The Project Gutenberg ebook of Darwinism (1889), by Alfred Russel Wallace

But as soon as the proboscis is withdrawn, the two pollen masses begin

to diverge till they are exactly as far apart as are the stigmas of the

flower; and then commences a second movement which brings them down

till they project straight forward nearly at right angles to their first

position, so as exactly to hit against the stigmatic surfaces of the

next flower visited on which they leave a portion of their pollen. The

whole of these motions take about half a minute, and in that time the

moth will usually have flown to another plant, and thus effect the most

beneficial kind of cross-fertilisation.[145] This description will be

better understood by referring to the illustration opposite, from

Darwin's _Fertilisation of Orchids_(Fig. 30).
[Illustration: FIG. 30.--Orchis pyramidalis.]

_The Interpretation of these Facts._

adaptations for cross-fertilisation, the details of which are to be

found in any of the numerous works on the subject,[146] we find

ourselves confronted with the very puzzling question--Why were these

innumerable highly complex adaptations produced, when the very same

result may be effected--and often is effected--by extremely simple

means? Supposing, as we must do, that all flowers were once of simple

and regular forms, like a buttercup or a rose, how did such irregular

and often complicated flowers as the papilionaceous or pea family, the

labiates or sage family, and the infinitely varied and fantastic orchids

ever come into existence? No cause has yet been suggested but the need

of attracting insects to cross-fertilise them; yet the attractiveness of

regular flowers with bright colours and an ample supply of nectar is

equally great, and cross-fertilisation can be quite as effectively

secured in these by any of the four simple methods already described.

Before attempting to suggest a possible solution of this difficult

problem, we have yet to pass in review a large body of curious

adaptations connected with insect fertilisation, and will first call

attention to that portion of the phenomena which throw some light upon

the special colours of flowers in their relation to the various kinds of

insects which visit them. For these facts we are largely indebted to

the exact and long-continued researches of Professor Hermann Müller.

_Summary of Additional Facts bearing on Insect Fertilisation._
1. That the size and colour of a flower are important factors in

determining the visits of insects, is shown by the general fact of more

insects visiting conspicuous than inconspicuous flowers. As a single

instance, the handsome Geranium palustre was observed by Professor

Müller to be visited by sixteen different species of insects, the

equally showy G. pratense by thirteen species, while the smaller and

much less conspicuous G. molle was visited by eight species, and G.

pusillum by only one. In many cases, however, a flower may be very

attractive to only a few species of insects; and Professor Müller

states, as the result of many years' assiduous observation, that "a

species of flower is the more visited by insects the more conspicuous it

is."
2. Sweet odour is usually supplementary to the attraction of colour.

Thus it is rarely present in the largest and most gaudily coloured

flowers which inhabit open places, such as poppies, paeonies,

sunflowers, and many others; while it is often the accompaniment of

inconspicuous flowers, as the mignonette; of such as grow in shady

places, as the violet and primrose; and especially of white or yellowish

flowers, as the white jasmine, clematis, stephanotis, etc.

out their scent only by night, as in our butterfly-orchis (Habenaria

chlorantha); and they sometimes open only at night, as do many of the

evening primroses and other flowers. These flowers are often long tubed

in accordance with the length of the moths' probosces, as in the genus

Pancratium, our butterfly orchis, white jasmine, and a host of others.

sometimes specially adapted to be fertilised by them, as in many pinks

(Dianthus deltoides, D. superbus, D. atrorubens), the corn-cockle

(Lychnis Githago), and many others. Blue flowers are especially

attractive to bees and other hymenoptera (though they frequent flowers

of all colours), no less than sixty-seven species of this order having

been observed to visit the common "sheep's-bit" (Jasione montana). Dull

yellow or brownish flowers, some of which smell like carrion, are

attractive to flies, as the Arum and Aristolochia; while the dull

purplish flowers of the Scrophularia are specially attractive to wasps.

by sham nectaries! In the herb-paris (Paris quadrifolia) the ovary

glistens as if moist, and flies alight on it and carry away pollen to

another flower; while in grass of parnassus (Parnassia palustris) there

are a number of small stalked yellow balls near the base of the flower,

which look like drops of honey but are really dry. In this case there is

a little nectar lower down, but the special attraction is a sham; and as

there are fresh broods of insects every year, it takes time for them to

learn by experience, and thus enough are always deceived to effect

cross-fertilisation.[147] This is analogous to the case of the young

birds, which have to learn by experience the insects that are inedible,

as explained at page 253.

beneficial, as it enables bees to avoid wasting time in visiting those

blossoms which have been already fertilised and their nectar exhausted.

The common lungwort (Pulmonaria officinalis), is at first red, but later

turns blue; and H. Müller observed bees visiting many red flowers in

succession, but neglecting the blue. In South Brazil there is a species

of Lantana, whose flowers are yellow the first day, orange the second,

and purple the third; and Dr. Fritz Müller observed that many

butterflies visited the yellow flowers only, some both the yellow and

the orange flowers, but none the purple.

cases a bright central eye, as in the borage and forget-me-not; or lines

or spots converging to the centre, as in geraniums, pinks, and many

others. This enables insects to go quickly and directly to the opening

of the flower, and is equally important in aiding them to obtain a

better supply of food, and to fertilise a larger number of flowers.

most abound where they grow. Thus the gentians of the lowlands are

adapted to bees, those of the high alps to butterflies only; and while

most species of Rhinanthus (a genus to which our common "yellow rattle"

belongs) are bee-flowers, one high alpine species (R. alpinus) has been

also adapted for fertilisation by butterflies only. The reason of this

is, that in the high alps butterflies are immensely more plentiful than

bees, and flowers adapted to be fertilised by bees can often have their

nectar extracted by butterflies without effecting cross-fertilisation.

It is, therefore, important to have a modification of structure which

shall make butterflies the fertilisers, and this in many cases has been

done.[148]

flowers, because the fine working days are comparatively few, and if no

time is wasted the bees will get more honey, and in doing so will

fertilise more flowers. Now, it has been ascertained by several

observers that many insects, bees especially, keep to one kind of flower

at a time, visiting hundreds of blossoms in succession, and passing over

other species that may be mixed with them. They thus acquire quickness

in going at once to the nectar, and the change of colour in the flower,

or incipient withering when fertilised, enables them to avoid those

flowers that have already had their honey exhausted. It is probably to

assist the insects in keeping to one flower at a time, which is of vital

importance to the perpetuation of the species, that the flowers which

bloom intermingled at the same season are usually very distinct both in

form and colour. In the sandy districts of Surrey, in the early spring,

the copses are gay with three flowers--the primrose, the wood-anemone,

and the lesser celandine, forming a beautiful contrast, while at the

same time the purple and the white dead-nettles abound on hedge banks. A

little later, in the same copses, we have the blue wild hyacinth (Scilla

nutans), the red campion (Lychnis dioica), the pure white great starwort

(Stellaria Holosteum), and the yellow dead-nettle (Lamium Galeobdolon),

all distinct and well-contrasted flowers. In damp meadows in summer we

have the ragged robin (Lychnis Floscuculi), the spotted orchis (O.

maculata), and the yellow rattle (Rhinanthus Crista-galli); while in

drier meadows we have cowslips, ox-eye daisies, and buttercups, all very

distinct both in form and colour. So in cornfields we have the scarlet

poppies, the purple corn-cockle, the yellow corn-marygold, and the blue

cornflower; while on our moors the purple heath and the dwarf gorse make

a gorgeous contrast. Thus the difference of colour which enables the

insect to visit with rapidity and unerring aim a number of flowers of

the same kind in succession, serves to adorn our meadows, banks, woods,

and heaths with a charming variety of floral colour and form at each

season of the year.[149]

_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).

nepenthoides.]

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.

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.

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._

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._

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