the attempts of opponents to find another
explanation of the phenomena, might well be disregarded in this book.
However, the most important of these objections and counter-theories
may here be briefly mentioned.
Plateau in Ghent was the first to collect _facts_ which appeared to
contradict the Darwinian theory of flowers; he observed that insects
avoided _artificial_ flowers, even when they were indistinguishable
in colour from natural ones as far as our eyes could perceive, and he
concluded from this that it is not the colour which guides the insects
to the flowers, that they find the blossoms less by their sense of
sight than by their sense of smell. But great caution is required
in drawing conclusions from experiments of this kind. I once placed
artificial marguerites (_Chrysanthemum leucanthemum_) among natural
ones in a roomy frame in the open air, and for a considerable time I
was unable to see any of the numerous butterflies (_Vanessa urticæ_),
which were flying about the real chrysanthemums, settle on one of the
artificial flowers. The insects often flew quite close to them without
paying them the least attention, and I was inclined to conclude that
they either perceived the difference at sight, or that they missed the
odour of the natural flowers in the artificial ones. But in the course
of a few days it happened twice in my presence that a butterfly settled
on one of the artificial blooms and _persistently groped about with
fully outstretched tube to find the entrance to the honey_. It was only
after prolonged futile attempts that it desisted and flew away. That
bees are guided by the eye in their visits to flowers has been shown by
A. Forel, who cut off the whole proboscis, together with the antennæ,
from humble-bees which were swarming eagerly about the flowers. He thus
robbed them of the whole apparatus of smell, and nevertheless they flew
down from a considerable height direct to the same flowers. An English
observer, Mr. G. N. Bulman, has been led to believe, with Plateau, that
it is a matter of entire indifference to the bees whether the flowers
are blue, or red, or simply green in colour, if only they contain
honey, and that therefore the bees could have played no part in the
development of blue flowers, as Hermann Müller assumed they had, and
that they could have no preference for blue or any other colour, as Sir
John Lubbock and others had concluded from their experiments. This is
correct in so far that bees feed as eagerly on the greenish blossoms of
the lime-tree as they do on the deep-blue gentian of the Alpine meadows
or the red blossoms of the Weigelia, the dog-roses of our gardens or
the yellow buttercups (_Ranunculus_) of our meadows; they despise
nothing that yields them honey. But it certainly does not follow
from this that the bees may not, under certain circumstances, have
exercised a selecting influence upon the fixation and intensification
of a new colour-variety of a flower. This is less a question of a
_colour-preference_, in the human sense, on the part of the bees
than of the _greater visibility_ of the colour in question in the
environment peculiar to the flower, and of the amount of rivalry the
bees meet with from other insects in regard to the same flower. In
individual cases this would be difficult to demonstrate, especially
since we can form only an approximate idea of the insect's power of
seeing colour, and cannot judge what the colours of the individual
blossoms count for in the mosaic picture of a flowery meadow. Yet this
is the important point, for, as soon as the bees perceive one colour
more readily than another, the preponderance of this colour-variety
over other variations is assured, since it will be more frequently
visited. In the same way we cannot guess in individual cases why one
species of flower should exhale perfume while a nearly related species
does not. But when we remember that many flowers adapted for the visits
of dipterous insects possess a nauseous carrion-like smell, by means
of which they not only attract flies but scare off other insects, we
can readily imagine cases in which it was of importance to a flower
to be able to be easily found by bees without betraying itself by its
pleasant fragrance to other less desirable visitors.
Thus, therefore, we can understand the odourless but intensely blue
species of gentian, if we may assume that its blue colour is more
visible to bees than to other insects. If I were to elaborate in
detail all the principles which here suggest themselves to me I should
require to write a complete section, and I am unwilling to do this
until I can bring forward a much larger number of new observations
than I am at present in a position to do. All I wish to do here is
to exhort doubters to modesty, and to remind them that these matters
are exceedingly complex, and that we should be glad and grateful that
expert observers like Darwin and Hermann Müller have given us some
insight into the principles interconnecting the facts, instead of
imagining whenever we meet with some little apparently contradictory
fact, which may indeed be quite correct in itself, that the whole
theory of the development of flowers through insects has been
overthrown. Let us rather endeavour to understand such facts, and to
arrange them in their places as stones of the new building.
Often the contradiction is merely the result of the imperfect
theoretical conceptions of its discoverer, as we have already shown in
regard to Nägeli. Bulman, too, fancies he has proved that bees do not
distinguish between the different varieties of a flower, but visit them
indiscriminately with the same eagerness, thus causing intercrossing
of all the varieties, and preventing any one from becoming dominant.
But are the varieties which we plant side by side in our gardens of the
kind that are evolved by bees? That is to say, are their _differences
such as will turn the scale for or against the visits of the bees_?
If one were less, another more easily seen by the bees; or if one
were more fragrant, or had a fragrance more agreeable to bees than
the other, the result of the experiment would probably have been very
different.
One more objection has been made. It is said that the bees, although
exclusively restricted, both themselves and their descendants, to
a diet of flowers, are not so constant _to a particular flower_ as
the theory requires. They do indeed exhibit a 'considerable amount
of constancy,' and often visit a large number of flowers of the same
species in succession, but the theory requires that they should
not only confine themselves to this one species, but to a _single
variety_ of this species. These views show that their authors have not
penetrated far towards an understanding of the nature of selection.
Nature does not operate with individual flowers, but with millions and
myriads of them, and not with the flowers of a single spring, but with
those of hundreds and thousands of years. How often a particular bee
may carry pollen uselessly to a strange flower without thereby lowering
the aggregate of seeds so far that the existence of the species seems
imperilled, or how often she may fertilize the pistil of a useful
variation with the pollen of the parent species, without interrupting
or hindering the process of the evolution of the variety, no mortal
can calculate, and what the theory requires can only be formulated in
this way: The constancy of the bees in their visits to the flowers must
be so great that, on an average, the quantity of seeds will be formed
which suffices for the preservation of the species. And in regard
to the transformation of a species, the attraction which the useful
variety has for the bees must, on an average, be _somewhat stronger_
than that of the parent species. As soon as this is the case the seeds
of the variety will be formed in preponderant numbers, although they
may not all be quite pure from the first, and by degrees, in the course
of generations, the plants of the new variety will preponderate more
and more over those of the parent form, and finally will alone remain.
In the first case we have before our eyes the proof that, in spite of
the imperfect constancy of the bees, a sufficient number of seeds is
produced to secure the existence of the species. Or does Mr. Bulman
conclude from the fact that the bees are _not absolutely constant_ that
flowers are not fertilized by bees at all?
I cannot conclude this note without touching briefly upon what the
opponents of the flower theory have contributed, and what explanation
of the facts they are prepared to offer.
In his important work, _Mechanische-physiologische Theorie der
Abstammungslehre_, published in 1884, Nägeli, as a convinced opponent
of the theory of selection, attempted an explanation. He was quite
aware that his assumption of an inward 'perfecting principle' would not
suffice to explain the mutual adaptations of flowers and insects, and
he refers the transformation of the first inconspicuous blossoms into
flowers to the mechanical stimulus which the visiting insects exerted
upon the parts of the blossom. By the pressure of their footsteps,
the pushing and probing with their proboscis, they have, he says,
transformed gradually, for instance, the little covering leaves at the
base of a pollen vessel into large flower petals, caused the conversion
of short flower-tubes into long ones, and of the pollen, once dry and
dusty, into the firmly adhesive mass formed in the anther lobes of
our modern flowers. The colour of the flowers depends, according to
him, upon the influence of light, which certainly no more explains the
yellow ring on a blue ground in the forget-me-not than it does the many
other nectar-guides which show the insect the way to the honey. Nägeli
works with the Lamarckian principle in the most daring way, and with
the same _naïveté_ as Lamarck himself in his time, that is, without
offering any sort of explanation as to how the minute impression made,
say by the foot or by the proboscis of an insect, upon a flower, is to
be handed on to the flowers of succeeding generations. He treats the
unending chain of generations as if it were a single individual, and
operates with his 'secular' stimulus, and with 'weak stimuli, lasting
through countless generations,' as though they were a proved fact. But
I have not even touched upon the question as to whether these 'stimuli'
could produce the changes he ascribes to them, even if they were
continually affecting the flower. How the scale-like covering leaves
of the pollen vessels could become larger and petal-like through the
treading of an insect's foot is as difficult to see as why a honey-tube
should _become longer_ because of the butterfly's honey-sucking: might
it not just as well become _wider_, _narrower_, or even _shorter_? I
see no convincing reason why it should become _longer_! And even if
it did so, it would necessarily continue to lengthen as time went on,
and this is not the case, for we find corolla-tubes of all possible
lengths, but, _it is to be noted, always in harmony with the length of
the proboscis of the visiting insect_. In a similar way Henslow has
recently attempted to refer the origin of flowers to the mechanical
stimulus exercised upon it by the visiting insects. 'An insect hanging
to the lower petal of a flower elongates the same by its weight, and
the lengthened petal is transmitted by heredity.'...'The irritation
caused by its feet in walking along the flower causes the appearance
of colouring matter, and the colour is likewise transmitted.'...'As it
probes for honey it causes a flow of sweet sap to that part, and this
also becomes hereditary!'
In this case, also, it is simply taken for granted that every little
passing irritation not only produces a perceptible effect, but that
this effect is transmissible. In a later lecture we shall have to
discuss in detail the question of the inheritance of functional
modifications. It is enough to say here that, if this kind of
transmission really took place even in the case of such minute and
transitory changes, there could be no dispute as to the correctness
of the 'Lamarckian principle,' since every fairly strong and lasting
irritation could be demonstrated with certainty to produce an effect.
When a butterfly, floating freely in the air, sucks honey from a tube,
the irritation must be almost analogous to that caused by a comb
lightly drawn by some one through our hair, and this is supposed to
effect the gradual lengthening of the corolla-tube of the flower!
The secretion of honey, too, depends upon the persistent irritation
of the proboscis! Then 'deceptive flowers,' like the Cypripedium we
have mentioned, could not exist at all, for they contain no honey,
although the proboscis of the bee must cause the same irritation in
them as in other orchids which do contain honey. This whole 'theory' of
direct effect is, moreover, only a crude and apparent interpretation,
which explains the conditions only in so far as they can be seen from
a distance; it fails as soon as they are more exactly examined; all
the great differences in the position of the honey, its concealment
from intelligent insects, its protection from rain by means of hairs,
and against unwelcome guests by a sticky secretion, the development
of a corolla-tube which corresponds in length to the length of the
visiting insect's proboscis, the development of spurs on the flower,
in short, all the numerous contrivances which have reference to
cross-fertilization by insects remain quite unintelligible in the light
of this theory--it is a mere _pis aller_ explanation for those who
continue to struggle against accepting the theory of selection.
LECTURE XI
SEXUAL SELECTION
Decorative colouring of male butterflies and birds--Wallace's
interpretation--Preponderance of males--Choice of the females--Sense
by sight in butterflies--Attractive odours--Scent-scales--Fragrance of
the females--The limits of natural and sexual selection not clearly
defined--Odours of particular species--Odours of other animals at
the breeding season--Song of the Cicadas, and of birds--Diversity
of decoration successively acquired--Humming-birds--Substitution
of other aids to wooing in place of personal decoration--Smelling
organs of male insects and crabs--Contrivances for seizing and
holding the female--Small size of certain males--Weapons of males
used in struggle for the females--Turban eyes of Ephemerids--Hoods
that can be inflated on the head of birds--Absence of secondary
sexual characters in lower animals--Transference of male characters
to the females--Lycæna--Parrots--Fashion operative in the phyletic
modifications of colour--Pattern of markings on the upper surface of a
butterfly's wing simpler than on the under side--Conclusion.
WE found in the process of Natural Selection an explanation of
numerous effective adaptations in plants and animals, as regards form,
colouring, and metabolism, of the most diverse weapons and protective
devices, of the existence of those forms of blossoms which we call
flowers, of instincts, and so on. The origin of the most characteristic
parts of whole orders of insects can only be understood as adaptations
to the environment brought about by means of natural selection.
Impressed by this, we have now to ask whether _all_ the transformations
of organisms may not be referred to adaptation to the continually
changing conditions of life? We shall return to this question later,
but in the meantime we are far from being able to answer it in the
affirmative, for there are undoubtedly a great many characters, at
least in animals, which cannot have owed their origin to natural
selection in the form in which we have studied it so far.
How could the splendid plumage of the humming-birds, of the pheasants,
of the parrots, the wonderful colour-patterns of so many diurnal
butterflies, be referred to the process of natural selection, since all
these characters can have no significance for their possessors in the
struggle for existence? Or of what use in the struggle for existence
could the possession of its gorgeous dress of feathers be to the bird
of Paradise; or of what service is the azure blue iridescence of the
_Morpho_ of Brazil, which makes it conspicuous from a distance when
it plays about the crowns of the palm-trees? We might indeed suppose
that they are warning signs of unpalatableness, like those of the
Heliconiides or of the gaily coloured caterpillars, but, in the first
place, these gay creatures are by no means inedible, and are indeed
much persecuted; and, secondly, the females have quite different and
very much darker and simpler colours. The gleaming splendour of all
these birds of Paradise and humming-birds, as well as that of many
butterflies, is found in the male sex only. The females of the birds
just mentioned are dark in colour and without the sparkling decorative
feathers of the males; they are plain--just like the females of many
butterflies. Alfred Russel Wallace has suggested that the explanation
of this lies in the greater need of the females for protection, since,
as is well known, they usually perform the labours of brooding, and are
thus frequently exposed to the attacks of enemies. It is undoubtedly
true that the dark and inconspicuous colouring of many birds and
butterflies depends on this need for protection, but this does not
explain the brilliant colours of the males of these species. Or can it
be that these require no explanation further than that they are, so to
speak, a chance secondary outcome of the structural relations of the
feathers and wing-scales respectively, which brought with it some other
advantage not known to us? Perhaps something in the same way as the red
colour of the blood in all vertebrates, from fishes upwards, cannot be
useful on the ground that it appears red to us, but because it is the
expression of the chemical constitution of the hæmoglobin, a body which
is indispensable to the metabolism, which here has the secondary and
intrinsically quite unimportant peculiarity of reflecting the red rays
of light.