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Chapter XII
GENERAL RESULTS.
Cross-fertilisation proved to be beneficial, and self-fertilisation
injurious.
Allied species differ greatly in the means by which cross-fertilisation
is favoured and self-fertilisation avoided.
The benefits and evils of the two processes depend on the degree of
differentiation in the sexual elements.
The evil effects not due to the combination of morbid tendencies in the
parents.
Nature of the conditions to which plants are subjected when growing near
together in a state of nature or under culture, and the effects of such
conditions.
Theoretical considerations with respect to the interaction of
differentiated sexual elements.
Practical lessons.
Genesis of the two sexes.
Close correspondence between the effects of cross-fertilisation and
self-fertilisation, and of the legitimate and illegitimate unions of
heterostyled plants, in comparison with hybrid unions.
The first and most important of the conclusions which may be drawn from
the observations given in this volume, is that cross-fertilisation is
generally beneficial, and self-fertilisation injurious. This is shown by
the difference in height, weight, constitutional vigour, and fertility
of the offspring from crossed and self-fertilised flowers, and in the
number of seeds produced by the parent-plants. With respect to the
second of these two propositions, namely, that self-fertilisation is
generally injurious, we have abundant evidence. The structure of the
flowers in such plants as Lobelia ramosa, Digitalis purpurea, etc.,
renders the aid of insects almost indispensable for their fertilisation;
and bearing in mind the prepotency of pollen from a distinct individual
over that from the same individual, such plants will almost certainly
have been crossed during many or all previous generations. So it must
be, owing merely to the prepotency of foreign pollen, with cabbages and
various other plants, the varieties of which almost invariably
intercross when grown together. The same inference may be drawn still
more surely with respect to those plants, such as Reseda and
Eschscholtzia, which are sterile with their own pollen, but fertile with
that from any other individual. These several plants must therefore have
been crossed during a long series of previous generations, and the
artificial crosses in my experiments cannot have increased the vigour of
the offspring beyond that of their progenitors. Therefore the difference
between the self-fertilised and crossed plants raised by me cannot be
attributed to the superiority of the crossed, but to the inferiority of
the self-fertilised seedlings, due to the injurious effects of
self-fertilisation.
With respect to the first proposition, namely, that cross-fertilisation
is generally beneficial, we likewise have excellent evidence. Plants of
Ipomoea were intercrossed for nine successive generations; they were
then again intercrossed, and at the same time crossed with a plant of a
fresh stock, that is, one brought from another garden; and the offspring
of this latter cross were to the intercrossed plants in height as 100 to
78, and in fertility as 100 to 51. An analogous experiment with
Eschscholtzia gave a similar result, as far as fertility was concerned.
In neither of these cases were any of the plants the product of
self-fertilisation. Plants of Dianthus were self-fertilised for three
generations, and this no doubt was injurious; but when these plants were
fertilised by a fresh stock and by intercrossed plants of the same
stock, there was a great difference in fertility between the two sets of
seedlings, and some difference in their height. Petunia offers a nearly
parallel case. With various other plants, the wonderful effects of a
cross with a fresh stock may be seen in Table 7/C. Several accounts have
also been published of the extraordinary growth of seedlings from a
cross between two varieties of the same species, some of which are known
never to fertilise themselves; so that here neither self-fertilisation
nor relationship even in a remote degree can have come into play. (12/1.
See 'Variation under Domestication' chapter 19 2nd edition volume 2 page
159.) We may therefore conclude that the above two propositions are
true,--that cross-fertilisation is generally beneficial and
self-fertilisation injurious to the offspring.
That certain plants, for instance, Viola tricolor, Digitalis purpurea,
Sarothamnus scoparius, Cyclamen persicum, etc., which have been
naturally cross-fertilised for many or all previous generations, should
suffer to an extreme degree from a single act of self-fertilisation is a
most surprising fact. Nothing of the kkind has been observed in our
domestic animals; but then we must remember that the closest possible
interbreeding with such animals, that is, between brothers and sisters,
cannot be considered as nearly so close a union as that between the
pollen and ovules of the same flower. Whether the evil from
self-fertilisation goes on increasing during successive generations is
not as yet known; but we may infer from my experiments that the increase
if any is far from rapid. After plants have been propagated by
self-fertilisation for several generations, a single cross with a fresh
stock restores their pristine vigour; and we have a strictly analogous
result with our domestic animals. (12/2. Ibid chapter 19 2nd edition
volume 2 page 159.) The good effects of cross-fertilisation are
transmitted by plants to the next generation; and judging from the
varieties of the common pea, to many succeeding generations. But this
may merely be that crossed plants of the first generation are extremely
vigorous, and transmit their vigour, like any other character, to their
successors.
Notwithstanding the evil which many plants suffer from
self-fertilisation, they can be thus propagated under favourable
conditions for many generations, as shown by some of my experiments, and
more especially by the survival during at least half a century of the
same varieties of the common pea and sweet-pea. The same conclusion
probably holds good with several other exotic plants, which are never or
most rarely cross-fertilised in this country. But all these plants, as
far as they have been tried, profit greatly by a cross with a fresh
stock. Some few plants, for instance, Ophrys apifera, have almost
certainly been propagated in a state of nature for thousands of
generations without having been once intercrossed; and whether they
would profit by a cross with a fresh stock is not known. But such cases
ought not to make us doubt that as a general rule crossing is
beneficial, any more than the existence of plants which, in a state of
nature, are propagated exclusively by rhizomes, stolons, etc. (their
flowers never producing seeds), (12/3. I have given several cases in my
'Variation under Domestication' chapter 18 2nd edition volume 2 page
152.) (their flowers never producing seeds), should make us doubt that
seminal generation must have some great advantage, as it is the common
plan followed by nature. Whether any species has been reproduced
asexually from a very remote period cannot, of course, be ascertained.
Our sole means for forming any judgment on this head is the duration of
the varieties of our fruit trees which have been long propagated by
grafts or buds. Andrew Knight formerly maintained that under these
circumstances they always become weakly, but this conclusion has been
warmly disputed by others. A recent and competent judge, Professor Asa
Gray, leans to the side of Andrew Knight, which seems to me, from such
evidence as I have been able to collect, the more probable view,
notwithstanding many opposed facts. (12/4. 'Darwiniana: Essays and
Reviews pertaining to Darwinism' 1876 page 338.)
The means for favouring cross-fertilisation and preventing
self-fertilisation, or conversely for favouring self-fertilisation and
preventing to a certain extent cross-fertilisation, are wonderfully
diversified; and it is remarkable that these differ widely in closely
allied plants,--in the species of the same genus, and sometimes in the
individuals of the same species. (12/5. Hildebrand has insisted strongly
to this effect in his valuable observations on the fertilisation of the
Gramineae: 'Monatsbericht K. Akad. Berlin' October 1872 page 763.) It is
not rare to find hermaphrodite plants and others with separated sexes
within the same genus; and it is common to find some of the species
dichogamous and others maturing their sexual elements simultaneously.
The dichogamous genus Saxifraga contains proterandrous and proterogynous
species. (12/6. Dr. Engler 'Botanische Zeitung' 1868 page 833.) Several
genera include both heterostyled (dimorphic or trimorphic forms) and
homostyled species. Ophrys offers a remarkable instance of one species
having its structure manifestly adapted for self-fertilisation, and
other species as manifestly adapted for cross-fertilisation. Some
con-generic species are quite sterile and others quite fertile with
their own pollen. From these several causes we often find within the
same genus species which do not produce seeds, while others produce an
abundance, when insects are excluded. Some species bear cleistogene
flowers which cannot be crossed, as well as perfect flowers, whilst
others in the same genus never produce cleistogene flowers. Some species
exist under two forms, the one bearing conspicuous flowers adapted for
cross-fertilisation, the other bearing inconspicuous flowers adapted for
self-fertilisation, whilst other species in the same genus present only
a single form. Even with the individuals of the same species, the degree
of self-sterility varies greatly, as in Reseda. With polygamous plants,
the distribution of the sexes differs in the individuals of the same
species. The relative period at which the sexual elements in the same
flower are mature, differs in the varieties of Pelargonium; and Carriere
gives several cases, showing that the period varies according to the
temperature to which the plants are exposed. (12/7. 'Des Varieties' 1865
page 30.)
This extraordinary diversity in the means for favouring or preventing
cross- and self-fertilisation in closely allied forms, probably depends
on the results of both processes being highly beneficial to the species,
but directly opposed in many ways to one another and dependent on
variable conditions. Self-fertilisation assures the production of a
large supply of seeds; and the necessity or advantage of this will be
determined by the average length of life of the plant, which largely
depends on the amount of destruction suffered by the seeds and
seedlings. This destruction follows from the most various and variable
causes, such as the presence of animals of several kinds, and the growth
of surrounding plants. The possibility of cross-fertilisation depends
mainly on the presence and number of certain insects, often of insects
belonging to special groups, and on the degree to which they are
attracted to the flowers of any particular species in preference to
other flowers,--all circumstances likely to change. Moreover, the
advantages which follow from cross-fertilisation differ much in
different plants, so that it is probable that allied plants would often
profit in different degrees by cross-fertilisation. Under these
extremely complex and fluctuating conditions, with two somewhat opposed
ends to be gained, namely, the safe propagation of the species and the
production of cross-fertilised, vigorous offspring, it is not surprising
that allied forms should exhibit an extreme diversity in the means which
favour either end. If, as there is reason to suspect, self-fertilisation
is in some respects beneficial, although more than counterbalanced by
the advantages derived from a cross with a fresh stock, the problem
becomes still more complicated.
As I only twice experimented on more than a single species in a genus, I
cannot say whether the crossed offspring of the several species within
the same genus differ in their degree of superiority over their
self-fertilised brethren; but I should expect that this would often
prove to be the case from what was observed with the two species of
Lobelia and with the individuals of the same species of Nicotiana. The
species belonging to distinct genera in the same family certainly differ
in this respect. The effects of cross- and self-fertilisation may be
confined either to the growth or to the fertility of the offspring, but
generally extends to both qualities. There does not seem to exist any
close correspondence between the degree to which their offspring profit
by this process; but we may easily err on this head, as there are two
means for ensuring cross-fertilisation which are not externally
perceptible, namely, self-sterility and the prepotent fertilising
influence of pollen from another individual. Lastly, it has been shown
in a former chapter that the effect produced by cross and
self-fertilisation on the fertility of the parent-plants does not always
correspond with that produced on the height, vigour, and fertility of
their offspring. The same remark applies to crossed and self-fertilised
seedlings when these are used as the parent-plants. This want of
correspondence probably depends, at least in part, on the number of
seeds produced being chiefly determined by the number of the
pollen-tubes which reach the ovules, and this will be governed by the
reaction between the pollen and the stigmatic secretion or tissues;
whereas the growth and constitutional vigour of the offspring will be
chiefly determined, not only by the number of pollen-tubes reaching the
ovules, but by the nature of the reaction between the contents of the
pollen-grains and ovules.
There are two other important conclusions which may be deduced from my
observations: firstly, that the advantages of cross-fertilisation do not
follow from some mysterious virtue in the mere union of two distinct
individuals, but from such individuals having been subjected during
previous generations to different conditions, or to their having varied
in a manner commonly called spontaneous, so that in either case their
sexual elements have been in some degree differentiated. And secondly,
that the injury from self-fertilisation follows from the want of such
differentiation in the sexual elements. These two propositions are fully
established by my experiments. Thus, when plants of the Ipomoea and of
the Mimulus, which had been self-fertilised for the seven previous
generations and had been kept all the time under the same conditions,
were intercrossed one with another, the offspring did not profit in the
least by the cross. Mimulus offers another instructive case, showing
that the benefit of a cross depends on the previous treatment of the
progenitors: plants which had been self-fertilised for the eight
previous generations were crossed with plants which had been
intercrossed for the same number of generations, all having been kept
under the same conditions as far as possible; seedlings from this cross
were grown in competition with others derived from the same
self-fertilised mother-plant crossed by a fresh stock; and the latter
seedlings were to the former in height as 100 to 52, and in fertility as
100 to 4. An exactly parallel experiment was tried on Dianthus, with
this difference, that the plants had been self-fertilised only for the
three previous generations, and the result was similar though not so
strongly marked. The foregoing two cases of the offspring of Ipomoea and
Eschscholtzia, derived from a cross with a fresh stock, being as much
superior to the intercrossed plants of the old stock, as these latter
were to the self-fertilised offspring, strongly supports the same
conclusion. A cross with a fresh stock or with another variety seems to
be always highly beneficial, whether or not the mother-plants have been
intercrossed or self-fertilised for several previous generations. The
fact that a cross between two flowers on the same plant does no good or
very little good, is likewise a strong corroboration of our conclusion;
for the sexual elements in the flowers on the same plant can rarely have
been differentiated, though this is possible, as flower-buds are in one
sense distinct individuals, sometimes varying and differing from one
another in structure or constitution. Thus the proposition that the
benefit from cross-fertilisation depends on the plants which are crossed
having been subjected during previous generations to somewhat different
conditions, or to their having varied from some unknown cause as if they
had been thus subjected, is securely fortified on all sides.
Before proceeding any further, the view which has been maintained by
several physiologists must be noticed, namely, that all the evils from
breeding animals too closely, and no doubt, as they would say, from the
self-fertilisation of plants, is the result of the increase of some
morbid tendency or weakness of constitution common to the closely
related parents, or to the two sexes of hermaphrodite plants.
Undoubtedly injury has often thus resulted; but it is a vain attempt to
extend this view to the numerous cases given in my Tables. It should be
remembered that the same mother-plant was both self-fertilised and
crossed, so that if she had been unhealthy she would have transmitted
half her morbid tendencies to her crossed offspring. But plants
appearing perfectly healthy, some of them growing wild, or the immediate
offspring of wild plants, or vigorous common garden-plants, were
selected for experiment. Considering the number of species which were
tried, it is nothing less than absurd to suppose that in all these cases
the mother-plants, though not appearing in any way diseased, were weak
or unhealthy in so peculiar a manner that their self-fertilised
seedlings, many hundreds in number, were rendered inferior in height,
weight, constitutional vigour and fertility to their crossed offspring.
Moreover, this belief cannot be extended to the strongly marked
advantages which invariably follow, as far as my experience serves, from
intercrossing the individuals of the same variety or of distinct
varieties, if these have been subjected during some generations to
different conditions.
It is obvious that the exposure of two sets of plants during several
generations to different conditions can lead to no beneficial results,
as far as crossing is concerned, unless their sexual elements are thus
affected. That every organism is acted on to a certain extent by a
change in its environment, will not, I presume, be disputed. It is
hardly necessary to advance evidence on this head; we can perceive the
difference between individual plants of the same species which have
grown in somewhat more shady or sunny, dry or damp places. Plants which
have been propagated for some generations under different climates or at
different seasons of the year transmit different constitutions to their
seedlings. Under such circumstances, the chemical constitution of their
fluids and the nature of their tissues are often modified. (12/8.
Numerous cases together with references are given in my 'Variation under
Domestication' chapter 23 2nd edition volume 2 page 264. With respect to
animals, Mr. Brackenridge 'A Contribution to the Theory of Diathesis'
Edinburgh 1869, has well shown that the different organs of animals are
excited into different degrees of activity by differences of temperature
and food, and become to a certain extent adapted to them.) Many other
such facts could be adduced. In short, every alteration in the function
of a part is probably connected with some corresponding, though often
quite imperceptible change in structure or composition.
Whatever affects an organism in any way, likewise tends to act on its
sexual elements. We see this in the inheritance of newly acquired
modifications, such as those from the increased use or disuse of a part,
and even from mutilations if followed by disease. (12/9. 'Variation
under Domestication' chapter 12 2nd edition volume 1 page 466.) We have
abundant evidence how susceptible the reproductive system is to changed
conditions, in the many instances of animals rendered sterile by
confinement; so that they will not unite, or if they unite do not
produce offspring, though the confinement may be far from close; and of
plants rendered sterile by cultivation. But hardly any cases afford more
striking evidence how powerfully a change in the conditions of life acts
on the sexual elements, than those already given, of plants which are
completely self-sterile in one country, and when brought to another,
yield, even in the first generation, a fair supply of self-fertilised
seeds.
But it may be said, granting that changed conditions act on the sexual
elements, how can two or more plants growing close together, either in
their native country or in a garden, be differently acted on, inasmuch
as they appear to be exposed to exactly the same conditions? Although
this question has been already considered, it deserves further
consideration under several points of view. In my experiments with
Digitalis purpurea, some flowers on a wild plant were self-fertilised,
and others were crossed with pollen from another plant growing within
two or three feet's distance. The crossed and self-fertilised plants
raised from the seeds thus obtained, produced flower-stems in number as
100 to 47, and in average height as 100 to 70. Therefore the cross
between these two plants was highly beneficial; but how could their
sexual elements have been differentiated by exposure to different
conditions? If the progenitors of the two plants had lived on the same
spot during the last score of generations, and had never been crossed
with any plant beyond the distance of a few feet, in all probability
their offspring would have been reduced to the same state as some of the
plants in my experiments,--such as the intercrossed plants of the ninth
generation of Ipomoea,--or the self-fertilised plants of the eighth
generation of Mimulus,--or the offspring from flowers on the same
plant,--and in this case a cross between the two plants of Digitalis
would have done no good. But seeds are often widely dispersed by natural
means, and one of the above two plants or one of their ancestors may
have come from a distance, from a more shady or sunny, dry or moist
place, or from a different kind of soil containing other organic or
inorganic matter. We know from the admirable researches of Messrs. Lawes
and Gilbert that different plants require and consume very different
amounts of inorganic matter. (12/10. 'Journal of the Royal Agricultural
Society of England' volume 24 part 1.) But the amount in the soil would
probably not make so great a difference to the several individuals of
any particular species as might at first be expected; for the
surrounding species with different requirements would tend, from
existing in greater or lesser numbers, to keep each species in a sort of
equilibrium, with respect to what it could obtain from the soil. So it
would be even with respect to moisture during dry seasons; and how
powerful is the influence of a little more or less moisture in the soil
on the presence and distribution of plants, is often well shown in old
pasture fields which still retain traces of former ridges and furrows.
Nevertheless, as the proportional numbers of the surrounding plants in
two neighbouring places is rarely exactly the same, the individuals of
the same species will be subjected to somewhat different conditions with
respect to what they can absorb from the soil. It is surprising how the
free growth of one set of plants affects others growing mingled with
them; I allowed the plants on rather more than a square yard of turf
which had been closely mown for several years, to grow up; and nine
species out of twenty were thus exterminated; but whether this was
altogether due to the kinds which grew up robbing the others of
nutriment, I do not know.
Seeds often lie dormant for several years in the ground, and germinate
when brought near the surface by any means, as by burrowing animals.
They would probably be affected by the mere circumstance of having long
lain dormant; for gardeners believe that the production of double
flowers and of fruit is thus influenced. Seeds, moreover, which were
matured during different seasons, will have been subjected during the
whole course of their development to different degrees of heat and
moisture.
It was shown in the last chapter that pollen is often carried by insects
to a considerable distance from plant to plant. Therefore one of the
parents or ancestors of our two plants of Digitalis may have been
crossed by a distant plant growing under somewhat different conditions.
Plants thus crossed often produce an unusually large number of seeds; a
striking instance of this fact is afforded by the Bignonia, previously
mentioned, which was fertilised by Fritz Muller with pollen from some
adjoining plants and set hardly any seed, but when fertilised with
pollen from a distant plant, was highly fertile. Seedlings from a cross
of this kind grow with great vigour, and transmit their vigour to their
descendants. These, therefore, in the struggle for life, will generally
beat and exterminate the seedlings from plants which have long grown
near together under the same conditions, and will thus tend to spread.
When two varieties which present well-marked differences are crossed,
their descendants in the later generations differ greatly from one
another in external characters; and this is due to the augmentation or
obliteration of some of these characters, and to the reappearance of
former ones through reversion; and so it will be, as we may feel almost
sure, with any slight differences in the constitution of their sexual
elements. Anyhow, my experiments indicate that crossing plants which
have been long subjected to almost though not quite the same conditions,
is the most powerful of all the means for retaining some degree of
differentiation in the sexual elements, as shown by the superiority in
the later generations of the intercrossed over the self-fertilised
seedlings. Nevertheless, the continued intercrossing of plants thus
treated does tend to obliterate such differentiation, as may be inferred
from the lessened benefit derived from intercrossing such plants, in
comparison with that from a cross with a fresh stock. It seems probable,
as I may add, that seeds have acquired their endless curious adaptations
for wide dissemination, not only that the seedlings would thus be
enabled to find new and fitting homes, but that the individuals which
have been long subjected to the same conditions should occasionally
intercross with a fresh stock. (12/11. See Professor Hildebrand's
excellent treatise 'Verbreitungsmittel der Pflanzen' 1873.)
From the foregoing several considerations we may, I think, conclude that
in the above case of the Digitalis, and even in that of plants which
have grown for thousands of generations in the same district, as must
often have occurred with species having a much restricted range, we are
apt to over-estimate the degree to which the individuals have been
subjected to absolutely the same conditions. There is at least no
difficulty in believing that such plants have been subjected to
sufficiently distinct conditions to differentiate their sexual elements;
for we know that a plant propagated for some generations in another
garden in the same district serves as a fresh stock and has high
fertilising powers. The curious cases of plants which can fertilise and
be fertilised by any other individual of the same species, but are
altogether sterile with their own pollen, become intelligible, if the
view here propounded is correct, namely, that the individuals of the
same species growing in a state of nature near together, have not really
been subjected during several previous generations to quite the same
conditions.
Some naturalists assume that there is an innate tendency in all beings
to vary and to advance in organisation, independently of external
agencies; and they would, I presume, thus explain the slight differences
which distinguish all the individuals of the same species both in
external characters and in constitution, as well as the greater
differences in both respects between nearly allied varieties. No two
individuals can be found quite alike; thus if we sow a number of seeds
from the same capsule under as nearly as possible the same conditions,
they germinate at different rates and grow more or less vigorously. They
resist cold and other unfavourable conditions differently. They would in
all probability, as we know to be the case with animals of the same
species, be somewhat differently acted on by the same poison, or by the
same disease. They have different powers of transmitting their
characters to their offspring; and many analogous facts could be given.
(12/12. Vilmorin as quoted by Verlot 'Des Varieties' pages 32, 38, 39.)
Now, if it were true that plants growing near together in a state of
nature had been subjected during many previous generations to absolutely
the same conditions, such differences as those just specified would be
quite inexplicable; but they are to a certain extent intelligible in
accordance with the views just advanced.
As most of the plants on which I experimented were grown in my garden or
in pots under glass, a few words must be added on the conditions to
which they were exposed, as well as on the effects of cultivation. When
a species is first brought under culture, it may or may not be subjected
to a change of climate, but it is always grown in ground broken up, and
more or less manured; it is also saved from competition with other
plants. The paramount importance of this latter circumstance is proved
by the multitude of species which flourish and multiply in a garden, but
cannot exist unless they are protected from other plants. When thus
saved from competition they are able to get whatever they require from
the soil, probably often in excess; and they are thus subjected to a
great change of conditions. It is probably in chief part owing to this
cause that all plants with rare exceptions vary after being cultivated
for some generations. The individuals which have already begun to vary
will intercross one with another by the aid of insects; and this
accounts for the extreme diversity of character which many of our long
cultivated plants exhibit. But it should be observed that the result
will be largely determined by the degree of their variability and by the
frequency of the intercrosses; for if a plant varies very little, like
most species in a state of nature, frequent intercrosses tend to give
uniformity of character to it.
I have attempted to show that with plants growing naturally in the same
district, except in the unusual case of each individual being surrounded
by exactly the same proportional numbers of other species having certain
powers of absorption, each will be subjected to slightly different
conditions. This does not apply to the individuals of the same species
when cultivated in cleared ground in the same garden. But if their
flowers are visited by insects, they will intercross; and this will give
to their sexual elements during a considerable number of generations a
sufficient amount of differentiation for a cross to be beneficial.
Moreover, seeds are frequently exchanged or procured from other gardens
having a different kind of soil; and the individuals of the same
cultivated species will thus be subjected to a change of conditions. If
the flowers are not visited by our native insects, or very rarely so, as
in the case of the common and sweet pea, and apparently in that of the
tobacco when kept in a hothouse, any differentiation in the sexual
elements caused by intercrosses will tend to disappear. This appears to
have occurred with the plants just mentioned, for they were not
benefited by being crossed one with another, though they were greatly
benefited by a cross with a fresh stock.
I have been led to the views just advanced with respect to the causes of
the differentiation of the sexual elements and of the variability of our
garden plants, by the results of my various experiments, and more
especially by the four cases in which extremely inconstant species,
after having been self-fertilised and grown under closely similar
conditions for several generations, produced flowers of a uniform and
constant tint. These conditions were nearly the same as those to which
plants, growing in a garden clear of weeds, are subjected, if they are
propagated by self-fertilised seeds on the same spot. The plants in pots
were, however, exposed to less severe fluctuations of climate than those
out of doors; but their conditions, though closely uniform for all the
individuals of the same generation, differed somewhat in the successive
generations. Now, under these circumstances, the sexual elements of the
plants which were intercrossed in each generation retained sufficient
differentiation during several years for their offspring to be superior
to the self-fertilised, but this superiority gradually and manifestly
decreased, as was shown by the difference in the result between a cross
with one of the intercrossed plants and with a fresh stock. These
intercrossed plants tended also in a few cases to become somewhat more
uniform in some of their external characters than they were at first.
With respect to the plants which were self-fertilised in each
generation, their sexual elements apparently lost, after some years, all
differentiation, for a cross between them did no more good than a cross
between the flowers on the same plant. But it is a still more remarkable
fact, that although the seedlings of Mimulus, Ipomoea, Dianthus, and
Petunia which were first raised, varied excessively in the colour of
their flowers, their offspring, after being self-fertilised and grown
under uniform conditions for some generations, bore flowers almost as
uniform in tint as those on a natural species. In one case also the
plants themselves became remarkably uniform in height.
The conclusion that the advantages of a cross depend altogether on the
differentiation of the sexual elements, harmonises perfectly with the
fact that an occasional and slight change in the conditions of life is
beneficial to all plants and animals. (12/13. I have given sufficient
evidence on this head in my 'Variation under Domestication' chapter 18
volume 2 2nd edition page 127.) But the offspring from a cross between
organisms which have been exposed to different conditions, profit in an
incomparably higher degree than do young or old beings from a mere
change in the conditions. In this latter case we never see anything like
the effect which generally follows from a cross with another individual,
especially from a cross with a fresh stock. This might, perhaps, have
been expected, for the blending together of the sexual elements of two
differentiated beings will affect the whole constitution at a very early
period of life, whilst the organisation is highly flexible. We have,
moreover, reason to believe that changed conditions generally act
differently on the several parts or organs of the same individual
(12/14. See, for instance, Brackenridge 'Theory of Diathesis' Edinburgh
1869.); and if we may further believe that these now slightly
differentiated parts react on one another, the harmony between the
beneficial effects on the individual due to changed conditions, and
those due to the interaction of differentiated sexual elements, becomes
still closer.
That wonderfully accurate observer, Sprengel, who first showed how
important a part insects play in the fertilisation of flowers, called
his book 'The Secret of Nature Displayed;' yet he only occasionally saw
that the object for which so many curious and beautiful adaptations have
been acquired, was the cross-fertilisation of distinct plants; and he
knew nothing of the benefits which the offspring thus receive in growth,
vigour, and fertility. But the veil of secrecy is as yet far from
lifted; nor will it be, until we can say why it is beneficial that the
sexual elements should be differentiated to a certain extent, and why,
if the differentiation be carried still further, injury follows. It is
an extraordinary fact that with many species, flowers fertilised with
their own pollen are either absolutely or in some degree sterile; if
fertilised with pollen from another flower on the same plant, they are
sometimes, though rarely, a little more fertile; if fertilised with
pollen from another individual or variety of the same species, they are
fully fertile; but if with pollen from a distinct species, they are
sterile in all possible degrees, until utter sterility is reached. We
thus have a long series with absolute sterility at the two ends;--at one
end due to the sexual elements not having been sufficiently
differentiated, and at the other end to their having been differentiated
in too great a degree, or in some peculiar manner.
The fertilisation of one of the higher plants depends, in the first
place, on the mutual action of the pollen-grains and the stigmatic
secretion or tissues, and afterwards on the mutual action of the
contents of the pollen-grains and ovules. Both actions, judging from the
increased fertility of the parent-plants and from the increased powers
of growth in the offspring, are favoured by some degree of
differentiation in the elements which interact and unite so as to form a
new being. Here we have some analogy with chemical affinity or
attraction, which comes into play only between atoms or molecules of a
different nature. As Professor Miller remarks: "Generally speaking, the
greater the difference in the properties of two bodies, the more intense
is their tendency to mutual chemical action...But between bodies of a
similar character the tendency to unite is feeble." (12/15. 'Elements of
Chemistry' 4th edition 1867 part 1 page 11. Dr. Frankland informs me
that similar views with respect to chemical affinity are generally
accepted by chemists.) This latter proposition accords well with the
feeble effects of a plant's own pollen on the fertility of the
mother-plant and on the growth of the offspring; and the former
proposition accords well with the powerful influence in both ways of
pollen from an individual which has been differentiated by exposure to
changed conditions, or by so-called spontaneous variation. But the
analogy fails when we turn to the negative or weak effects of pollen
from one species on a distinct species; for although some substances
which are extremely dissimilar, for instance, carbon and chlorine, have
a very feeble affinity for each other, yet it cannot be said that the
weakness of the affinity depends in such cases on the extent to which
the substances differ. It is not known why a certain amount of
differentiation is necessary or favourable for the chemical affinity or
union of two substances, any more than for the fertilisation or union of
two organisms.
Mr. Herbert Spencer has discussed this whole subject at great length,
and after stating that all the forces throughout nature tend towards an
equilibrium, remarks, "that the need of this union of sperm-cell and
germ-ccell is the need for overthrowing this equilibrium and
re-establishing active molecular change in the detached germ--a result
which is probably effected by mixing the slightly-different
physiological units of slightly-different individuals." (12/16.
'Principles of Biology' volume 1 page 274 1864. In my 'Origin of
Species' published in 1859, I spoke of the good effects from slight
changes in the condition of life and from cross-fertilisation, and of
the evil effects from great changes in the conditions and from crossing
widely distinct forms (i.e., species), as a series of facts "connected
together by some common but unknown bond, which is essentially related
to the principle of life.") But we must not allow this highly
generalised view, or the analogy of chemical affinity, to conceal from
us our ignorance. We do not know what is the nature or degree of the
differentiation in the sexual elements which is favourable for union,
and what is injurious for union, as in the case of distinct species. We
cannot say why the individuals of certain species profit greatly, and
others very little by being crossed. There are some few species which
have been self-fertilised for a vast number of generations, and yet are
vigorous enough to compete successfully with a host of surrounding
plants. We can form no conception why the advantage from a cross is
sometimes directed exclusively to the vegetative system, and sometimes
to the reproductive system, but commonly to both. It is equally
inconceivable why some individuals of the same species should be
sterile, whilst others are fully fertile with their own pollen; why a
change of climate should either lessen or increase the sterility of
self-sterile species; and why the individuals of some species should be
even more fertile with pollen from a distinct species than with their
own pollen. And so it is with many other facts, which are so obscure
that we stand in awe before the mystery of life.
Under a practical point of view, agriculturists and horticulturists may
learn something from the conclusions at which we have arrived. Firstly,
we see that the injury from the close breeding of animals and from the
self-fertilisation of plants, does not necessarily depend on any
tendency to disease or weakness of constitution common to the related
parents, and only indirectly on their relationship, in so far as they
are apt to resemble each other in all respects, including their sexual
nature. And, secondly, that the advantages of cross-fertilisation depend
on the sexual elements of the parents having become in some degree
differentiated by the exposure of their progenitors to different
conditions, or from their having intercrossed with individuals thus
exposed, or, lastly, from what we call in our ignorance spontaneous
variation. He therefore who wishes to pair closely related animals ought
to keep them under conditions as different as possible. Some few
breeders, guided by their keen powers of observation, have acted on this
principle, and have kept stocks of the same animals at two or more
distant and differently situated farms. They have then coupled the
individuals from these farms with excellent results. (12/17. 'Variation
of Animals and Plants under Domestication' chapter 17 2nd edition volume
2 pages 98, 105.) This same plan is also unconsciously followed whenever
the males, reared in one place, are let out for propagation to breeders
in other places. As some kinds of plants suffer much more from
self-fertilisation than do others, so it probably is with animals from
too close interbreeding. The effects of close interbreeding on animals,
judging again from plants, would be deterioration in general vigour,
including fertility, with no necessary loss of excellence of form; and
this seems to be the usual result.
It is a common practice with horticulturists to obtain seeds from
another place having a very different soil, so as to avoid raising
plants for a long succession of generations under the same conditions;
but with all the species which freely intercross by aid of insects or
the wind, it would be an incomparably better plan to obtain seeds of the
required variety, which had been raised for some generations under as
different conditions as possible, and sow them in alternate rows with
seeds matured in the old garden. The two stocks would then intercross,
with a thorough blending of their whole organisations, and with no loss
of purity to the variety; and this would yield far more favourable
results than a mere exchange of seeds. We have seen in my experiments
how wonderfully the offspring profited in height, weight, hardiness, and
fertility, by crosses of this kind. For instance, plants of Ipomoea thus
crossed were to the intercrossed plants of the same stock, with which
they grew in competition, as 100 to 78 in height, and as 100 to 51 in
fertility; and plants of Eschscholtzia similarly compared were as 100 to
45 in fertility. In comparison with self-fertilised plants the results
are still more striking; thus cabbages derived from a cross with a fresh
stock were to the self-fertilised as 100 to 22 in weight.
Florists may learn from the four cases which have been fully described,
that they have the power of fixing each fleeting variety of colour, if
they will fertilise the flowers of the desired kind with their own
pollen for half-a-dozen generations, and grow the seedlings under the
same conditions. But a cross with any other individual of the same
variety must be carefully prevented, as each has its own peculiar
constitution. After a dozen generations of self-fertilisation, it is
probable that the new variety would remain constant even if grown under
somewhat different conditions; and there would no longer be any
necessity to guard against intercrosses between the individuals of the
same variety.
With respect to mankind, my son George has endeavoured to discover by a
statistical investigation whether the marriages of first cousins are at
all injurious, although this is a degree of relationship which would not
be objected to in our domestic animals; and he has come to the
conclusion from his own researches and those of Dr. Mitchell that the
evidence as to any evil thus caused is conflicting, but on the whole
points to its being very small. From the facts given in this volume we
may infer that with mankind the marriages of nearly related persons,
some of whose parents and ancestors had lived under very different
conditions, would be much less injurious than that of persons who had
always lived in the same place and followed the same habits of life. Nor
can I see reason to doubt that the widely different habits of life of
men and women in civilised nations, especially amongst the upper
classes, would tend to counterbalance any evil from marriages between
healthy and somewhat closely related persons.
Under a theoretical point of view it is some gain to science to know
that numberless structures in hermaphrodite plants, and probably in
hermaphrodite animals, are special adaptations for securing an
occasional cross between two individuals; and that the advantages from
such a cross depend altogether on the beings which are united, or their
progenitors, having had their sexual elements somewhat differentiated,
so that the embryo is benefited in the same manner as is a mature plant
or animal by a slight change in its conditions of life, although in a
much higher degree.
Another and more important result may be deduced from my observations.
Eggs and seeds are highly serviceable as a means of dissemination, but
we now know that fertile eggs can be produced without the aid of the
male. There are also many other methods by which organisms can be
propagated asexually. Why then have the two sexes been developed, and
why do males exist which cannot themselves produce offspring? The answer
lies, as I can hardly doubt, in the great good which is derived from the
fusion of two somewhat differentiated individuals; and with the
exception of the lowest organisms this is possible only by means of the
sexual elements, these consisting of cells separated from the body,
containing the germs of every part, and capable of being fused
completely together.
It has been shown in the present volume that the offspring from the
union of two distinct individuals, especially if their progenitors have
been subjected to very different conditions, have an immense advantage
in height, weight, constitutional vigour and fertility over the
self-fertilised offspring from one of the same parents. And this fact is
amply sufficient to account for the development of the sexual elements,
that is, for the genesis of the two sexes.
It is a different question why the two sexes are sometimes combined in
the same individual and are sometimes separated. As with many of the
lowest plants and animals the conjugation of two individuals which are
either quite similar or in some degree different, is a common
phenomenon, it seems probable, as remarked in the last chapter, that the
sexes were primordially separate. The individual which receives the
contents of the other, may be called the female; and the other, which is
often smaller and more locomotive, may be called the male; though these
sexual names ought hardly to be applied as long as the whole contents of
the two forms are blended into one. The object gained by the two sexes
becoming united in the same hermaphrodite form probably is to allow of
occasional or frequent self-fertilisation, so as to ensure the
propagation of the species, more especially in the case of organisms
affixed for life to the same spot. There does not seem to be any great
difficulty in understanding how an organism, formed by the conjugation
of two individuals which represented the two incipient sexes, might have
given rise by budding first to a monoecious and then to an hermaphrodite
form; and in the case of animals even without budding to an
hermaphrodite form, for the bilateral structure of animals perhaps
indicates that they were aboriginally formed by the fusion of two
individuals.
It is a more difficult problem why some plants and apparently all the
higher animals, after becoming hermaphrodites, have since had their
sexes re-separated. This separation has been attributed by some
naturalists to the advantages which follow from a division of
physiological labour. The principle is intelligible when the same organ
has to perform at the same time diverse functions; but it is not obvious
why the male and female glands when placed in different parts of the
same compound or simple individual, should not perform their functions
equally well as when placed in two distinct individuals. In some
instances the sexes may have been re-separated for the sake of
preventing too frequent self-fertilisation; but this explanation does
not seem probable, as the same end might have been gained by other and
simpler means, for instance dichogamy. It may be that the production of
the male and female reproductive elements and the maturation of the
ovules was too great a strain and expenditure of vital force for a
single individual to withstand, if endowed with a highly complex
organisation; and that at the same time there was no need for all the
individuals to produce young, and consequently that no injury, on the
contrary, good resulted from half of them, or the males, failing to
produce offspring.
There is another subject on which some light is thrown by the facts
given in this volume, namely, hybridisation. It is notorious that when
distinct species of plants are crossed, they produce with the rarest
exceptions fewer seeds than the normal number. This unproductiveness
varies in different species up to sterility so complete that not even an
empty capsule is formed; and all experimentalists have found that it is
much influenced by the conditions to which the crossed species are
subjected. The pollen of each species is strongly prepotent over that of
any other species, so that if a plant's own pollen is placed on the
stigma some time after foreign pollen has been applied to it, any effect
from the latter is quite obliterated. It is also notorious that not only
the parent species, but the hybrids raised from them are more or less
sterile; and that their pollen is often in a more or less aborted
condition. The degree of sterility of various hybrids does not always
strictly correspond with the degree of difficulty in uniting the parent
forms. When hybrids are capable of breeding inter se, their descendants
are more or less sterile, and they often become still more sterile in
the later generations; but then close interbreeding has hitherto been
practised in all such cases. The more sterile hybrids are sometimes much
dwarfed in stature, and have a feeble constitution. Other facts could be
given, but these will suffice for us. Naturalists formerly attributed
all these results to the difference between species being fundamentally
distinct from that between the varieties of the same species; and this
is still the verdict of some naturalists.
The results of my experiments in self-fertilising and cross-fertilising
the individuals or the varieties of the same species, are strikingly
analogous with those just given, though in a reversed manner. With the
majority of species flowers fertilised with their own pollen yield
fewer, sometimes much fewer seeds, than those fertilised with pollen
from another individual or variety. Some self-fertilised flowers are
absolutely sterile; but the degree of their sterility is largely
determined by the conditions to which the parent plants have been
exposed, as was well exemplified in the case of Eschscholtzia and
Abutilon. The effects of pollen from the same plant are obliterated by
the prepotent influence of pollen from another individual or variety,
although the latter may have been placed on the stigma some hours
afterwards. The offspring from self-fertilised flowers are themselves
more or less sterile, sometimes highly sterile, and their pollen is
sometimes in an imperfect condition; but I have not met with any case of
complete sterility in self-fertilised seedlings, as is so common with
hybrids. The degree of their sterility does not correspond with that of
the parent-plants when first self-fertilised. The offspring of
self-fertilised plants suffer in stature, weight, and constitutional
vigour more frequently and in a greater degree than do the hybrid
offspring of the greater number of crossed species. Decreased height is
transmitted to the next generation, but I did not ascertain whether this
applies to decreased fertility.
I have elsewhere shown that by uniting in various ways dimorphic or
trimorphic heterostyled plants, which belong to the same undoubted
species, we get another series of results exactly parallel with those
from crossing distinct species. (12/18. 'Journal of the Linnean Society
Botany' volume 10 1867 page 393.) Plants illegitimately fertilised with
pollen from a distinct plant belonging to the same form, yield fewer,
often much fewer seeds, than they do when legitimately fertilised with
pollen from a plant belonging to a distinct form. They sometimes yield
no seed, not even an empty capsule, like a species fertilised with
pollen from a distinct genus. The degree of sterility is much affected
by the conditions to which the plants have been subjected. (12/19.
'Journal of the Linnean Society Botany' volume 8 1864 page 180.) The
pollen from a distinct form is strongly prepotent over that from the
same form, although the former may have been placed on the stigma many
hours afterwards. The offspring from a union between plants of the same
form are more or less sterile, like hybrids, and have their pollen in a
more or less aborted condition; and some of the seedlings are as barren
and as dwarfed as the most barren hybrid. They also resemble hybrids in
several other respects, which need not here be specified in
detail,--such as their sterility not corresponding in degree with that
of the parent plants,--the unequal sterility of the latter, when
reciprocally united,--and the varying sterility of the seedlings raised
from the same seed-capsule.
We thus have two grand classes of cases giving results which correspond
in the most striking manner with those which follow from the crossing of
so-called true and distinct species. With respect to the difference
between seedlings raised from cross and self-fertilised flowers, there
is good evidence that this depends altogether on whether the sexual
elements of the parents have been sufficiently differentiated, by
exposure to different conditions or by spontaneous variation. It is
probable that nearly the same conclusion may be extended to heterostyled
plants; but this is not the proper place for discussing the origin of
the long-styled, short-styled and mid-styled forms, which all belong to
the same species as certainly as do the two sexes of the same species.
We have therefore no right to maintain that the sterility of species
when first crossed and of their hybrid offspring, is determined by some
cause fundamentally different from that which determines the sterility
of the individuals both of ordinary and of heterostyled plants when
united in various ways. Nevertheless, I am aware that it will take many
years to remove this prejudice.
There is hardly anything more wonderful in nature than the sensitiveness
of the sexual elements to external influences, and the delicacy of their
affinities. We see this in slight changes in the conditions of life
being favourable to the fertility and vigour of the parents, while
certain other and not great changes cause them to be quite sterile
without any apparent injury to their health. We see how sensitive the
sexual elements of those plants must be, which are completely sterile
with their own pollen, but are fertile with that of any other individual
of the same species. Such plants become either more or less self-sterile
if subjected to changed conditions, although the change may be far from
great. The ovules of a heterostyled trimorphic plant are affected very
differently by pollen from the three sets of stamens belonging to the
same species. With ordinary plants the pollen of another variety or
merely of another individual of the same variety is often strongly
prepotent over its own pollen, when both are placed at the same time on
the same stigma. In those great families of plants containing many
thousand allied species, the stigma of each distinguishes with unerring
certainty its own pollen from that of every other species.
There can be no doubt that the sterility of distinct species when first
crossed, and of their hybrid offspring, depends exclusively on the
nature or affinities of their sexual elements. We see this in the want
of any close correspondence between the degree of sterility and the
amount of external difference in the species which are crossed; and
still more clearly in the wide difference in the results of crossing
reciprocally the same two species;--that is, when species A is crossed
with pollen from B, and then B is crossed with pollen from A. Bearing in
mind what has just been said on the extreme sensitiveness and delicate
affinities of the reproductive system, why should we feel any surprise
at the sexual elements of those forms, which we call species, having
been differentiated in such a manner that they are incapable or only
feebly capable of acting on one another? We know that species have
generally lived under the same conditions, and have retained their own
proper characters, for a much longer period than varieties.
Long-continued domestication eliminates, as I have shown in my
'Variation under Domestication,' the mutual sterility which distinct
species lately taken from a state of nature almost always exhibit when
intercrossed; and we can thus understand the fact that the most
different domestic races of animals are not mutually sterile. But
whether this holds good with cultivated varieties of plants is not
known, though some facts indicate that it does. The elimination of
sterility through long-continued domestication may probably be
attributed to the varying conditions to which our domestic animals have
been subjected; and no doubt it is owing to this same cause that they
withstand great and sudden changes in their conditions of life with far
less loss of fertility than do natural species. From these several
considerations it appears probable that the difference in the affinities
of the sexual elements of distinct species, on which their mutual
incapacity for breeding together depends, is caused by their having been
habituated for a very long period each to its own conditions, and to the
sexual elements having thus acquired firmly fixed affinities. However
this may be, with the two great classes of cases before us, namely,
those relating to the self-fertilisation and cross-fertilisation of the
individuals of the same species, and those relating to the illegitimate
and legitimate unions of heterostyled plants, it is quite unjustifiable
to assume that the sterility of species when first crossed and of their
hybrid offspring, indicates that they differ in some fundamental manner
from the varieties or individuals of the same species.
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