leaf-cutting ants, for how should the Aztec ants living inside notice
the presence of the lightly climbing leaf-cutters? But that is
provided for, for the Aztecs also frequent the outside of the trunk,
and just where attack would be most disastrous, namely, at the stalks
of the young leaves. At these places there is a peculiar velvet-like
cushion of hair (_P_), from which grow little stalked white papillæ
(Fig. 37, _B_), which are rich in nourishment, and are not only eaten
by the ants, but are harvested by them, being carried off into the
ants' dwellings, presumably to feed their larvæ. In this case, then,
a particular organ, offering special attraction to ants, has been
developed by the plant at the places more especially threatened; while,
as regards the ants, it is probable that only the instincts of feeding
and habitat require to be modified, since courage and thirst for battle
are present in all ants, almost any species being ready at any time to
throw itself on any other which intrudes into its domain.
It should be noted that not all the candelabra-trees live in symbiosis
with ants, and so secure a means of defence against the leaf-cutters.
Schimper found in the primitive forests of South America several
species of _Cecropia_ which never had ants in the chambers of their
hollow stem. But these species did not exhibit the nutritive cushions
at the base of the leaf-stalk; these contrivances for attracting and
retaining the presence of partner ants were altogether absent. Indeed,
only one species, _Cecropia peltata_, has produced these peculiar
structures, and, as they are of no _direct_ use to the tree, we must
say that it has produced them only for the ants. Here, again, natural
selection must have gradually brought about the development of these
nutritive cushions, though as yet we do not know what the beginnings of
the process may have been. In no case can the origin of these cushions
be referred to any direct influence of the environmental conditions.
We may now pass to the association of two species of plants, of
which the lichens furnish the best-known and probably most complete
illustration. Till about twenty years ago the lichens, which in so many
diverse forms clothe the bark of trees, the stones, and the rocks,
were regarded as simple plants like the flowering plants, the ferns,
or the mosses; and many lichenologists occupied themselves with the
exact systematic distinction of about a thousand species, each of which
could be as well and exactly classified, according to form, colour,
habitat, and minute structure, as any other kind of plant. Then De Bary
and Schwendener discovered that the lichens were made up of two kinds
of plants, fungi and algæ, so intimately associated with and adapted
to one another, that on coming together they always assume the same
specific form.
[Illustration: FIG. 38. A fragment of a Lichen (_Ephebe kerneri_),
magnified 450 times. _a_, the green alga-cells. _P_, the fungoid
filaments. After Kerner.]
The framework, and therefore the largest part, and the one which
determines the form of a lichen, is due to the fungus (Fig. 38).
Colourless threads of fungus ramify in a definite manner according
to the species of fungus, and in the network of spaces left by this
ramification green alga-cells (_a_) lie singly, or in rows, or groups.
The fungus is propagated by multitudes of minute spores, which it
produces periodically, and these are disseminated in the air by the
bursting of the sporangia and are carried away by the wind in the form
of fine dust; the alga multiplies simply by continual division into
two, but it also, like the whole lichen, can survive desiccation,
and, after falling to pieces, is likewise carried through the air as
microscopic dust.
The partnership of the two plants rests on a basis of mutual benefit;
the fungus, like all fungi, is without chlorophyll, and cannot
therefore decompose carbonic acid gas or elaborate its own organic
food-stuffs; it receives these from the alga. The alga has in the
network of the fungus a safe shelter and basis of attachment, for the
fungus is able to bore into the bark of trees and even into stones;
besides which it absorbs water and salts, and supplies these to the
partner alga. We here see the mutual advantage derived from the
partnership, which is really an extremely intimate one. Fungus spores,
sown by themselves, spring up and develop some branchings of fungoid
hyphæ, a so-called mycelium, but without the requisite partner alga
these remain weak and soon die away. The alga, on the other hand,
can, in some cases, though not in all, survive without the fungus if
the necessary conditions of its life be supplied to it, but it grows
differently and more luxuriantly in association with the fungus.
The same species of alga may be found associated with different species
of fungi, and then each partnership forms a distinct species of lichen
of definite and characteristic appearance; Stahl even succeeded in
making new species of lichen artificially by bringing the spores of a
lichen-forming fungus into contact with alga-cells, with which they had
never been associated in free nature.
The most remarkable feature of this remarkable association seems to me
to be the formation of common reproductive bodies--an adaptation in
face of which all doubt as to the theory of selection must disappear.
Periodically there are developed in the substance of the lichen small
corpuscles, the so-called soredia, each of which consists of one or
more alga-cells surrounded and kept together by threads of the fungus.
When they are developed in large numbers they form a floury dust over
the maternal lichen, which 'breaks up' and leaves them, like the spores
of the fungus, to be carried away by the wind. If these alight on
favourable soil nothing more is needed than the external conditions
of development, light, warmth, and water, to enable the lichen to
spring up anew. The great advantage to the preservation of 'species' is
obvious, for, when multiplication by the ordinary method occurs among
lichens, the spores of the fungus, even if they have fallen on good
ground, can only develop into a new lichen if chance bring to them the
proper partner alga.
Obviously there must be, in the formation of the soredia, great
advantage for the species, or rather 'for the two species,' for the
fungi as well as the algæ benefit by the arrangement, which ensures the
continuance of the partnership. It was not without reason, however,
that the dual organism was so long regarded as a simple species in the
natural history sense, _for that is what it really is_, although it has
arisen in a manner quite different from the usual origin of species. As
we know species which consist only of single cells, and others which
consist of many cells, differentiated in different ways, and forming
a cell-community or 'person,' and, finally, others which consist of a
community of diversely differentiated personæ, making up a 'stock'; so
in the lichens we see that even different species may combine to form
a new physiological whole, a vital unit, an individual of the highest
order. When, at the outset of these lectures, I said that the theory
of evolution was now no longer a mere hypothesis, and that its general
truth could no longer be doubted by any one acquainted with the facts
available, I had in my mind, among other facts, especially that of
symbiosis, and above all the case of the lichens.
There are many other interesting cases of symbiosis between two
different kinds of plants, and one side of the partnership is
represented by fungi in a relatively large number of instances. The
reason is not far to seek: fungi must always be dependent on other
plants for their food; they must be parasitic, because they cannot
themselves produce the organic substances they require. They must
therefore associate themselves in some way with other organisms, living
or dead, and as a general rule they simply prey upon their associate,
sucking up its juices and killing it. But in not a few cases they can
render services in return, and, as we have seen in the case of the
lichens, symbiosis may then occur. Fungi in general have the power of
discovering and absorbing the least trace of water in the soil, and
with it they absorb the salts necessary to the plant, and in this,
apparently, consists the service which they are able to render even to
large plants fixed deep in the earth, such as shrubs and trees. The
roots of many of our forest trees, e.g. beech, oak, fir, silver poplar,
and bushes like broom, heaths, and rhododendrons, are thickly wrapped
round with a network of fungoid threads, and the mutual relations just
indicated exist between these and the plants in question (Fig. 39,
_A_ and _B_). The plants give to the fungi some contribution from the
superfluity of their food-stuffs, and receive in return water and
salts, which are of value especially in times of drought. Perhaps there
is some connexion between this and the fact that limes wither and lose
their leaves so quickly during great summer-heat; these and many other
of our trees possess no root-fungi or mycorhizæ.
It is easy to understand, therefore, that genuine 'symbiosis' may have
arisen from parasitism. But that this is not the only path that leads
to symbiosis is shown by the cases of animal symbiosis we have already
discussed.
[Illustration: FIG. 39. _A_, fragment of a Silver Poplar root, with an
envelope of symbiotic fungoid filaments (mycelium); after Kerner. _B_,
apex of a Beech root, with the closely enveloping mantle of mycelium;
enlarged 480 times.]
The partnership between polyps and hermit-crabs may have arisen from a
one-sided commensalism, since polyps establishing themselves on mollusc
shells which were often made use of by hermit-crabs would be better fed
than those which settled down on stones. There are still species which
make use of both modes of settlement. Then followed the adaptation
of the crustacean to the polyp, for, first, those hermit-crabs would
thrive best which tolerated the presence of the polyp; then those which
sought its presence, that is to say, which gave a preference to shells
covered with polyps; and, finally, those which would take no others,
and even themselves fixed the sea-anemone upon it, if it chanced to be
removed. Intelligence need not be taken into account in the matter at
all, not even in the hermit-crab's case. We have only to recall the
complex instincts, exercised only once in a lifetime, which compel the
silkworm and the emperor moth to elaborate their effective cocoons.
The elaboration of the spinning-instinct can only be due to natural
selection, for the insect can have had no idea of the utility of its
performance, and the same is true in the case of the sea-anemones
or the hydroid polyps and the hermit-crab. The sea-anemone is quite
unconscious that it is defending its partner, the hermit-crab, when it
lashes out its stinging acontia on any disturbance, and the hermit-crab
is equally unaware that the sea-anemone is contributing to its safety;
both animals act quite unconsciously, purely instinctively, and the
origin of these instincts, on which the symbiosis is based, must be
due, not to intelligent activities which have become habitual, but only
to the survival of the fittest.
According to the principle of natural selection nothing can arise
but that which is of use directly or indirectly to its possessor.
Nevertheless, there are cases in which it appears as if something had
arisen, which was of no use to the species in which the variation
appeared, but only to the species protected by it. This is the case
in the remarkable symbiosis between algæ of the family Nostocaceæ and
the floating, moss-like water-fern _Azolla_. This plant, in external
appearance almost like duckweed, has on the under surface of its leaves
a minute opening, leading into a relatively roomy hair-lined cavity,
and in this cavity there is always, enclosed in jelly, a bluish green
unicellular alga, _Anabæna_. The cavity is present in every leaf, and
the alga is present in every cavity, making its way in from a deposit
of alga-cells which is found on the incurved tip of every young shoot.
As soon as a young leaf of _Azolla_ unfolds from the bud it receives
its _Anabæna_ cells from this deposit, and no one has yet found
either twigs or leaves which were free from the algæ. But no one has
succeeded in discovering any benefit derived by the _Azolla_ from this
partnership.
This looks like a contradiction of the theory of selection, but there
remains the possibility that there is some benefit rendered to the
_Azolla_ by the alga, though we cannot see it as yet. There is also
the possibility that the cavity is an organ which was of use to the
plant at an earlier time, perhaps as an insect-trap, but has now lost
its significance, and is utilized by the alga as a dwelling-place.
This, however, is contradicted by the remarkable distribution of the
four known species of _Azolla_. Two of these are widely distributed in
America; the third lives in Australia, Asia, and Africa; the fourth
in the region of the Nile: all four have cavities in their leaves,
and in all these forms the cavity is inhabited by the same species of
_Anabæna_. This indicates that the leaf-cavity and the partnership
with the alga must have originated in remote antiquity; the symbiosis
must date from a time before the four modern species of _Azolla_ had
split off from a single parent-species. But no rudimentary organ,
that is to say, no organ not of use to the plant itself, would have
been preserved through such a vast period of time, as we shall see
later, for useless organs disappear in the course of ages. As the
cavity has not yet disappeared, we may assume with some probability
that it is useful to the plant, whether by means of the _Anabæna_, or
in some other unknown way. To draw an argument against the reality of
the processes of selection from our lack of knowledge of what this
advantage may be would be as unreasonable as if, notwithstanding our
experience that stones sink in the water, we were to assume of a
particular stone which we did not see sink, because it was hidden from
our sight by bushes, that perhaps it had not sunk, but was capable of
floating.
LECTURE X
THE ORIGIN OF FLOWERS
Introduction--Precursors of Darwin--Pollination
by wind--Arrangements in flowers for securing
cross-fertilization--Salvia, Pedicularis--Flowers visited by
flies--Aristolochia--Pinguicula--Daphne--Orchids--Flowers are
built up of adaptations--Mouth-parts of insects--Proboscis of
butterflies--Mouth-parts of the cockroach--Of the bee--Pollen
baskets of bees--Origin of flowers--Attraction of insects by
colour--Limitation of the area visited--Nägeli's objection to
the theory of selection--Other interpretations excluded--_Viola
calcarata_--Only those changes which are useful to their possessors
have persisted--Deceptive flowers--Cypripedium--Pollinia of
Orchis--The case of the Yucca-moth--The relative imperfection of
the adaptations tells in favour of their origin through natural
selection--Honey thieves.
WHEN one species is so intimately bound up with another that neither
can live for any length of time except in partnership, that is
certainly an example of far-reaching mutual adaptation, but there
are innumerable cases of mutual adaptation, in which, although there
is no common life in the same place, yet the first form of life is
adjusted in relation to the peculiarities of the second, and the second
to those of the first. One of the most beautiful, and, in regard to
natural selection, the most instructive of these cases is illustrated
by the relations between insects and the higher plants, relations which
have grown out of the fact that many insects have formed the habit of
visiting the flowers of the plants for the sake of the pollen. In this
connexion the theory of selection has made the most unexpected and
highly interesting disclosures, for it has informed us how the flowers
have arisen.
In earlier times the beauty, the splendour of colour, and the fragrance
of flowers were regarded as phenomena created for the delight of
mankind, or as an outcome of the infinite creative power of Mother
Nature, who loves to run riot in form and colour. Without allowing our
pleasure in all this manifold beauty to be spoilt, we must nowadays
form quite a different conception of the way in which the flowers
have been called into being. Although here, as everywhere else in
Nature, we cannot go back to ultimate causes, yet we can show, on
very satisfactory evidence, that the flowers illustrate the reaction
of the plants to the visits of insects, and that they have been in
large measure evoked by these visits. There might, indeed, have
been blossoms, but there would have been no flowers--that is to say,
blossoms with large, coloured, outer parts, with fragrance, and with
nectar inside, unless the blossoms had been sought out by insects
during the long ages. Flowers are adaptations of the higher flowering
plants to the visits of insects. There can be no doubt about that now,
for--thanks to the numerous and very detailed studies of a small number
of prominent workers--we need not only suppose it, we can prove it
with all the certainty that can be desired. The mutual adaptations of
insects and flowers afford one of the clearest examples of the mode
of operation and the power of natural selection, and the case cannot
therefore be omitted from lectures on the theory of descent.
That bees and many other insects visit flowers for the sake of the
nectar and pollen has been known to men from very early times. But this
fact by itself would only explain why adaptations to flower-visiting
have taken place in these insects to enable them, for instance, to
reach the nectar out of deep corolla-tubes, or to load themselves with
a great quantity of pollen, and to carry it to their hives, as happens
in the case of the bees. But what causes the plants to produce nectar,
and offer it to the insects, since it is of no use to themselves? And
further, what induces them to make the pillage easier to the insects,
by making their blossoms visible from afar through their brilliant
colours, or by sending forth a stream of fragrance that, even during
the night, guides their visitors towards them?
As far back as the end of the eighteenth century a thoughtful and
clear-sighted Berlin naturalist, Christian Konrad Sprengel, took
a great step towards answering this question. In the year 1793 he
published a paper entitled 'The Newly Discovered Secret of Nature in
the Structure and Fertilization of Flowers[8],' in which he quite
correctly recognized and interpreted a great many of the remarkable
adaptations of flowers to the visits of insects. Unfortunately, the
value of these discoveries was not appreciated in Sprengel's own time,
and his work had to wait more than half a century for recognition.
[8] _Das neu-entdeckte Geheimniss der Natur im Bau u. der Befruchtung
der Blumen_, Berlin, 1793.
Sprengel was completely dominated by the idea of an all-wise Creator,
who 'has not created even a single hair without intention,' and, guided
by this idea, he endeavoured to penetrate into the significance of
many little details in the structure of flowers. Thus he recognized
that the hairs which cover the lower surface of the petals of the
wood-cranesbill (_Geranium sylvaticum_) protect the nectar of the
flower from being diluted with rain, and he drew the conclusion,
correct enough, though far removed from our modern ideas as regards the
directly efficient cause, that the nectar was there for the insects.
He was also impressed by the fact that the sky-blue corolla of the
forget-me-not (_Myosotis palustris_) has a beautiful yellow ring round
the entrance to the corolla-tube, and he interpreted this as a means by
which insects were shown the way to the nectar which is concealed in
the depths of the tube.
[Illustration: FIG. 40. _Potentilla verna_, after Hermann Müller. _A_,
seen from above. _Kbl_, sepals. _Bl_, petals. _Nt_, nectaries near the
base of the stamens. _B_, section through the flower. _Gr_, stigma.
_St_, stamen. _Nt_, nectary.]
We now know that such 'honey-guides' are present in most of the flowers
visited by insects, in the form of spots, lines, or other marking,
usually of conspicuous colour, that is, of a colour contrasting with
the ground colour of the flower. Thus, in species of Iris, regular
paths of short hairs lead the way to the place where the nectar lies.
In the spring potentilla (_Potentilla verna_) (Fig. 40) the yellow
petals (_A_, _Bl_) become bright orange-red towards their bases,
and this shows the way to the nectaries, which lie at the bases
of the stamens (_st_), and are protected by hairs, the so-called
'nectar-covers' (_Saftdecke_) of Sprengel, from being washed by rain.
The recognition of the honey-guides led Sprengel on to the idea that
the general colouring of the flower effects on a large scale what the
honey-guides do in a more detailed way--it attracts the attention of
passing insects to where nectar is to be found; indeed, he went an
important step further by recognizing that there are flowers which
cannot fertilize themselves, in which the insect, in its search for
honey, covers itself with pollen, which is then rubbed off on the
stigma of the next flower visited, fertilization being thus effected.
He demonstrated this not only for the Iris, but for many other
flowers, and he drew the conclusion that 'Nature does not seem to have
wished that any flower should be fertilized by its own pollen.' How
near Sprengel was to reaching a complete solution of the problem is
now plain to us, for he even discovered that many flowers, such as
_Hemerocallis fulva_, remained infertile if they were dusted with their
own pollen.
Even the numerous experiments of that admirable German botanist, C. F.
Gärtner, although they advanced matters further, did not suffice to
make the relations between insects and flowers thoroughly clear; for
this the basis of the theory of Descent and Selection was necessary.
Here, again, it was reserved for Charles Darwin to lead the way where
both contemporaries and predecessors had been blindly groping. He
recognized that, _in general_, self-fertilization is disadvantageous
to plants; that they produce fewer seeds, and that these produce
feebler plants, than when they are cross-fertilized; that, therefore,
those flowers which are arranged to secure cross-fertilization have an
advantage over those which are self-fertilized. In many species, as
Sprengel had already pointed out, self-fertilization leads to actual
infertility; only a few plants are as fertile with their own pollen as
with that of another plant; and Darwin believed that, in all flowering
plants, crossing with others of the same kind, at least from time to
time, is necessary if they are not to degenerate.
Thus the advantage which the flowers derive from the visits of
insects lies in the fact that insects are instrumental in the
cross-fertilization of the flowers, and we can now understand how the
plant was able to vary in a manner favourable to the insect-visits, and
to exhibit adaptations which serve exclusively to make these visits
easier; we understand how it was possible that there should develop
among flowers an endless number of contrivances which served solely
to attract insects, and even how, for the same end, the insignificant
blossoms of the oldest Phanerogams must have been transformed into real
flowers.
We must not imagine, however, that the obviously important crossing of
plant-individuals, usually called 'cross-pollination,' can be effected
only by means of insects. There were numerous plants in earlier times,
and there is still a whole series in which cross-fertilization is
effected through the air by the wind; these are the anemophilous or
wind-pollinated Angiosperms.
To these belong most of the catkin-bearers, such as hazel and birch,
and also the grasses and sedges, the hemp and the hop, and so forth.
In these plants there is no real flower, but only an inconspicuous
blossom, without brightly-coloured outer envelopes, without fragrance
or nectar; all of them have smooth pollen grains, which easily
separate into fine dust and are carried away by the wind until they
fall, by chance, far from their place of origin, on the stigma of a
female blossom.
[Illustration: FIG. 41. Flower of Meadow Sage (_Salvia pratensis_),
after H. Müller. _st´_, immature anthers concealed in the 'helmet' of
the flower. _st´´_, mature anther lowered. _gr´_, immature stigma.
_gr´´_, mature stigma. _U_, the lower lip of the corolla, the
landing-stage for the bee.]