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Darwinism (1889) by Alfred Russel Wallace

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_Fertilisation of Flowers by Birds._

In the temperate regions of the Northern Hemisphere, insects are the
chief agents in cross-fertilisation when this is not effected by the
wind; but in warmer regions, and in the Southern hemisphere, birds are
found to take a considerable part in the operation, and have in many
cases led to modifications in the form and colour of flowers. Each part
of the globe has special groups of birds which are flower-haunters.
America has the humming-birds (Trochilidae), and the smaller group of
the sugar-birds (Caerebidae). In the Eastern tropics the sun-birds
(Nectarineidae) take the place of the humming-birds, and another small
group, the flower-peckers (Dicaeidae), assist them. In the Australian
region there are also two flower-feeding groups, the Meliphagidae, or
honey-suckers, and the brush-tongued lories (Trichoglossidae). Recent
researches by American naturalists have shown that many flowers are
fertilised by humming-birds, such as passion-flowers, trumpet-flowers,
fuchsias, and lobelias; while some, as the Salvia splendens of Mexico,
are specially adapted to their visits. We may thus perhaps explain the
number of very large tubular flowers in the tropics, such as the huge
brugmansias and bignonias; while in the Andes and in Chile, where
humming-birds are especially plentiful, we find great numbers of red
tubular flowers, often of large size and apparently adapted to these
little creatures. Such are the beautiful Lapageria and Philesia, the
grand Pitcairneas, and the genera Fuchsia, Mitraria, Embothrium,
Escallonia, Desfontainea, Eccremocarpus, and many Gesneraceae. Among the
most extraordinary modifications of flower structure adapted to bird
fertilisation are the species of Marcgravia, in which the pedicels and
bracts of the terminal portion of a pendent bunch of flowers have been
modified into pitchers which secrete nectar and attract insects, while
birds feeding on the nectar, or insects, have the pollen of the
overhanging flowers dusted on their backs, and, carrying it to other
flowers, thus cross-fertilise them (see Illustration).

[Illustration: FIG. 31.--Humming-bird fertilising Marcgravia
nepenthoides.]

In Australia and New Zealand the fine "glory peas" (Clianthus), the
Sophora, Loranthus, many Epacrideae and Myrtaceae, and the large flowers
of the New Zealand flax (Phormium tenax), are cross-fertilised by
birds; while in Natal the fine trumpet-creeper (Tecoma capensis) is
fertilised by Nectarineas.

The great extent to which insect and bird agency is necessary to flowers
is well shown by the case of New Zealand. The entire country is
comparatively poor in species of insects, especially in bees and
butterflies which are the chief flower fertilisers; yet according to the
researches of local botanists no less than one-fourth of all the
flowering plants are incapable of self-fertilisation, and, therefore,
wholly dependent on insect or bird agency for the continuance of the
species.

The facts as to the cross-fertilisation of flowers which have now been
very briefly summarised, taken in connection with Darwin's experiments
proving the increased vigour and fertility given by cross-fertilisation,
seem amply to justify his aphorism that "Nature abhors
self-fertilisation," and his more precise statement, that, "No plant is
perpetually self-fertilised;" and this view has been upheld by
Hildebrand, Delpino, and other botanists.[150]


_Self-Fertilisation of Flowers._

But all this time we have been only looking at one side of the question,
for there exists an abundance of facts which seem to imply, just as
surely, the utter uselessness of cross-fertilisation. Let us, then, see
what these facts are before proceeding further.

1. An immense variety of plants are habitually self-fertilised, and
their numbers probably far exceed those which are habitually
cross-fertilised by insects. Almost all the very small or obscure
flowered plants with hermaphrodite flowers are of this kind. Most of
these, however, may be insect fertilised occasionally, and may,
therefore, come under the rule that no species are perpetually
self-fertilised.

2. There are many plants, however, in which special arrangements exist
to secure self-fertilisation. Sometimes the corolla closes and brings
the anthers and stigma into contact; in others the anthers cluster round
the stigmas, both maturing together, as in many buttercups, stitchwort
(Stellaria media), sandwort (Spergula), and some willow-herbs
(Epilobium); or they arch over the pistil, as in Galium aparine and
Alisma Plantago. The style is also modified to bring it into contact
with the anthers, as in the dandelion, groundsel, and many other
plants.[151] All these, however, may be occasionally cross-fertilised.

3. In other cases precautions are taken to prevent cross-fertilisation,
as in the numerous cleistogamous or closed flowers. These occur in no
less than fifty-five different genera, belonging to twenty-four natural
orders, and in thirty-two of these genera the normal flowers are
irregular, and have therefore been specially modified for insect
fertilisation.[152] These flowers appear to be degradations of the
normal flowers, and are closed up by various modifications of the petals
or other parts, so that it is impossible for insects to reach the
interior, yet they produce seed in abundance, and are often the chief
means by which the species is continued. Thus, in our common dog-violet
the perfect flowers rarely produce seed, while the rudimentary
cleistogamic flowers do so in abundance. The sweet violet also produces
abundance of seed from its cleistogamic flowers, and few from its
perfect flowers; but in Liguria it produces only perfect flowers which
seed abundantly. No case appears to be known of a plant which has
cleistogamic flowers only, but a small rush (Juncus bufonius) is in this
condition in some parts of Russia, while in other parts perfect flowers
are also produced.[153] Our common henbit dead-nettle (Lamium
amplexicaule) produces cleistogamic flowers, as do also some orchids.
The advantage gained by the plant is great economy of specialised
material, since with very small flowers and very little expenditure of
pollen an abundance of seed is produced.

4. A considerable number of plants which have evidently been specially
modified for insect fertilisation have, by further modification, become
quite self-fertile. This is the case with the garden-pea, and also with
our beautiful bee-orchis, in which the pollen-masses constantly fall on
to the stigmas, and the flower, being thus self-fertilised, produces
abundance of capsules and of seed. Yet in many of its close allies
insect agency is absolutely required; but in one of these, the
fly-orchis, comparatively very little seed is produced, and
self-fertilisation would therefore be advantageous to it. When
garden-peas were artificially cross-fertilised by Mr. Darwin, it seemed
to do them no good, as the seeds from these crosses produced less
vigorous plants than seed from those which were self-fertilised; a fact
directly opposed to what usually occurs in cross-fertilised plants.

5. As opposed to the theory that there is any absolute need for
cross-fertilisation, it has been urged by Mr. Henslow and others that
many self-fertilised plants are exceptionally vigorous, such as
groundsel, chickweed, sow-thistle, buttercups, and other common weeds;
while most plants of world-wide distribution are self-fertilised, and
these have proved themselves to be best fitted to survive in the battle
of life. More than fifty species of common British plants are very
widely distributed, and all are habitually self-fertilised.[154] That
self-fertilisation has some great advantage is shown by the fact that it
is usually the species which have the smallest and least conspicuous
flowers which have spread widely, while the large and showy flowered
species of the same genera or families, which require insects to
cross-fertilise them, have a much more limited distribution.

6. It is now believed by some botanists that many inconspicuous and
imperfect flowers, including those that are wind-fertilised, such as
plantains, nettles, sedges, and grasses, do not represent primitive or
undeveloped forms, but are degradations from more perfect flowers which
were once adapted to insect fertilisation. In almost every order we find
some plants which have become thus reduced or degraded for wind or
self-fertilisation, as Poterium and Sanguisorba among the Rosaceae;
while this has certainly been the case in the cleistogamic flowers. In
most of the above-mentioned plants there are distinct rudiments of
petals or other floral organs, and as the chief use of these is to
attract insects, they could hardly have existed in primitive
flowers.[155] We know, moreover, that when the petals cease to be
required for the attraction of insects, they rapidly diminish in size,
lose their bright colour or almost wholly disappear.[156]


_Difficulties and Contradictions._

The very bare summary that has now been given of the main facts relating
to the fertilisation of flowers, will have served to show the vast
extent and complexity of the inquiry, and the extraordinary
contradictions and difficulties which it presents. We have direct proof
of the beneficial results of intercrossing in a great number of cases;
we have an overwhelming mass of facts as to the varied and complex
structure of flowers evidently adapted to secure this intercrossing by
insect agency; yet we see many of the most vigorous plants which spread
widely over the globe, with none of these adaptations, and evidently
depending on self-fertilisation for their continued existence and
success in the battle of life. Yet more extraordinary is it to find
numerous cases in which the special arrangements for cross-fertilisation
appear to have been a failure, since they have either been supplemented
by special means for self-fertilisation, or have reverted back in
various degrees to simpler forms in which self-fertilisation becomes the
rule. There is also a further difficulty in the highly complex modes by
which cross-fertilisation is often brought about; for we have seen that
there are several very effective yet very simple modes of securing
intercrossing, involving a minimum of change in the form and structure
of the flower; and when we consider that the result attained with so
much cost of structural modification is by no means an unmixed good, and
is far less certain in securing the perpetuation of the species than is
self-fertilisation, it is most puzzling to find such complex methods
resorted to, sometimes to the extent of special precautions against the
possibility of self-fertilisation ever taking place. Let us now see
whether any light can be thrown on these various anomalies and
contradictions.


_Intercrossing not necessarily Advantageous._

No one was more fully impressed than Mr. Darwin with the beneficial
effects of intercrossing on the vigour and fertility of the species or
race, yet he clearly saw that it was not always and necessarily
advantageous. He says: "The most important conclusion at which I have
arrived is, that the mere act of intercrossing by itself does no good.
The good depends on the individuals which are crossed differing slightly
in constitution, owing to their progenitors having been subjected during
several generations to slightly different conditions. This conclusion,
as we shall hereafter see, is closely connected with various important
physiological problems, such as the benefit derived from slight changes
in the conditions of life."[157] Mr. Darwin has also adduced much direct
evidence proving that slight changes in the conditions of life are
beneficial to both animals and plants, maintaining or restoring their
vigour and fertility in the same way as a favourable cross seems to
restore it.[158] It is, I believe, by a careful consideration of these
two classes of facts that we shall find the clue to the labyrinth in
which this subject has appeared to involve us.


_Supposed Evil Results of Close Interbreeding._

Just as we have seen that intercrossing is not necessarily good, we
shall be forced to admit that close interbreeding is not necessarily
bad. Our finest breeds of domestic animals have been thus produced, and
by a careful statistical inquiry Mr. George Darwin has shown that the
most constant and long-continued intermarriages among the British
aristocracy have produced no prejudicial results. The rabbits on Porto
Santo are all the produce of a single female; they have lived on the
same small island for 470 years, and they still abound there and appear
to be vigorous and healthy (see p. 161).

We have, however, on the other hand, overwhelming evidence that in many
cases, among our domestic animals and cultivated plants, close
interbreeding does produce bad results, and the apparent contradiction
may perhaps be explained on the same general principles, and under
similar limitations, as were found to be necessary in defining the value
of intercrossing. It appears probable, then, that it is not
interbreeding in itself that is hurtful, but interbreeding without
rigid selection or some change of conditions. Under nature, as in the
case of the Porto Santo rabbits, the rapid increase of these animals
would in a very few years stock the island with a full population, and
thereafter natural selection would act powerfully in the preservation
only of the healthiest and the most fertile, and under these conditions
no deterioration would occur. Among the aristocracy there has been a
constant selection of beauty, which is generally synonymous with health,
while any constitutional infertility has led to the extinction of the
family. With domestic animals the selection practised is usually neither
severe enough nor of the right kind. There is no natural struggle for
existence, but certain points of form and colour characteristic of the
breed are considered essential, and thus the most vigorous or the most
fertile are not always those which are selected to continue the stock.
In nature, too, the species always extends over a larger area and
consists of much greater numbers, and thus a difference of constitution
soon arises in different parts of the area, which is wanting in the
limited numbers of pure bred domestic animals. From a consideration of
these varied facts we conclude that an occasional disturbance of the
organic equilibrium is what is essential to keep up the vigour and
fertility of any organism, and that this disturbance may be equally well
produced either by a cross between individuals of somewhat different
constitutions, or by occasional slight changes in the conditions of
life. Now plants which have great powers of dispersal enjoy a constant
change of conditions, and can, therefore, exist permanently, or at all
events, for very long periods, without intercrossing; while those which
have limited powers of dispersal, and are restricted to a comparatively
small and uniform area, need an occasional cross to keep up their
fertility and general vigour. We should, therefore, expect that those
groups of plants which are adapted both for cross-and
self-fertilisation, which have showy flowers and possess great powers of
seed-dispersal, would be the most abundant and most widely distributed;
and this we find to be the case, the Compositae possessing all these
characteristics in the highest degree, and being the most generally
abundant group of plants with conspicuous flowers in all parts of the
world.


_How the Struggle for Existence Acts among Flowers._

Let us now consider what will be the action of the struggle for
existence under the conditions we have seen to exist.

Everywhere and at all times some species of plants will be dominant and
aggressive; while others will be diminishing in numbers, reduced to
occupy a smaller area, and generally having a hard struggle to maintain
themselves. Whenever a self-fertilising plant is thus reduced in numbers
it will be in danger of extinction, because, being limited to a small
area, it will suffer from the effects of too uniform conditions which
will produce weakness and infertility. But while this change is in
progress, any crosses between individuals of slightly different
constitution will be beneficial, and all variations favouring either
insect agency on the one hand, or wind-dispersal of pollen on the other,
will lead to the production of a somewhat stronger and more fertile
stock. Increased size or greater brilliancy of the flower, more abundant
nectar, sweeter odour, or adaptations for more effectual
cross-fertilisation would all be preserved, and thus would be initiated
some form of specialisation for insect agency in cross-fertilisation;
and in every different species so circumstanced the result would be
different, depending as it would on many and complex combinations of
variation of parts of the flower, and of the insect species which most
abounded in the district.

Species thus favourably modified might begin a new era of development,
and, while spreading over a somewhat wider area, give rise to new
varieties or species, all adapted in various degrees and modes to secure
cross-fertilisation by insect agency. But in course of ages some change
of conditions might prove adverse. Either the insects required might
diminish in numbers or be attracted by other competing flowers, or a
change of climate might give the advantage to other more vigorous
plants. Then self-fertilisation with greater means of dispersal might be
more advantageous; the flowers might become smaller and more numerous;
the seeds smaller and lighter so as to be more easily dispersed by the
wind, while some of the special adaptations for insect fertilisation
being useless would, by the absence of selection and by the law of
economy of growth, be reduced to a rudimentary form. With these
modifications the species might extend its range into new districts,
thereby obtaining increased vigour by the change of conditions, as
appears to have been the case with so many of the small flowered
self-fertilised plants. Thus it might continue to exist for a long
series of ages, till under other changes--geographical or biological--it
might again suffer from competition or from other adverse circumstances,
and be at length again confined to a limited area, or reduced to very
scanty numbers.

But when this cycle of change had taken place, the species would be very
different from the original form. The flower would have been at one time
modified to favour the visits of insects and to secure
cross-fertilisation by their aid, and when the need for this passed
away, some portions of these structures would remain, though in a
reduced or rudimentary condition. But when insect agency became of
importance a second time, the new modifications would start from a
different or more advanced basis, and thus a more complex result might
be produced. Owing to the unequal rates at which the reduction of the
various parts might occur, some amount of irregularity in the flower
might arise, and on a second development towards insect
cross-fertilisation this irregularity, if useful, might be increased by
variation and selection.

The rapidity and comparative certainty with which such changes as are
here supposed do really take place, are well shown by the great
differences in floral structure, as regards the mode of fertilisation,
in allied genera and species, and even in some cases in varieties of the
same species. Thus in the Ranunculaceae we find the conspicuous part of
the flower to be the petals in Ranunculus, the sepals in Helleborus,
Anemone, etc., and the stamens in most species of Thalictrum. In all
these we have a simple regular flower, but in Aquilegia it is made
complex by the spurred petals, and in Delphinium and Aconitum it becomes
quite irregular. In the more simple class self-fertilisation occurs
freely, but it is prevented in the more complex flowers by the stamens
maturing before the pistil. In the Caprifoliaceae we have small and
regular greenish flowers, as in the moschatel (Adoxa); more conspicuous
regular open flowers without honey, as in the elder (Sambucus); and
tubular flowers increasing in length and irregularity, till in some,
like our common honeysuckle, they are adapted for fertilisation by moths
only, with abundant honey and delicious perfume to attract them. In the
Scrophulariaceae we find open, almost regular flowers, as Veronica and
Verbascum, fertilised by flies and bees, but also self-fertilised;
Scrophularia adapted in form and colour to be fertilised by wasps; and
the more complex and irregular flowers of Linaria, Rhinanthus,
Melampyrum, Pedicularis, etc., mostly adapted to be fertilised by bees.

In the genera Geranium, Polygonum, Veronica, and several others there is
a gradation of forms from large and bright to small and obscure coloured
flowers, and in every case the former are adapted for insect
fertilisation, often exclusively, while in the latter self-fertilisation
constantly occurs. In the yellow rattle (Rhinanthus Crista-galli) there
are two forms (which have been named _major_ and _minor_), the larger
and more conspicuous adapted to insect fertilisation only, the smaller
capable of self-fertilisation; and two similar forms exist in the
eyebright (Euphrasia officinalis). In both these cases there are special
modifications in the length and curvature of the style as well as in the
size and shape of the corolla; and the two forms are evidently becoming
each adapted to special conditions, since in some districts the one, in
other districts the other is most abundant.[159]

These examples show us that the kind of change suggested above is
actually going on, and has presumably always been going on in nature
throughout the long geological epochs during which the development of
flowers has been progressing. The two great modes of gaining increased
vigour and fertility--intercrossing and dispersal over wider areas--have
been resorted to again and again, under the pressure of a constant
struggle for existence and the need for adaptation to ever-changing
conditions. During all the modifications that ensued, useless parts were
reduced or suppressed, owing to the absence of selection and the
principle of economy of growth; and thus at each fresh adaptation some
rudiments of old structures were re-developed, but not unfrequently in
a different form and for a distinct purpose.

The chief types of flowering plants have existed during the millions of
ages of the whole tertiary period, and during this enormous lapse of
time many of them may have been modified in the direction of insect
fertilisation, and again into that of self-fertilisation, not once or
twice only, but perhaps scores or even hundreds of times; and at each
such modification a difference in the environment may have led to a
distinct line of development. At one epoch the highest specialisation of
structure in adaptation to a single species or group of insects may have
saved a plant from extinction; while, at other times, the simplest mode
of self-fertilisation, combined with greater powers of dispersal and a
constitution capable of supporting diverse physical conditions, may have
led to a similar result. With some groups the tendency seems to have
been almost continuously to greater and greater specialisation, while
with others a tendency to simplification and degradation has resulted in
such plants as the grasses and sedges.

We are now enabled dimly to perceive how the curious anomaly of very
simple and very complex methods of securing cross-fertilisation--both
equally effective--may have been brought about. The simple modes may be
the result of a comparatively direct modification from the more
primitive types of flowers, which were occasionally, and, as it were,
accidentally visited and fertilised by insects; while the more complex
modes, existing for the most part in the highly irregular flowers, may
result from those cases in which adaptation to insect-fertilisation, and
partial or complete degradation to self-fertilisation or to
wind-fertilisation, have again and again recurred, each time producing
some additional complexity, arising from the working up of old rudiments
for new purposes, till there have been reached the marvellous flower
structures of the papilionaceous tribes, of the asclepiads, or of the
orchids.

We thus see that the existing diversity of colour and of structure in
flowers is probably the ultimate result of the ever-recurring struggle
for existence, combined with the ever-changing relations between the
vegetable and animal kingdoms during countless ages. The constant
variability of every part and organ, with the enormous powers of
increase possessed by plants, have enabled them to become again and
again readjusted to each change of condition as it occurred, resulting
in that endless variety, that marvellous complexity, and that exquisite
colouring which excite our admiration in the realm of flowers, and
constitute them the perennial charm and crowning glory of nature.

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