was probably a thick covering of hair on the ventral surface of the
body, which served as a starting-point for the selection, and, in
consequence, the further course of the adaptation would be concerned
solely with this hair-covered surface, while variations in other less
hairy spots would remain un-utilized.
After all this it will no longer seem a paradoxical statement that the
existence of gaily coloured, diversely formed, and fragrant flowers is
due to the visits of insects, and that, on the other hand, many insects
have undergone essential transformations in their mouth-parts and
otherwise as an adaptation to a flower diet, and that an entire order
of insects with thousands of species--the Lepidoptera--would not be
in existence at all if there had been no flowers. We must now attempt
to show, in a more detailed way, how, by what steps, and under what
conditions, our modern flowers have arisen from the earlier flowering
plants. In this I follow closely the classic exposition which we owe to
Hermann Müller.
The ancestral forms of the modern higher plants, the so-called
'primitive seed plants' or 'Archisperms,' were all anemophilous, as the
Conifers and Cycads are still. Their smooth pollen-grains, produced
in enormous quantities, fell like clouds of dust into the air, were
carried by the wind hither and thither, and some occasionally alighted
on the stigma of a female flower. In these plants the sexes often occur
separately on different trees or individuals, and there must be a
certain advantage in this when the pollination is effected by the wind.
The male flowers of the Archisperms would be visited by insects in
remote ages, just as they are now; but the visitors came to feed upon
the pollen, and did not render any service to the plant in return;
they rather did it harm by reducing its store of pollen. If it was
possible to cause the insect to benefit the plant at the same time
as it was pillaging the pollen, by carrying some of it to female
blossoms and thereby securing cross-fertilization, it would be of
great advantage, for the plant would no longer require to produce such
enormous quantities of pollen, and the fertilization would be much
more certain than when it depended on the wind. It is obvious that the
successful pollination of anemophilous plants implies good weather and
a favourable wind.
[Illustration: FIG. 50. Flowers of the Willow (_Salix cinerea_); after
H. Müller. _A_, the male. _B_, the female catkin. _C_, individual male
flower; _n_, nectary. _D_, individual female flower; _n_, nectary. _E_,
Poplar, an exceptional hermaphrodite flower.]
It is plain that the utilization of the insect-visitors in
fertilization might be secured in either of two ways; the female
blossoms might also offer something attractive to the insects, or
hermaphrodite flowers might be formed. As a matter of fact, both ways
have been followed by Nature. An example of the former is the willow,
the cross-fertilization of which was forced upon the insects by the
development in both female and male blossoms of a nectary (Fig. 50,
_C_ and _D_), a little pit or basin in which nectar was secreted. The
insects flew now to male and now to female willow-catkins, and in doing
so they carried to the stigma of the female blossom the pollen, which
in this case was not dusty but sticky, so that it readily adhered to
their bodies.
The securing of cross-fertilization by the development of hermaphrodite
flowers has, however, occurred much more frequently, and we can
understand that this method secured the advantageous crossing much more
perfectly, for the pollen had necessarily to be carried from blossom
to blossom, while, in cases like that of the willow, countless male
blossoms might be visited for nectar one after the other before the
insect made up its mind to fly to a female blossom of the same species.
The beginnings of the modification of the unisexual flowers in this
direction may be seen in variations which occur even now, for we not
infrequently find, in a male catkin, individual blossoms, which, in
addition to the stamens, possess also a pistil with a stigma. (Fig. 50
_E_ shows such an abnormal hermaphrodite flower from a poplar.)
As soon as hermaphrodite flowers came into existence the struggle
to attract insects began in a more intense degree. Every little
improvement in this direction would form the starting-point of a
process of selection, and would be carried on and increased to the
highest possible pitch of perfection.
It was probably the outer envelopes of the blossoms which first changed
their original green into other colours, usually those which contrasted
strongly with the green, and thus directed the attention of the insects
to the flowers. Variations in the colour of ordinary leaves are
always cropping up from time to time, whether it be that the green is
transformed into yellow or that the chlorophyll disappears more or less
completely and red or blue coloured juices take its place. Many insects
can undoubtedly see colour, and are attracted by the size of coloured
flowers, as Hermann Müller found by counting the visits of insects to
two nearly related species of mallow, one of which, _Malva silvestris_,
has very large bright rose-red flowers visible from afar, while the
other, _Malva rotundifolia_, has very inconspicuous small pale-red
flowers. To the former there were thirty-one different visitors, to the
latter he could only make sure of four. The second species, as is to be
expected, depends chiefly on self-fertilization.
It has recently been disputed from various quarters that insects are
attracted by the colours of the flowers, and these objections are
based chiefly on experiments with artificial flowers. But when, for
instance, Plateau, in the course of such experiments saw bees and
butterflies first fly towards the artificial flowers, and then turn
away and concern themselves no more about them, that only proves that
their sight is sharper than we have given them credit for; for though
they may be deceived at a distance, they are not so when they are near;
it is possible, too, that the sense of smell turns the scale[9]. I
have myself made similar experiments with diurnal butterflies, before
which I placed a single artificial chrysanthemum midst a mass of
natural flowers. It rarely happened indeed that a butterfly settled on
the artificial flower; they usually flew first above it, but did not
alight. Twice, however, I saw them alight on the artificial flower,
and eagerly grope about with the proboscis for a few moments, then fly
quickly away. They had visited the real chrysanthemums or horse-daisies
with evident delight, and eagerly sucked up the honey from the many
individual florets of every flower, and they now endeavoured to do
the same in the artificial flower, and only desisted when the attempt
proved unsuccessful. In this experiment the colours were of course
only white and yellow; with red and blue it is probably more difficult
to give the exact impression of the natural flower-colours; and in
addition there is the absence of the delicate fragrance exhaled by the
flower.
[9] The experiments of Plateau have since been criticized by
Kienitz-Gerloff, who altogether denies their value (1903).
It must be allowed that the colour is certainly not the sole attraction
to the flower; the fragrance helps in most cases, and even this is
not the object of the insect's visits. The real object is the nectar,
to which colour and fragrance only show the way. The development of
fragrance and nectar must, like that of the colour, have been carried
on and increased by processes of selection, which had their basis in
the necessity for securing insect-visits, and as soon as these main
qualities of the flower were established greater refinements would
begin, and flower-forms would be evolved, which would diverge farther
and farther, especially in shape, from the originally simple and
regular form of the blossom.
The reason for this must have lain chiefly in the fact that, after
insect-visits in general were secured by a flower, it would be
advantageous to exclude all insects which would pillage the nectar
without rendering in return the service of cross-fertilization--all
those, therefore, which were unsuited either because of their minute
size or because of the inconstancy of their visits. Before the
butterflies and the bees existed, the regularly formed flat flower with
unconcealed nectar would be visited by a mixed company of caddis-flies,
saw-flies, and ichneumon-flies. But as the nectar changed its place to
the deeper recesses of the flower it was withdrawn from all but the
more intelligent insects, and thus the circle of visitors was already
narrowed to some extent. But when in a particular species the petals
fused into a short tube, all visitors were excluded whose mouth-parts
were too short to reach the nectar; while among those which could
reach it the process of proboscis-formation began; the under lip, or
the first maxillæ, or both parts together, lengthened step for step
with the corolla-tube of the flower, and thus from the caddis-flies
came the butterflies, and from the ichneumon-flies the burrowing-wasps
(_Sphegidæ_) and the bees.
At first sight one might perhaps imagine that it would have been more
advantageous to the flowers to attract a great many visitors, but
this is obviously not the case. On the contrary, specialized flowers,
accessible only to a few visitors, have a much greater certainty of
being pollinated by them, because insects which only fly to a few
species are more certain to visit these, and above all to visit many
flowers of the same species one after another. Hermann Müller observed
that, in four minutes, one of the humming-bird hawk-moths (_Macroglossa
stellatarum_) visited 108 different flowers of the same species, the
beautiful Alpine violet (_Viola calcarata_), one after the other, and
it may have effected an equal number of pollinations in that short time.
It was, therefore, a real advantage to the flowers to narrow their
circle of visitors more and more by varying so that only the useful
visitors could gain access to their nectar, and that the rest should
be excluded. Thus there arose 'bee-flowers,' 'butterfly-flowers,'
'hawk-moth flowers,' and, indeed, in many cases, a species of flower
has become so highly specialized that its fertilization can only
be brought about by a single species of insect. This explains the
remarkable adaptations of the orchids and the enormous length of the
proboscis in certain butterflies. Even our own hawk-moths _Macroglossa
stellatarum_ and _Sphinx convolvuli_ show an astonishing length of
proboscis, which measures 8 cm. in the latter species. In _Macrosilia
cluentius_, in Brazil, the proboscis is 20 cm. in length; and in
Madagascar there grows an orchid with nectaries 30 cm. in length,
filled with nectar to a depth of 2 cm., but the fertilizing hawk-moth
is not yet known.
Thus we may say that the flowers, by varying in one direction or
another, have selected a definite circle of visitors, and, conversely,
that particular insect-groups have selected particular flowers for
themselves, for those transformations of the flowers were always most
advantageous which secured to them the exclusive visits of their best
crossing agents, and these transformations were, on the one hand,
such as kept off unwelcome visitors, and, on the other hand, such as
attracted the most suitable ones.
From the botanical point of view the assumption that flowers and
flower-visiting insects have been adapted to each other by means
of processes of selection has been regarded as untenable, because
every variation in the flower presupposes a corresponding one in the
insect. I should not have mentioned this objection had it not come
from such a famous naturalist as Nägeli, and if it were not both
interesting and useful in our present discussion. Nägeli maintained
that selection could not, for instance, have effected a lengthening
of the corolla-tube of a flower, because the proboscis of the insects
must have lengthened _simultaneously_ with it. If the corolla-tube
had lengthened alone, without the proboscis of the butterfly being
at the same time elongated, the flower would no longer be fertilized
at all, and if the lengthening of the proboscis preceded that of the
corolla-tube it would have no value for the butterfly, and could not
therefore have been the object of a process of selection.
This objection overlooks the facts that a species of plant and of
butterfly consists not of one individual but of thousands or millions,
and that these are not absolutely uniform, but in fact heterogeneous.
It is precisely in this that the struggle for existence consists--that
the individuals of every species differ from one another, and that
some are better, others less well constituted. The elimination of the
latter and the preferring of the former constitutes the process of
selection, which always secures the fitter by continually rejecting the
less fit. In the case we are considering, then, there would be, among
the individuals of the plant-species concerned, flowers with a longer
and flowers with a shorter corolla-tube, and among the butterflies
some with a longer and some with a shorter proboscis. If among the
flowers the longer ones were more certain to be cross-fertilized than
the shorter ones, because hurtful visitors were better excluded, the
longer ones would produce more and better seeds, and would transmit
their character to more descendants; and if, among the butterflies,
those with the longer proboscis had an advantage, because the nectar
in the longer tubes would, so to speak, be reserved for them, and they
would thus be better nourished than those with the shorter proboscis,
the number of individuals with long proboscis must have increased from
generation to generation. Thus the length of the corolla-tube and the
length of the proboscis would go on increasing as long as there was
any advantage in it for the flower, and both parties must of necessity
have varied _pari passu_, since every lengthening of the corolla
was accompanied by a preferring of the longest proboscis variation.
The augmentation of the characters depended on, and could only have
depended on, a guiding of the variations in the direction of utility.
But this is exactly what we call, after Darwin and Wallace, Natural
Selection.
We have, however, in the history of flowers, a means of demonstrating
the reality of the processes of selection in two other ways. In the
first place, it is obvious that no other interpretation can be given
of such simultaneous mutual adaptations of two different kinds of
organisms. If we were to postulate, as Nägeli, for instance, did, an
intrinsic Power of Development in organisms, which produces and guides
their variations, we should, as I have already said, be compelled
also to take for granted a kind of pre-established harmony, such as
Leibnitz assumed to account for the correlation of body and mind:
plant and insect must always have been correspondingly altered so that
they bore the same relation to each other as two clocks which were
so exactly fashioned that they always kept time, though they did not
influence each other. But the case would be more complicated than that
of the clocks, because the changes which must have taken place on both
sides were quite different, and yet at the same time such that they
corresponded as exactly as Will and Action. The whole history of the
earth and of the forms of life must, therefore, have been foreseen
down to the smallest details, and embodied in the postulated Power of
Development.
But such an assumption could hardly lay claim to the rank of a
scientific hypothesis. Although every grain of sand blown about by
the wind on this earth could certainly only have fallen where it
actually did fall, yet it is in the power of any of us to throw a
handful of sand wherever it pleases us, and although even this act of
throwing must have had its sufficient reason in us, yet no one could
maintain that its direction and the places where the grains fell were
predestined in the history of the earth. In other words: That which we
call chance plays a part also in the evolution of organisms, and the
assumption of a Power of Development, predestinating even in detail,
is contradicted by the fact that species are transformed in accordance
with the chance conditions of their life.
This can be clearly demonstrated in the case of flowers. That the wild
pansy (_Viola tricolor_), which lives in the plains and on mountains of
moderate elevation, is fertilized by bees, and the nearly allied _Viola
calcarata_ of the High Alps by Lepidoptera, is readily intelligible,
since bees are very abundant in the lower region, and make the
fertilization of the species a certainty, while this is not so in the
High Alps. There the Lepidoptera are greatly in the majority, as every
one knows who has traversed the flower-decked meads of the High Alps in
July, and has seen the hundreds and thousands of butterflies and moths
which fly from flower to flower. Thus the viola of the High Alps has
become a 'butterfly-flower' by the development of its nectaries into a
long spur, accessible only to the proboscis of a moth or butterfly. The
chance which led certain individuals of the ancestral species to climb
the Alps must also have supplied the incentive to the production of the
changes adapted to the visits of the prevalent insect. The hypothesis
of a predestinating Power of Development suffers utter shipwreck in
face of facts like these.
We have, furthermore, an excellent touchstone for the reality of the
processes of selection in the _quality_ of the variations in flowers
and insects. Natural selection can only bring about those changes
which are of use to the possessors themselves; we should therefore
expect to find among flowers only such arrangements as are, directly or
indirectly, of use to them, and, conversely, among insects only such as
are useful to the insect.
And this is what we actually do find. All the arrangements of the
flowers--their colour, their form, their honey-guides, their hairy
honey-paths (Iris), their fragrance, and their honey itself--are all
indirectly useful to the plant itself, because they all co-operate in
compelling the honey-seeking insect to effect the fertilization of
the flower. This is most clearly seen in the case of the so-called
'Deceptive' flowers, which attract insects by their size and beauty,
their fragrance, and their resemblance to other flowers, and force
their visitors to be the means of their cross-fertilization, although
they contain no nectar at all. This is the case, according to Hermann
Müller, with the most beautiful of our indigenous orchids, the lady's
slipper (_Cypripedium calceolaris_). This flower is visited by bees
of the genus _Andrena_, which creep into the large wooden-shoe-shaped
under lip in the search for honey, only to find themselves prisoners,
for they cannot get out, at least by the way they came in, because of
the steep and smoothly polished walls of the flower. There is only one
way for the bee; it must force itself under the stigma, which it can
only do with great exertion, and not without being smeared with pollen,
which it carries to the next flower into which it creeps. It can only
leave this one in the same way, and thus the pollen is transferred to
the stigma by a mechanical necessity.
Such remarkable cases remind us in some ways of those cases of mimicry
in which the deceptions have to be used with caution or they lose their
effect. One might be disposed to imagine that such an intelligent
insect as a bee would not be deceived by the lady's slipper more than
once, and would not creep into a second flower after discovering that
there was no nectar in the first. But this conclusion is not correct,
for the bees are well accustomed in many flowers to find that the
nectar has already been taken by other bees; they could therefore not
conclude from one unsuccessful visit that the _Cypripedium_ did not
produce nectar at all, but would try again in a second, a third, and a
fourth flower. If these orchids had abundantly covered flower-spikes
like many species of _Orchis_, and if the species were common, the bees
would probably soon learn not to visit them, but the reverse is the
case. There is usually only one or, at most, two open flowers on the
lady's slipper, and the plant is rare, and probably occurs nowhere in
large numbers.
If we could find a flower in which the nectar lay open and accessible
to all insects, and which did not require any service from them
in return, the case could not be interpreted in terms of natural
selection; but we do not know of any such case.
[Illustration: FIG. 51. The Yucca-moth (_Pronuba yuccasella_). _M_,
laying eggs in the ovary of the Yucca flower. _n_, the stigma. After
Riley.]
Conversely, too, there are no adaptations in the insects which are
useful only to the flowers, and which are not of some use, directly or
indirectly, to the insect itself. Bees and butterflies certainly carry
the pollen from one flower to the stigma of another, but they are not
impelled to do this by a special instinct; they are forced to do it
by the structure of the flower, which has its stamens so placed and
arranged that they must shake their pollen over the visitor, or it may
be that the anthers are modified into stalked, viscid pollinia which
spring off at a touch, and fix themselves, so to speak, on the insect's
head. And even this is not all in the case of the orchis, for the
insect would never of its own accord transfer these pollinia on to the
stigma of the next flower; this is effected by the physical peculiarity
which causes the pollinia, after a short time, to bend forwards on the
insect's head.
All this fits in as well as possible with the hypothesis: how could an
instinct to carry pollen from one flower to the stigma of another have
been developed in an insect through natural selection, since the insect
itself has nothing to gain from this proceeding? Accordingly, we never
find in the insect any pincers or any kind of grasping organ adapted
for seizing and transmitting the pollen.
There is, however, one very remarkable case in which this appears to
be so, indeed really is so, and nevertheless it is not contradictory
to, but is corroborative of, the theory of selection. The excellent
American entomologist, Riley, established by means of careful
observations that the large white flowers of the Yucca are fertilized
by a little moth which behaves in a manner otherwise unheard of among
insects. Only the females visit the flowers, and they at once busy
themselves collecting a large ball of pollen. To this end they have
on the maxillary palps (Fig. 52, _C_, _mxp_) a long process (_si_),
curved in the form of a sickle, and covered with hairs, which probably
no other Lepidopteron possesses, with the help of which the moth
very quickly sweeps together a ball of pollen, it may be three times
the size of her own head. With this ball the insect flies to the
next flower, and there she lays her egg, by means of an ovipositor
otherwise unknown among Lepidoptera (Fig. 52, _A_, _op_), in the pods
of the flower. Finally, she pushes the ball of pollen deep into the
funnel-shaped stigmatic opening on the pistil (Fig. 51, _n_), and so
effects the cross-fertilization. The ovules develop, and when the
caterpillars emerge from the egg four to five days later they feed on
these until they are ready to enter on the pupa stage. Each little
caterpillar requires about eighteen or twenty seeds for its nourishment
(Fig. 52, _B_, _r_).