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Chapter I
INTRODUCTORY REMARKS.
Various means which favour or determine the cross-fertilisation of plants.
Benefits derived from cross-fertilisation.
Self-fertilisation favourable to the propagation of the species.
Brief history of the subject.
Object of the experiments, and the manner in which they were tried.
Statistical value of the measurements.
The experiments carried on during several successive generations.
Nature of the relationship of the plants in the later generations.
Uniformity of the conditions to which the plants were subjected.
Some apparent and some real causes of error.
Amount of pollen employed.
Arrangement of the work.
Importance of the conclusions.
There is weighty and abundant evidence that the flowers of most kinds of
plants are constructed so as to be occasionally or habitually
cross-fertilised by pollen from another flower, produced either by the
same plant, or generally, as we shall hereafter see reason to believe,
by a distinct plant. Cross-fertilisation is sometimes ensured by the
sexes being separated, and in a large number of cases by the pollen and
stigma of the same flower being matured at different times. Such plants
are called dichogamous, and have been divided into two sub-classes:
proterandrous species, in which the pollen is mature before the stigma,
and proterogynous species, in which the reverse occurs; this latter form
of dichogamy not being nearly so common as the other.
Cross-fertilisation is also ensured, in many cases, by mechanical
contrivances of wonderful beauty, preventing the impregnation of the
flowers by their own pollen. There is a small class of plants, which I
have called dimorphic and trimorphic, but to which Hildebrand has given
the more appropriate name of heterostyled; this class consists of plants
presenting two or three distinct forms, adapted for reciprocal
fertilisation, so that, like plants with separate sexes, they can hardly
fail to be intercrossed in each generation. The male and female organs
of some flowers are irritable, and the insects which touch them get
dusted with pollen, which is thus transported to other flowers. Again,
there is a class, in which the ovules absolutely refuse to be fertilised
by pollen from the same plant, but can be fertilised by pollen from any
other individual of the same species. There are also very many species
which are partially sterile with their own pollen. Lastly, there is a
large class in which the flowers present no apparent obstacle of any
kind to self-fertilisation, nevertheless these plants are frequently
intercrossed, owing to the prepotency of pollen from another individual
or variety over the plant's own pollen.
As plants are adapted by such diversified and effective means for
cross-fertilisation, it might have been inferred from this fact alone
that they derived some great advantage from the process; and it is the
object of the present work to show the nature and importance of the
benefits thus derived. There are, however, some exceptions to the rule
of plants being constructed so as to allow of or to favour
cross-fertilisation, for some few plants seem to be invariably
self-fertilised; yet even these retain traces of having been formerly
adapted for cross-fertilisation. These exceptions need not make us doubt
the truth of the above rule, any more than the existence of some few
plants which produce flowers, and yet never set seed, should make us
doubt that flowers are adapted for the production of seed and the
propagation of the species.
We should always keep in mind the obvious fact that the production of
seed is the chief end of the act of fertilisation; and that this end can
be gained by hermaphrodite plants with incomparably greater certainty by
self-fertilisation, than by the union of the sexual elements belonging
to two distinct flowers or plants. Yet it is as unmistakably plain that
innumerable flowers are adapted for cross-fertilisation, as that the
teeth and talons of a carnivorous animal are adapted for catching prey;
or that the plumes, wings, and hooks of a seed are adapted for its
dissemination. Flowers, therefore, are constructed so as to gain two
objects which are, to a certain extent, antagonistic, and this explains
many apparent anomalies in their structure. The close proximity of the
anthers to the stigma in a multitude of species favours, and often
leads, to self-fertilisation; but this end could have been gained far
more safely if the flowers had been completely closed, for then the
pollen would not have been injured by the rain or devoured by insects,
as often happens. Moreover, in this case, a very small quantity of
pollen would have been sufficient for fertilisation, instead of millions
of grains being produced. But the openness of the flower and the
production of a great and apparently wasteful amount of pollen are
necessary for cross-fertilisation. These remarks are well illustrated by
the plants called cleistogene, which bear on the same stock two kinds of
flowers. The flowers of the one kind are minute and completely closed,
so that they cannot possibly be crossed; but they are abundantly
fertile, although producing an extremely small quantity of pollen. The
flowers of the other kind produce much pollen and are open; and these
can be, and often are, cross-fertilised. Hermann Muller has also made
the remarkable discovery that there are some plants which exist under
two forms; that is, produce on distinct stocks two kinds of
hermaphrodite flowers. The one form bears small flowers constructed for
self-fertilisation; whilst the other bears larger and much more
conspicuous flowers plainly constructed for cross-fertilisation by the
aid of insects; and without their aid these produce no seed.
The adaptation of flowers for cross-fertilisation is a subject which has
interested me for the last thirty-seven years, and I have collected a
large mass of observations, but these are now rendered superfluous by
the many excellent works which have been lately published. In the year
1857 I wrote a short paper on the fertilisation of the kidney bean (1/1.
'Gardeners' Chronicle' 1857 page 725 and 1858 pages 824 and 844. 'Annals
and Magazine of Natural History' 3rd series volume 2 1858 page 462.);
and in 1862 my work 'On the Contrivances by which British and Foreign
Orchids are Fertilised by Insects' appeared. It seemed to me a better
plan to work out one group of plants as carefully as I could, rather
than to publish many miscellaneous and imperfect observations. My
present work is the complement of that on Orchids, in which it was shown
how admirably these plants are constructed so as to permit of, or to
favour, or to necessitate cross-fertilisation. The adaptations for
cross-fertilisation are perhaps more obvious in the Orchideae than in
any other group of plants, but it is an error to speak of them, as some
authors have done, as an exceptional case. The lever-like action of the
stamens of Salvia (described by Hildebrand, Dr. W. Ogle, and others), by
which the anthers are depressed and rubbed on the backs of bees, shows
as perfect a structure as can be found in any orchid. Papilionaceous
flowers, as described by various authors--for instance, by Mr. T.H.
Farrer--offer innumerable curious adaptations for cross-fertilisation.
The case of Posoqueria fragrans (one of the Rubiaceae), is as wonderful
as that of the most wonderful orchid. The stamens, according to Fritz
Muller, are irritable, so that as soon as a moth visits a flower, the
anthers explode and cover the insect with pollen; one of the filaments
which is broader than the others then moves and closes the flower for
about twelve hours, after which time it resumes its original position.
(1/2. 'Botanische Zeitung' 1866 page 129.) Thus the stigma cannot be
fertilised by pollen from the same flower, but only by that brought by a
moth from some other flower. Endless other beautiful contrivances for
this same purpose could be specified.
Long before I had attended to the fertilisation of flowers, a remarkable
book appeared in 1793 in Germany, 'Das Entdeckte Geheimniss der Natur,'
by C.K. Sprengel, in which he clearly proved by innumerable
observations, how essential a part insects play in the fertilisation of
many plants. But he was in advance of his age, and his discoveries were
for a long time neglected. Since the appearance of my book on Orchids,
many excellent works on the fertilisation of flowers, such as those by
Hildebrand, Delpino, Axell and Hermann Muller, and numerous shorter
papers, have been published. (1/3. Sir John Lubbock has given an
interesting summary of the whole subject in his 'British Wild Flowers
considered in relation to Insects' 1875. Hermann Muller's work 'Die
Befruchtung der Blumen durch Insekten' 1873, contains an immense number
of original observations and generalisations. It is, moreover,
invaluable as a repertory with references to almost everything which has
been published on the subject. His work differs from that of all others
in specifying what kinds of insects, as far as known, visit the flowers
of each species. He likewise enters on new ground, by showing not only
that flowers are adapted for their own good to the visits of certain
insects; but that the insects themselves are excellently adapted for
procuring nectar or pollen from certain flowers. The value of H.
Muller's work can hardly be over-estimated, and it is much to be desired
that it should be translated into English. Severin Axell's work is
written in Swedish, so that I have not been able to read it.) A list
would occupy several pages, and this is not the proper place to give
their titles, as we are not here concerned with the means, but with the
results of cross-fertilisation. No one who feels interest in the
mechanism by which nature effects her ends, can read these books and
memoirs without the most lively interest.
From my own observations on plants, guided to a certain extent by the
experience of the breeders of animals, I became convinced many years ago
that it is a general law of nature that flowers are adapted to be
crossed, at least occasionally, by pollen from a distinct plant.
Sprengel at times foresaw this law, but only partially, for it does not
appear that he was aware that there was any difference in power between
pollen from the same plant and from a distinct plant. In the
introduction to his book (page 4) he says, as the sexes are separated in
so many flowers, and as so many other flowers are dichogamous, "it
appears that nature has not willed that any one flower should be
fertilised by its own pollen." Nevertheless, he was far from keeping
this conclusion always before his mind, or he did not see its full
importance, as may be perceived by anyone who will read his observations
carefully; and he consequently mistook the meaning of various
structures. But his discoveries are so numerous and his work so
excellent, that he can well afford to bear a small amount of blame. A
most capable judge, H. Muller, likewise says: "It is remarkable in how
very many cases Sprengel rightly perceived that pollen is necessarily
transported to the stigmas of other flowers of the same species by the
insects which visit them, and yet did not imagine that this
transportation was of any service to the plants themselves." (1/4. 'Die
Befruchtung der Blumen' 1873 page 4. His words are: "Es ist merkwurdig,
in wie zahlreichen Fallen Sprengel richtig erkannte, dass durch die
Besuchenden Insekten der Bluthenstaub mit Nothwendigkeit auf die Narben
anderer Bluthen derselben Art ubertragen wird, ohne auf die Vermuthung
zu kommen, dass in dieser Wirkung der Nutzen des Insektenbesuches fur
die Pflanzen selbst gesucht werden musse.")
Andrew Knight saw the truth much more clearly, for he remarks, "Nature
intended that a sexual intercourse should take place between
neighbouring plants of the same species." (1/5. 'Philosophical
Transactions' 1799 page 202.) After alluding to the various means by
which pollen is transported from flower to flower, as far as was then
imperfectly known, he adds, "Nature has something more in view than that
its own proper males would fecundate each blossom." In 1811 Kolreuter
plainly hinted at the same law, as did afterwards another famous
hybridiser of plants, Herbert. (1/6. Kolreuter 'Mem. de l'Acad. de St.
Petersbourg' tome 3 1809 published 1811 page 197. After showing how well
the Malvaceae are adapted for cross-fertilisation, he asks, "An id
aliquid in recessu habeat, quod hujuscemodi flores nunquam proprio suo
pulvere, sed semper eo aliarum suae speciei impregnentur, merito
quaeritur? Certe natura nil facit frustra." Herbert 'Amaryllidaceae,
with a Treatise on Cross-bred Vegetables' 1837.) But none of these
distinguished observers appear to have been sufficiently impressed with
the truth and generality of the law, so as to insist on it and impress
their beliefs on others.
In 1862 I summed up my observations on Orchids by saying that nature
"abhors perpetual self-fertilisation." If the word perpetual had been
omitted, the aphorism would have been false. As it stands, I believe
that it is true, though perhaps rather too strongly expressed; and I
should have added the self-evident proposition that the propagation of
the species, whether by self-fertilisation or by cross-fertilisation, or
asexually by buds, stolons, etc. is of paramount importance. Hermann
Muller has done excellent service by insisting repeatedly on this latter
point.
It often occurred to me that it would be advisable to try whether
seedlings from cross-fertilised flowers were in any way superior to
those from self-fertilised flowers. But as no instance was known with
animals of any evil appearing in a single generation from the closest
possible interbreeding, that is between brothers and sisters, I thought
that the same rule would hold good with plants; and that it would be
necessary at the sacrifice of too much time to self-fertilise and
intercross plants during several successive generations, in order to
arrive at any result. I ought to have reflected that such elaborate
provisions favouring cross-fertilisation, as we see in innumerable
plants, would not have been acquired for the sake of gaining a distant
and slight advantage, or of avoiding a distant and slight evil.
Moreover, the fertilisation of a flower by its own pollen corresponds to
a closer form of interbreeding than is possible with ordinary bi-sexual
animals; so that an earlier result might have been expected.
I was at last led to make the experiments recorded in the present volume
from the following circumstance. For the sake of determining certain
points with respect to inheritance, and without any thought of the
effects of close interbreeding, I raised close together two large beds
of self-fertilised and crossed seedlings from the same plant of Linaria
vulgaris. To my surprise, the crossed plants when fully grown were
plainly taller and more vigorous than the self-fertilised ones. Bees
incessantly visit the flowers of this Linaria and carry pollen from one
to the other; and if insects are excluded, the flowers produce extremely
few seeds; so that the wild plants from which my seedlings were raised
must have been intercrossed during all previous generations. It seemed
therefore quite incredible that the difference between the two beds of
seedlings could have been due to a single act of self-fertilisation; and
I attributed the result to the self-fertilised seeds not having been
well ripened, improbable as it was that all should have been in this
state, or to some other accidental and inexplicable cause. During the
next year, I raised for the same purpose as before two large beds close
together of self-fertilised and crossed seedlings from the carnation,
Dianthus caryophyllus. This plant, like the Linaria, is almost sterile
if insects are excluded; and we may draw the same inference as before,
namely, that the parent-plants must have been intercrossed during every
or almost every previous generation. Nevertheless, the self-fertilised
seedlings were plainly inferior in height and vigour to the crossed.
My attention was now thoroughly aroused, for I could hardly doubt that
the difference between the two beds was due to the one set being the
offspring of crossed, and the other of self-fertilised flowers.
Accordingly I selected almost by hazard two other plants, which happened
to be in flower in the greenhouse, namely, Mimulus luteus and Ipomoea
purpurea, both of which, unlike the Linaria and Dianthus, are highly
self-fertile if insects are excluded. Some flowers on a single plant of
both species were fertilised with their own pollen, and others were
crossed with pollen from a distinct individual; both plants being
protected by a net from insects. The crossed and self-fertilised seeds
thus produced were sown on opposite sides of the same pots, and treated
in all respects alike; and the plants when fully grown were measured and
compared. With both species, as in the cases of the Linaria and
Dianthus, the crossed seedlings were conspicuously superior in height
and in other ways to the self-fertilised. I therefore determined to
begin a long series of experiments with various plants, and these were
continued for the following eleven years; and we shall see that in a
large majority of cases the crossed beat the self-fertilised plants.
Several of the exceptional cases, moreover, in which the crossed plants
were not victorious, can be explained.
It should be observed that I have spoken for the sake of brevity, and
shall continue to do so, of crossed and self-fertilised seeds,
seedlings, or plants; these terms implying that they are the product of
crossed or self-fertilised flowers. Cross-fertilisation always means a
cross between distinct plants which were raised from seeds and not from
cuttings or buds. Self-fertilisation always implies that the flowers in
question were impregnated with their own pollen.
My experiments were tried in the following manner. A single plant, if it
produced a sufficiency of flowers, or two or three plants were placed
under a net stretched on a frame, and large enough to cover the plant
(together with the pot, when one was used) without touching it. This
latter point is important, for if the flowers touch the net they may be
cross-fertilised by bees, as I have known to happen; and when the net is
wet the pollen may be injured. I used at first "white cotton net," with
very fine meshes, but afterwards a kind of net with meshes one-tenth of
an inch in diameter; and this I found by experience effectually excluded
all insects excepting Thrips, which no net will exclude. On the plants
thus protected several flowers were marked, and were fertilised with
their own pollen; and an equal number on the same plants, marked in a
different manner, were at the same time crossed with pollen from a
distinct plant. The crossed flowers were never castrated, in order to
make the experiments as like as possible to what occurs under nature
with plants fertilised by the aid of insects. Therefore, some of the
flowers which were crossed may have failed to be thus fertilised, and
afterwards have been self-fertilised. But this and some other sources of
error will presently be discussed. In some few cases of spontaneously
self-fertile species, the flowers were allowed to fertilise themselves
under the net; and in still fewer cases uncovered plants were allowed to
be freely crossed by the insects which incessantly visited them. There
are some great advantages and some disadvantages in my having
occasionally varied my method of proceeding; but when there was any
difference in the treatment, it is always so stated under the head of
each species.
Care was taken that the seeds were thoroughly ripened before being
gathered. Afterwards the crossed and self-fertilised seeds were in most
cases placed on damp sand on opposite sides of a glass tumbler covered
by a glass plate, with a partition between the two lots; and the glass
was placed on the chimney-piece in a warm room. I could thus observe the
germination of the seeds. Sometimes a few would germinate on one side
before any on the other, and these were thrown away. But as often as a
pair germinated at the same time, they were planted on opposite sides of
a pot, with a superficial partition between the two; and I thus
proceeded until from half-a-dozen to a score or more seedlings of
exactly the same age were planted on the opposite sides of several pots.
If one of the young seedlings became sickly or was in any way injured,
it was pulled up and thrown away, as well as its antagonist on the
opposite side of the same pot.
As a large number of seeds were placed on the sand to germinate, many
remained after the pairs had been selected, some of which were in a
state of germination and others not so; and these were sown crowded
together on the opposite sides of one or two rather larger pots, or
sometimes in two long rows out of doors. In these cases there was the
most severe struggle for life among the crossed seedlings on one side of
the pot, and the self-fertilised seedlings on the other side, and
between the two lots which grew in competition in the same pot. A vast
number soon perished, and the tallest of the survivors on both sides
when fully grown were measured. Plants treated in this manner, were
subjected to nearly the same conditions as those growing in a state of
nature, which have to struggle to maturity in the midst of a host of
competitors.
On other occasions, from the want of time, the seeds, instead of being
allowed to germinate on damp sand, were sown on the opposite sides of
pots, and the fully grown plants measured. But this plan is less
accurate, as the seeds sometimes germinated more quickly on one side
than on the other. It was however necessary to act in this manner with
some few species, as certain kinds of seeds would not germinate well
when exposed to the light; though the glasses containing them were kept
on the chimney-piece on one side of a room, and some way from the two
windows which faced the north-east. (1/7. This occurred in the plainest
manner with the seeds of Papaver vagum and Delphinium consolida, and
less plainly with those of Adonis aestivalis and Ononis minutissima.
Rarely more than one or two of the seeds of these four species
germinated on the bare sand, though left there for some weeks; but when
these same seeds were placed on earth in pots, and covered with a thin
layer of sand, they germinated immediately in large numbers.)
The soil in the pots in which the seedlings were planted, or the seeds
sown, was well mixed, so as to be uniform in composition. The plants on
the two sides were always watered at the same time and as equally as
possible; and even if this had not been done, the water would have
spread almost equally to both sides, as the pots were not large. The
crossed and self-fertilised plants were separated by a superficial
partition, which was always kept directed towards the chief source of
the light, so that the plants on both sides were equally illuminated. I
do not believe it possible that two sets of plants could have been
subjected to more closely similar conditions, than were my crossed and
self-fertilised seedlings, as grown in the above described manner.
In comparing the two sets, the eye alone was never trusted. Generally
the height of every plant on both sides was carefully measured, often
more than once, namely, whilst young, sometimes again when older, and
finally when fully or almost fully grown. But in some cases, which are
always specified, owing to the want of time, only one or two of the
tallest plants on each side were measured. This plan, which is not a
good one, was never followed (except with the crowded plants raised from
the seeds remaining after the pairs had been planted) unless the tallest
plants on each side seemed fairly to represent the average difference
between those on both sides. It has, however, some great advantages, as
sickly or accidentally injured plants, or the offspring of ill-ripened
seeds, are thus eliminated. When the tallest plants alone on each side
were measured, their average height of course exceeds that of all the
plants on the same side taken together. But in the case of the much
crowded plants raised from the remaining seeds, the average height of
the tallest plants was less than that of the plants in pairs, owing to
the unfavourable conditions to which they were subjected from being
greatly crowded. For our purpose, however, of the comparison of the
crossed and self-fertilised plants, their absolute height signifies
little.
As the plants were measured by an ordinary English standard divided into
inches and eighths of an inch, I have not thought it worth while to
change the fractions into decimals. The average or mean heights were
calculated in the ordinary rough method by adding up the measurements of
all, and dividing the product by the number of plants measured; the
result being here given in inches and decimals. As the different species
grow to various heights, I have always for the sake of easy comparison
given in addition the average height of the crossed plants of each
species taken as 100, and have calculated the average height of the
self-fertilised plant in relation to this standard. With respect to the
crowded plants raised from the seeds remaining after the pairs had been
planted, and of which only some of the tallest on each side were
measured, I have not thought it worth while to complicate the results by
giving separate averages for them and for the pairs, but have added up
all their heights, and thus obtained a single average.
I long doubted whether it was worth while to give the measurements of
each separate plant, but have decided to do so, in order that it may be
seen that the superiority of the crossed plants over the
self-fertilised, does not commonly depend on the presence of two or
three extra fine plants on the one side, or of a few very poor plants on
the other side. Although several observers have insisted in general
terms on the offspring from intercrossed varieties being superior to
either parent-form, no precise measurements have been given (1/8. A
summary of these statements, with references, may be found in my
'Variation of Animals and Plants under Domestication' chapter 17 2nd
edition 1875 volume 2 page 109.); and I have met with no observations on
the effects of crossing and self-fertilising the individuals of the same
variety. Moreover, experiments of this kind require so much time--mine
having been continued during eleven years--that they are not likely soon
to be repeated.
As only a moderate number of crossed and self-fertilised plants were
measured, it was of great importance to me to learn how far the averages
were trustworthy. I therefore asked Mr. Galton, who has had much
experience in statistical researches, to examine some of my tables of
measurements, seven in number, namely, those of Ipomoea, Digitalis,
Reseda lutea, Viola, Limnanthes, Petunia, and Zea. I may premise that if
we took by chance a dozen or score of men belonging to two nations and
measured them, it would I presume be very rash to form any judgment from
such small numbers on their average heights. But the case is somewhat
different with my crossed and self-fertilised plants, as they were of
exactly the same age, were subjected from first to last to the same
conditions, and were descended from the same parents. When only from two
to six pairs of plants were measured, the results are manifestly of
little or no value, except in so far as they confirm and are confirmed
by experiments made on a larger scale with other species. I will now
give the report on the seven tables of measurements, which Mr. Galton
has had the great kindness to draw up for me.
["I have examined the measurements of the plants with care, and by many
statistical methods, to find out how far the means of the several sets
represent constant realities, such as would come out the same so long as
the general conditions of growth remained unaltered. The principal
methods that were adopted are easily explained by selecting one of the
shorter series of plants, say of Zea mays, for an example."
TABLE 1/1. Zea mays (young plants). (Mr. Galton.)
Heights of Plants in inches:
Column 1: Number (Name) of Pot.
Column 2: Crossed, as recorded by Mr. Darwin.
Column 3: Self-fertilised, as recorded by Mr. Darwin.
Column 4: Crossed, in Separate Pots, arranged in order of magnitude.
Column 5: Self-fertilised, in Separate Pots, arranged in order of magnitude.
Column 6: Crossed, in a Single Series, arranged in order of magnitude.
Column 7: Self-fertilised, in a Single Series, arranged in order of
magnitude.
Column 8: Difference, in a Single Series, arranged in order of magnitude.
Pot 1 : 23 4/8 : 17 3/8 :: 23 4/8 : 20 3/8 :: 23 4/8 : 20 3/8 : -3 1/8.
Pot 1 : 12 : 20 3/8 :: 21 : 20 :: 23 2/8 : 20 : -3 2/8.
Pot 1 : 21 : 20 :: 12 : 17 3/8 :: 23 : 20 : -3.
Pot 1 : - : - :: - : - :: 22 1/8 : 18 5/8 : -3 4/8.
Pot 1 : 22 : 20 :: 22 : 20 :: 22 1/8 : 18 5/8 : -3 4/8.
Pot 2 : 19 1/8 : 18 3/8 :: 21 4/8 : 18 5/8 :: 22 : 18 3/8 : -3 5/8.
Pot 2 : 21 4/8 : 18 5/8 :: 19 1/8 : 18 3/8 :: 21 5/8 : 18 : -3 5/8.
Pot 2 : - : - :: - : - :: 21 4/8 : 18 : -3 4/8.
Pot 2 : 22 1/8 : 18 5/8 :: 23 2/8 : 18 5/8 :: 21 : 18 : -3.
Pot 2 : 20 3/8 : 15 2/8 :: 22 1/8 : 18 :: 21 : 17 3/8 : -3 5/8.
Pot 3 : 18 2/8 : 16 4/8 :: 21 5/8 : 16 4/8 :: 20 3/8 : 16 4/8 : -3 7/8.
Pot 3 : 21 5/8 : 18 :: 20 3/8 : 16 2/8 :: 19 1/8 : 16 2/8 : -2 7/8.
Pot 3 : 23 2/8 : 16 2/8 :: 18 2/8 : 15 2/8 :: 18 2/8 : 15 4/8 : -2 6/8.
Pot 3 : - : - :: - : - :: 12 : 15 2/8 : +3 2/8.
Pot 3 : 21 : 18 :: 23 : 18 :: 12 : 12 6/8 : +0 6/8.
Pot 4 : 22 1/8 : 12 6/8 :: 22 1/8 : 18.
Pot 4 : 23 : 15 4/8 :: 21 : 15 4/8.
Pot 4 : 12 : 18 :: 12 : 12 6/8.
"The observations as I received them are shown in Table 1/1, Columns 2
and 3, where they certainly have no prima facie appearance of
regularity. But as soon as we arrange them the in order of their
magnitudes, as in columns 4 and 5, the case is materially altered. We
now see, with few exceptions, that the largest plant on the crossed side
in each pot exceeds the largest plant on the self-fertilised side, that
the second exceeds the second, the third the third, and so on. Out of
the fifteen cases in the table, there are only two exceptions to this
rule. We may therefore confidently affirm that a crossed series will
always be found to exceed a self-fertilised series, within the range of
the conditions under which the present experiment has been made."
TABLE 1/2.
Column 1: Number (Name) of Pot.
Column 2: Crossed.
Column 3: Self-fertilised.
Column 4: Difference.
Pot 1 : 18 7/8 : 19 2/8 : +0 3/8.
Pot 2 : 20 7/8 : 19 : -1 7/8.
Pot 3 : 21 1/8 : 16 7/8 : -4 2/8.
Pot 4 : 19 6/8 : 16 : -3 6/8.
"Next as regards the numerical estimate of this excess. The mean values
of the several groups are so discordant, as is shown in Table 1/2, that
a fairly precise numerical estimate seems impossible. But the
consideration arises, whether the difference between pot and pot may not
be of much the same order of importance as that of the other conditions
upon which the growth of the plants has been modified. If so, and only
on that condition, it would follow that when all the measurements,
either of the crossed or the self-fertilised plants, were combined into
a single series, that series would be statistically regular. The
experiment is tried in Table 1/1, columns 7 and 8, where the regularity
is abundantly clear, and justifies us in considering its mean as
perfectly reliable. I have protracted these measurements, and revised
them in the usual way, by drawing a curve through them with a free hand,
but the revision barely modifies the means derived from the original
observations. In the present, and in nearly all the other cases, the
difference between the original and revised means is under 2 per cent of
their value. It is a very remarkable coincidence that in the seven kinds
of plants, whose measurements I have examined, the ratio between the
heights of the crossed and of the self-fertilised ranges in five cases
within very narrow limits. In Zea mays it is as 100 to 84, and in the
others it ranges between 100 to 76 and 100 to 86."
"The determination of the variability (measured by what is technically
called the 'probable error') is a problem of more delicacy than that of
determining the means, and I doubt, after making many trials, whether it
is possible to derive useful conclusions from these few observations. We
ought to have measurements of at least fifty plants in each case, in
order to be in a position to deduce fair results. One fact, however,
bearing on variability, is very evident in most cases, though not in Zea
mays, namely, that the self-fertilised plants include the larger number
of exceptionally small specimens, while the crossed are more generally
full grown."
"Those groups of cases in which measurements have been made of a few of
the tallest plants that grew in rows, each of which contained a
multitude of plants, show very clearly that the crossed plants exceed
the self-fertilised in height, but they do not tell by inference
anything about their respective mean values. If it should happen that a
series is known to follow the law of error or any other law, and if the
number of individuals in the series is known, it would be always
possible to reconstruct the whole series when a fragment of it has been
given. But I find no such method to be applicable in the present case.
The doubt as to the number of plants in each row is of minor importance;
the real difficulty lies in our ignorance of the precise law followed by
the series. The experience of the plants in pots does not help us to
determine that law, because the observations of such plants are too few
to enable us to lay down more than the middle terms of the series to
which they belong with any sort of accuracy, whereas the cases we are
now considering refer to one of its extremities. There are other special
difficulties which need not be gone into, as the one already mentioned
is a complete bar."]
Mr. Galton sent me at the same time graphical representations which he
had made of the measurements, and they evidently form fairly regular
curves. He appends the words "very good" to those of Zea and Limnanthes.
He also calculated the average height of the crossed and self-fertilised
plants in the seven tables by a more correct method than that followed
by me, namely, by including the heights, as estimated in accordance with
statistical rules, of a few plants which died before they were measured;
whereas I merely added up the heights of the survivors, and divided the
sum by their number. The difference in our results is in one way highly
satisfactory, for the average heights of the self-fertilised plants, as
deduced by Mr. Galton, is less than mine in all the cases excepting one,
in which our averages are the same; and this shows that I have by no
means exaggerated the superiority of the crossed over the
self-fertilised plants.
After the heights of the crossed and self-fertilised plants had been
taken, they were sometimes cut down close to the ground, and an equal
number of both weighed. This method of comparison gives very striking
results, and I wish that it had been oftener followed. Finally a record
was often kept of any marked difference in the rate of germination of
the crossed and self-fertilised seeds,--of the relative periods of
flowering of the plants raised from them,--and of their productiveness,
that is, of the number of seed-capsules which they produced and of the
average number of seeds which each capsule contained.
When I began my experiments I did not intend to raise crossed and
self-fertilised plants for more than a single generation; but as soon as
the plants of the first generation were in flower I thought that I would
raise one more generation, and acted in the following manner. Several
flowers on one or more of the self-fertilised plants were again
self-fertilised; and several flowers on one or more of the crossed
plants were fertilised with pollen from another crossed plant of the
same lot. Having thus once begun, the same method was followed for as
many as ten successive generations with some of the species. The seeds
and seedlings were always treated in exactly the same manner as already
described. The self-fertilised plants, whether originally descended from
one or two mother-plants, were thus in each generation as closely
interbred as was possible; and I could not have improved on my plan. But
instead of crossing one of the crossed plants with another crossed
plant, I ought to have crossed the self-fertilised plants of each
generation with pollen taken from a non-related plant--that is, one
belonging to a distinct family or stock of the same species and variety.
This was done in several cases as an additional experiment, and gave
very striking results. But the plan usually followed was to put into
competition and compare intercrossed plants, which were almost always
the offspring of more or less closely related plants, with the
self-fertilised plants of each succeeding generation;--all having been
grown under closely similar conditions. I have, however, learnt more by
this method of proceeding, which was begun by an oversight and then
necessarily followed, than if I had always crossed the self-fertilised
plants of each succeeding generation with pollen from a fresh stock.
I have said that the crossed plants of the successive generations were
almost always inter-related. When the flowers on an hermaphrodite plant
are crossed with pollen taken from a distinct plant, the seedlings thus
raised may be considered as hermaphrodite brothers or sisters; those
raised from the same capsule being as close as twins or animals of the
same litter. But in one sense the flowers on the same plant are distinct
individuals, and as several flowers on the mother-plant were crossed by
pollen taken from several flowers on the father-plant, such seedlings
would be in one sense half-brothers or sisters, but more closely related
than are the half-brothers and sisters of ordinary animals. The flowers
on the mother-plant were, however, commonly crossed by pollen taken from
two or more distinct plants; and in these cases the seedlings might be
called with more truth half-brothers or sisters. When two or three
mother-plants were crossed, as often happened, by pollen taken from two
or three father-plants (the seeds being all intermingled), some of the
seedlings of the first generation would be in no way related, whilst
many others would be whole or half-brothers and sisters. In the second
generation a large number of the seedlings would be what may be called
whole or half first-cousins, mingled with whole and half-brothers and
sisters, and with some plants not at all related. So it would be in the
succeeding generations, but there would also be many cousins of the
second and more remote degrees. The relationship will thus have become
more and more inextricably complex in the later generations; with most
of the plants in some degree and many of them closely related.
I have only one other point to notice, but this is one of the highest
importance; namely, that the crossed and self-fertilised plants were
subjected in the same generation to as nearly similar and uniform
conditions as was possible. In the successive generations they were
exposed to slightly different conditions as the seasons varied, and they
were raised at different periods. But in other respects all were treated
alike, being grown in pots in the same artificially prepared soil, being
watered at the same time, and kept close together in the same greenhouse
or hothouse. They were therefore not exposed during successive years to
such great vicissitudes of climate as are plants growing out of doors.
ON SOME APPARENT AND REAL CAUSES OF ERROR IN MY EXPERIMENTS.
It has been objected to such experiments as mine, that covering plants
with a net, although only for a short time whilst in flower, may affect
their health and fertility. I have seen no such effect except in one
instance with a Myosotis, and the covering may not then have been the
real cause of injury. But even if the net were slightly injurious, and
certainly it was not so in any high degree, as I could judge by the
appearance of the plants and by comparing their fertility with that of
neighbouring uncovered plants, it would not have vitiated my
experiments; for in all the more important cases the flowers were
crossed as well as self-fertilised under a net, so that they were
treated in this respect exactly alike.
As it is impossible to exclude such minute pollen-carrying insects as
Thrips, flowers which it was intended to fertilise with their own pollen
may sometimes have been afterwards crossed with pollen brought by these
insects from another flower on the same plant; but as we shall hereafter
see, a cross of this kind does not produce any effect, or at most only a
slight one. When two or more plants were placed near one another under
the same net, as was often done, there is some real though not great
danger of the flowers which were believed to be self-fertilised being
afterwards crossed with pollen brought by Thrips from a distinct plant.
I have said that the danger is not great because I have often found that
plants which are self-sterile, unless aided by insects, remained sterile
when several plants of the same species were placed under the same net.
If, however, the flowers which had been presumably self-fertilised by me
were in any case afterwards crossed by Thrips with pollen brought from a
distinct plant, crossed seedlings would have been included amongst the
self-fertilised; but it should be especially observed that this
occurrence would tend to diminish and not to increase any superiority in
average height, fertility, etc., of the crossed over the self-fertilised
plants.
As the flowers which were crossed were never castrated, it is probable
or even almost certain that I sometimes failed to cross-fertilise them
effectually, and that they were afterwards spontaneously
self-fertilised. This would have been most likely to occur with
dichogamous species, for without much care it is not easy to perceive
whether their stigmas are ready to be fertilised when the anthers open.
But in all cases, as the flowers were protected from wind, rain, and the
access of insects, any pollen placed by me on the stigmatic surface
whilst it was immature, would generally have remained there until the
stigma was mature; and the flowers would then have been crossed as was
intended. Nevertheless, it is highly probable that self-fertilised
seedlings have sometimes by this means got included amongst the crossed
seedlings. The effect would be, as in the former case, not to exaggerate
but to diminish any average superiority of the crossed over the
self-fertilised plants.
Errors arising from the two causes just named, and from others,--such as
some of the seeds not having been thoroughly ripened, though care was
taken to avoid this error--the sickness or unperceived injury of any of
the plants,--will have been to a large extent eliminated, in those cases
in which many crossed and self-fertilised plants were measured and an
average struck. Some of these causes of error will also have been
eliminated by the seeds having been allowed to germinate on bare damp
sand, and being planted in pairs; for it is not likely that ill-matured
and well-matured, or diseased and healthy seeds, would germinate at
exactly the same time. The same result will have been gained in the
several cases in which only a few of the tallest, finest, and healthiest
plants on each side of the pots were measured.
Kolreuter and Gartner have proved that with some plants several, even as
many as from fifty to sixty, pollen-grains are necessary for the
fertilisation of all the ovules in the ovarium. (1/9. 'Kentniss der
Befruchtung' 1844 page 345. Naudin 'Nouvelles Archives du Museum' tome 1
page 27.) Naudin also found in the case of Mirabilis that if only one or
two of its very large pollen-grains were placed on the stigma, the
plants raised from such seeds were dwarfed. I was therefore careful to
give an amply sufficient supply of pollen, and generally covered the
stigma with it; but I did not take any special pains to place exactly
the same amount on the stigmas of the self-fertilised and crossed
flowers. After having acted in this manner during two seasons, I
remembered that Gartner thought, though without any direct evidence,
that an excess of pollen was perhaps injurious; and it has been proved
by Spallanzani, Quatrefages, and Newport, that with various animals an
excess of the seminal fluid entirely prevents fertilisation. (1/10.
'Transactions of the Philosophical Society' 1853 pages 253-258.) It was
therefore necessary to ascertain whether the fertility of the flowers
was affected by applying a rather small and an extremely large quantity
of pollen to the stigma. Accordingly a very small mass of pollen-grains
was placed on one side of the large stigma in sixty-four flowers of
Ipomoea purpurea, and a great mass of pollen over the whole surface of
the stigma in sixty-four other flowers. In order to vary the experiment,
half the flowers of both lots were on plants produced from
self-fertilised seeds, and the other half on plants from crossed seeds.
The sixty-four flowers with an excess of pollen yielded sixty-one
capsules; and excluding four capsules, each of which contained only a
single poor seed, the remainder contained on an average 5.07 seeds per
capsule. The sixty-four flowers with only a little pollen placed on one
side of the stigma yielded sixty-three capsules, and excluding one from
the same cause as before, the remainder contained on an average 5.129
seeds. So that the flowers fertilised with little pollen yielded rather
more capsules and seeds than did those fertilised with an excess; but
the difference is too slight to be of any significance. On the other
hand, the seeds produced by the flowers with an excess of pollen were a
little heavier of the two; for 170 of them weighed 79.67 grains, whilst
170 seeds from the flowers with very little pollen weighed 79.20 grains.
Both lots of seeds having been placed on damp sand presented no
difference in their rate of germination. We may therefore conclude that
my experiments were not affected by any slight difference in the amount
of pollen used; a sufficiency having been employed in all cases.
The order in which our subject will be treated in the present volume is
as follows. A long series of experiments will first be given in Chapters
2 to 6. Tables will afterwards be appended, showing in a condensed form
the relative heights, weights, and fertility of the offspring of the
various crossed and self-fertilised species. Another table exhibits the
striking results from fertilising plants, which during several
generations had either been self-fertilised or had been crossed with
plants kept all the time under closely similar conditions, with pollen
taken from plants of a distinct stock and which had been exposed to
different conditions. In the concluding chapters various related points
and questions of general interest will be discussed.
Anyone not specially interested in the subject need not attempt to read
all the details (marked []); though they possess, I think, some value,
and cannot be all summarised. But I would suggest to the reader to take
as an example the experiments on Ipomoea in Chapter 2; to which may be
added those on Digitalis, Origanum, Viola, or the common cabbage, as in
all these cases the crossed plants are superior to the self-fertilised
in a marked degree, but not in quite the same manner. As instances of
self-fertilised plants being equal or superior to the crossed, the
experiments on Bartonia, Canna, and the common pea ought to be read; but
in the last case, and probably in that of Canna, the want of any
superiority in the crossed plants can be explained.
Species were selected for experiment belonging to widely distinct
families, inhabiting various countries. In some few cases several genera
belonging to the same family were tried, and these are grouped together;
but the families themselves have been arranged not in any natural order,
but in that which was the most convenient for my purpose. The
experiments have been fully given, as the results appear to me of
sufficient value to justify the details. Plants bearing hermaphrodite
flowers can be interbred more closely than is possible with bisexual
animals, and are therefore well-fitted to throw light on the nature and
extent of the good effects of crossing, and on the evil effects of close
interbreeding or self-fertilisation. The most important conclusion at
which I have arrived is that the mere act of crossing 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,
or to what we call in our ignorance spontaneous variation. 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, and this stands in the closest
connection with life itself. It throws light on the origin of the two
sexes and on their separation or union in the same individual, and
lastly on the whole subject of hybridism, which is one of the greatest
obstacles to the general acceptance and progress of the great principle
of evolution.
In order to avoid misapprehension, I beg leave to repeat that throughout
this volume a crossed plant, seedling, or seed, means one of crossed
PARENTAGE, that is, one derived from a flower fertilised with pollen
from a distinct plant of the same species. And that a self-fertilised
plant, seedling, or seed, means one of self-fertilised PARENTAGE, that
is, one derived from a flower fertilised with pollen from the same
flower, or sometimes, when thus stated, from another flower on the same
plant.
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