Chapter XVII (2)

As these orchids had grown under unnatural conditions, in {134}
hot-houses, I concluded without hesitation that their self-sterility
was due to this cause. But Fritz Müller informs me that at Desterro, in
Brazil, he fertilised above one hundred flowers of the above-mentioned
_Oncidium flexuosum_, which is there endemic, with its own pollen, and
with that taken from distinct plants; all the former were sterile,
whilst those fertilised by pollen from any _other plant_ of the same
species were fertile. During the first three days there was no
difference in the action of the two kinds of pollen: that placed on the
stigma of the same plant separated in the usual manner into grains, and
emitted tubes which penetrated the column, and the stigmatic chamber
shut itself; but the flowers alone which had been fertilised by pollen
taken from a distinct plant produced seed-capsules. On a subsequent
occasion these experiments were repeated on a large scale with the same
result. Fritz Müller found that four other endemic species of Oncidium
were in like manner utterly sterile with their own pollen, but fertile
with that from any other plant: some of them likewise produced
seed-capsules when impregnated with pollen of widely distinct genera,
such as Leptotes, Cyrtopodium, and Rodriguezia! _Oncidium crispum_,
however, differs from the foregoing species in varying much in its
self-sterility; some plants producing fine pods with their own pollen,
others failing to do so; in two or three instances, Fritz Müller
observed that the pods produced by pollen taken from a distinct flower
on the same plant, were larger than those produced by the flower's own
pollen. In _Epidendrum cinnabarinum_, an orchid belonging to another
division of the family, fine pods were produced by the plant's own
pollen, but they contained by weight only about half as much seed as
the capsules which had been fertilized by pollen from a distinct plant,
and in one instance from a distinct species; moreover, a very large
proportion, and in some cases nearly all the seed produced by the
plant's own pollen, was embryonless and worthless. Some self-fertilized
capsules of a Maxillaria were in a similar state.

Another observation made by Fritz Müller is highly remarkable, namely,
that with various orchids the plant's own pollen not only fails to
impregnate the flower, but acts on the stigma, and is acted on, in an
injurious or poisonous manner. This is shown by the surface of the
stigma in contact with the pollen, and by the pollen itself, becoming
in from three to five days dark brown, and then decaying. The
discolouration and decay are not caused by parasitic cryptogams, which
were observed by Fritz Müller in only a single instance. These changes
are well shown by placing on the same stigma, at the same time, the
plant's own pollen and that from a distinct plant of the same species,
or of another species, or even of another and widely remote genus.
Thus, on the stigma of _Oncidium flexuosum_, the plant's own pollen and
that from a distinct plant were placed side by side, and in five days'
time the latter was perfectly fresh, whilst the plant's own pollen was
brown. On the other hand, when the pollen of a distinct plant of the
_Oncidium flexuosum_, and of the _Epidendrum zebra_ (_nov. spec.?_),
were placed together on the same stigma, they behaved in exactly the
same manner, the grains separating, emitting tubes, and penetrating the
stigma, so that the two {135} pollen-masses, after an interval of
eleven days, could not be distinguished except by the difference of
their caudicles, which, of course, undergo no change. Fritz Müller has,
moreover, made a large number of crosses between orchids belonging to
distinct species and genera, and he finds that in all cases when the
flowers are not fertilised their footstalks first begin to wither; and
the withering slowly spreads upwards until the germens fall off, after
an interval of one or two weeks, and in one instance of between six and
seven weeks; but even in this latter case, and in most other cases, the
pollen and stigma remained in appearance fresh. Occasionally, however,
the pollen becomes brownish, generally on the external surface, and not
in contact with the stigma, as is invariably the case when the plant's
own pollen is applied.

Fritz Müller observed the poisonous action of the plant's own pollen in
the above-mentioned _Oncidium flexuosum_, _O. unicorne, pubes_ (_?_),
and in two other unnamed species. Also in two species of Rodriguezia,
in two of Notylia, in one of Burlingtonia, and of a fourth genus in the
same group. In all these cases, except the last, it was proved that the
flowers were, as might have been expected, fertile with pollen from a
distinct plant of the same species. Numerous flowers of one species of
Notylia were fertilized with pollen from the same raceme; in two days'
time they all withered, the germens began to shrink, the pollen-masses
became dark brown, and not one pollen-grain emitted a tube. So that in
this orchid the injurious action of the plant's own pollen is more
rapid than with _Oncidium flexuosum_. Eight other flowers on the same
raceme were fertilized with pollen from a distinct plant of the same
species: two of these were dissected, and their stigmas were found to
be penetrated by numberless pollen-tubes; and the germens of the other
six flowers became well developed. On a subsequent occasion many other
flowers were fertilized with their own pollen, and all fell off dead in
a few days; whilst some flowers on the same raceme which had been left
simply unfertilised adhered and long remained fresh. We have seen that
in cross-unions between extremely distinct orchids the pollen long
remains undecayed; but Notylia behaved in this respect differently; for
when its pollen was placed on the stigma of _Oncidium flexuosum_, both
the stigma and pollen quickly became dark brown, in the same manner as
if the plant's own pollen had been applied.

Fritz Müller suggests that, as in all these cases the plant's own
pollen is not only impotent (thus effectually preventing
self-fertilization), but likewise prevents, as was ascertained in the
case of the Notylia and _Oncidium flexuosum_, the action of
subsequently applied pollen from a distinct individual, it would be an
advantage to the plant to have its own pollen rendered more and more
deleterious; for the germens would thus quickly be killed, and,
dropping off, there would be no further waste in nourishing a part
which ultimately could be of no avail. Fritz Müller's discovery that a
plant's own pollen and stigma in some cases act on each other as if
mutually poisonous, is certainly most remarkable.

We now come to cases closely analogous with those just {136} given, but different, inasmuch as individual plants alone of the species are self-impotent. This self-impotence does not depend on the pollen or ovules being in a state unfit for fertilisation, for both have been found effective in union with other plants of the same or of a distinct species. The fact of these plants having spontaneously acquired so peculiar a constitution, that they can be fertilised more readily by the pollen of a distinct species than by their own, is remarkable. These abnormal cases, as well as the foregoing normal cases, in which certain orchids, for instance, can be much more easily fertilised by the pollen of a distinct species than by their own, are exactly the reverse of what occurs with all ordinary species. For in these latter the two sexual elements of the same individual plant are capable of freely acting on each other; but are so constituted that they are more or less impotent when brought into union with the sexual elements of a distinct species, and produce more or less sterile hybrids. It would appear that the pollen or ovules, or both, of the individual plants which are in this abnormal state, have been affected in some strange manner by the conditions to which they themselves or their parents have been exposed; but whilst thus rendered self-sterile, they have retained the capacity common to most species of partially fertilizing and being partially fertilized by allied forms. However this may be, the subject, to a certain extent, is related to our general conclusion that good is derived from the act of crossing.

Gärtner experimented on two plants of _Lobelia fulgens_, brought from
separate places, and found[308] that their pollen was good, for he
fertilised with it _L. cardinalis_ and _syphilitica_; their ovules were
likewise good, for they were fertilised by the pollen of these same two
species; but these two plants of _L. fulgens_ could not be fertilised
by their own pollen, as can generally be effected with perfect ease
with this species. Again, the pollen of a plant of _Verbascum nigrum_
grown in a pot was found by Gärtner[309] capable of fertilising _V.
lychnitis_ and _V. Austriacum_; the ovules could be fertilised by the
pollen of _V. thapsus_; but the flowers could not be fertilised by
their own pollen. Kölreuter, also,[310] gives the case of three {137}
garden plants of _Verbascum phoeniceum_, which bore during two years
many flowers; these he successfully fertilised by the pollen of no less
than four distinct species, but they produced not a seed with their own
apparently good pollen; subsequently these same plants, and others
raised from seed, assumed a strangely fluctuating condition, being
temporarily sterile on the male or female side, or on both sides, and
sometimes fertile on both sides; but two of the plants were perfectly
fertile throughout the summer.

It appears[311] that certain flowers on certain plants of _Lilium
candidum_ can be fertilised more easily by pollen from a distinct
individual than by their own. So, again, with the varieties of the
potato. Tinzmann,[312] who made many trials with this plant, says that
pollen from another variety sometimes "exerts a powerful influence, and
I have found sorts of potatoes which would not bear seed from
impregnation with the pollen of their own flowers, would bear it when
impregnated with other pollen." It does not, however, appear to have
been proved that the pollen which failed to act on the flower's own
stigma was in itself good.

In the genus Passiflora it has long been known that several species do
not produce fruit, unless fertilised by pollen taken from distinct
species: thus, Mr. Mowbray[313] found that he could not get fruit from
_P. alata_ and _racemosa_ except by reciprocally fertilising them with
each other's pollen. Similar facts have been observed in Germany and
France;[314] and I have received two authentic accounts of _P.
quadrangularis_, which never produced fruit with its own pollen, but
would do so freely when fertilised in one case with the pollen of _P.
coerulea_, and in another case with that of _P. edulis_. So again, with
respect to _P. laurifolia_, a cultivator of much experience has
recently remarked[315] that the flowers "must be fertilised with the
pollen of _P. coerulea_, or of some other common kind, as their own
pollen will not fertilise them." But the fullest details on this
subject have been given by Mr. Scott:[316] plants of _Passiflora
racemosa_, _coerulea_, and _alata_ flowered profusely during many years
in the Botanic Gardens of Edinburgh, and, though repeatedly fertilised
by Mr. Scott and by others with their own pollen, never produced any
seed; yet this occurred at once with all three species when they were
crossed together in various ways. But in the case of _P. coerulea_,
three plants, two of which grew in the Botanic Gardens, were all
rendered fertile, merely by impregnating the one with pollen of the
other. The same result was attained in the same manner with _P. alata_,
but only with one plant out of three. As so many self-sterile species
have been mentioned, it may be stated that in the case of _P.
gracilis_, which is an annual, the flowers are nearly as fertile with
their own pollen as with that from a distinct plant; thus sixteen
flowers {138} spontaneously self-fertilised produced fruit, each
containing on an average 21.3 seed, whilst fruit from fourteen crossed
flowers contained 24.1 seed.

Returning to _P. alata_, I have received (1866) some interesting
details from Mr. Robinson Munro. Three plants, including one in
England, have already been mentioned which were inveterately
self-sterile, and Mr. Munro informs me of several others which, after
repeated trials during many years, have been found in the same
predicament. At some other places, however, this species fruits readily
when fertilised with its own pollen. At Taymouth Castle there is a
plant which was formerly grafted by Mr. Donaldson on a distinct
species, name unknown, and ever since the operation it has produced
fruit in abundance by its own pollen; so that this small and unnatural
change in the state of this plant has restored its self-fertility! Some
of the seedlings from the Taymouth Castle plant were found to be not
only sterile with their own pollen, but with each other's pollen, and
with the pollen of distinct species. Pollen from the Taymouth plant
failed to fertilise certain plants of the same species, but was
successful on one plant in the Edinburgh Botanic Gardens. Seedlings
were raised from this latter union, and some of their flowers were
fertilised by Mr. Munro with their own pollen; but they were found to
be as self-impotent as the mother-plant had always proved, except when
fertilised by the grafted Taymouth plant, and except, as we shall see,
when fertilised by her own seedlings. For Mr. Munro fertilised eighteen
flowers on the self-impotent mother-plant with pollen from these her
own self-impotent seedlings, and obtained, remarkable as the fact is,
eighteen fine capsules full of excellent seed! I have met with no case
in regard to plants which shows so well as this of _P. alata_, on what
small and mysterious causes complete fertility or complete sterility
depends.

The facts hitherto given relate to the much-lessened or completely destroyed fertility of pure species when impregnated with their own pollen, in comparison with their fertility when impregnated by distinct individuals or distinct species; but closely analogous facts have been observed with hybrids.

Herbert states[317] that having in flower at the same time nine hybrid
Hippeastrums, of complicated origin, descended from several species, he
found that "almost every flower touched with pollen from another cross
produced seed abundantly, and those which were touched with their own
pollen either failed entirely, or formed slowly a pod of inferior size,
with fewer seeds." In the 'Horticultural Journal' he adds that, "the
admission of the pollen of another cross-bred Hippeastrum (however
complicated the cross) to any _one_ flower of the number, is almost
sure to check the fructification of the others." In a letter written to
me in 1839, Dr. Herbert says that he had already tried these
experiments during five consecutive years, and he subsequently repeated
them, with the same invariable result. {139} He was thus led to make an
analogous trial on a pure species, namely, on the _Hippeastrum
aulicum_, which he had lately imported from Brazil: this bulb produced
four flowers, three of which were fertilised by their own pollen, and
the fourth by the pollen of a triple cross between _H. bulbulosum_,
_reginæ_, and _vittatum_; the result was, that "the ovaries of the
three first flowers soon ceased to grow, and after a few days perished
entirely: whereas the pod impregnated by the hybrid made vigorous and
rapid progress to maturity, and bore good seed, which vegetated
freely." This is, indeed, as Herbert remarks, "a strange truth," but
not so strange as it then appeared.

As a confirmation of these statements, I may add that Mr. M.
Mayes,[318] after much experience in crossing the species of Amaryllis
(Hippeastrum), says, "neither the species nor the hybrids will, we are
well aware, produce seed so abundantly from their own pollen as from
that of others." So, again, Mr. Bidwell, in New South Wales,[319]
asserts that _Amaryllis belladonna_ bears many more seeds when
fertilised by the pollen of _Brunswigia_ (_Amaryllis_ of some authors)
_Josephinæ_ or of _B. multiflora_, than when fertilised by its own
pollen. Mr. Beaton dusted four flowers of a Cyrtanthus with their own
pollen, and four with the pollen of _Vallota_ (_Amaryllis_) _purpurea_;
on the seventh day "those which received their own pollen slackened
their growth, and ultimately perished; those which were crossed with
the Vallota held on."[320] These latter cases, however, relate to
uncrossed species, like those before given with respect to Passiflora,
Orchids, &c., and are here referred to only because the plants belong
to the same group of Amaryllidaceæ.

In the experiments on the hybrid Hippeastrums, if Herbert had found
that the pollen of two or three kinds alone had been more efficient on
certain kinds than their own pollen, it might have been argued that
these, from their mixed parentage, had a closer mutual affinity than
the others; but this explanation is inadmissible, for the trials were
made reciprocally backwards and forwards on nine different hybrids; and
a cross, whichever way taken, always proved highly beneficial. I can
add a striking and analogous case from experiments made by the Rev. A.
Rawson, of Bromley Common, with some complex hybrids of Gladiolus. This
skilful horticulturist possessed a number of French varieties,
differing from each other only in the colour and size of the flowers,
all descended from Gandavensis, a well-known old hybrid, said to be
descended from _G. Natalensis_ by the pollen of _G.
oppositiflorus_.[321] Mr. Rawson, after repeated trials, found that
none of the varieties would set seed with their own pollen, although
{140} taken from distinct plants of the same variety, which had, of
course, been propagated by bulbs, but that they all seeded freely with
pollen from any other variety. To give two examples: Ophir did not
produce a capsule with its own pollen, but when fertilised with that of
Janire, Brenchleyensis, Vulcain, and Linné, it produced ten fine
capsules; but the pollen of Ophir was good, for when Linné was
fertilised by it seven capsules were produced. This later variety, on
the other hand, was utterly barren with its own pollen, which we have
seen was perfectly efficient on Ophir. Altogether, Mr. Rawson, in the
year 1861, fertilised twenty-six flowers borne by four varieties with
pollen taken from other varieties, and every single flower produced a
fine seed-capsule; whereas fifty-two flowers on the same plants,
fertilised at the same time with their own pollen, did not yield a
single seed-capsule. Mr. Rawson fertilised, in some cases, the
alternate flowers, and in other cases all those down one side of the
spike, with pollen of other varieties, and the remaining flowers with
their own pollen; I saw these plants when the capsules were nearly
mature, and their curious arrangement at once brought full conviction
to the mind that an immense advantage had been derived from crossing
these hybrids.

Lastly, I have heard from Dr. E. Bornet, of Antibes, who has made
numerous experiments in crossing the species of Cistus, but as not yet
published the results, that, when any of these hybrids are fertile,
they may be said to be, in regard to function, dioecious; "for the
flowers are always sterile when the pistil is fertilised by pollen
taken from the same flower or from flowers on the same plant. But they
are often fertile if pollen be employed from a distinct individual of
the same hybrid nature, or from a hybrid made by a reciprocal cross."

_Conclusion._--The facts just given, which show that certain plants are self-sterile, although both sexual elements are in a fit state for reproduction when united with distinct individuals of the same or other species, appear at first sight opposed to all analogy. The sexual elements of the same flower have become, as already remarked, differentiated in relation to each other, almost like those of two distinct species.

With respect to the species which, whilst living under their natural conditions, have their reproductive organs in this peculiar state, we may conclude that it has been naturally acquired for the sake of effectually preventing self-fertilisation. The case is closely analous with dimorphic and trimorphic plants, which can be fully fertilised only by plants belong to the opposite form, and not, as in the foregoing cases, in differently by any other plant. Some of these dimorphic plants are completely sterile with pollen taken from the same plant or from the same {141} form. It is interesting to observe the graduated series from plants which, when fertilised by their own pollen, yield the full number of seed, but with the seedlings a little dwarfed in stature--to plants which when self-fertilised yield few seeds--to those with yield none--and, lastly, to those in which the plant's own pollen and stigma act on each other like poison. This peculiar state of the reproductive organs, when occurring in certain individuals alone, is evidently abnormal; and as it chiefly affects exotic plants, or indigenous plants cultivated in pots, we may attribute it to some change in the conditions of life, acting on the plants themselves or on their parents. The self-impotent _Passiflora alata_, which recovered its self-fertility after having been grafted on a distinct stock, shows how small a change is sufficient to act powerfully on the reproductive system. The possibility of a plant becoming under culture self-impotent is interesting as throwing light on the occurrence of this same condition in natural species. A cultivated plant in this state generally remains so during its whole life; and from this fact we may infer that the state is probably congenital.

Kölreuter, however, has described some plants of Verbascum which varied in this respect even during the same season. As in all the normal cases, and in many, probably in most, of the abnormal cases, any two self-impotent plants can reciprocally fertilize each other, we may infer that a very slight difference in the nature of their sexual elements suffices to give fertility; but in other instances, as with some Passifloras and the hybrid Gladioli, a greater degree of differentiation appears to be necessary, for with these plants fertility is gained only by the union of distinct species, or of hybrids of distinct parentage. These facts all point to the same general conclusion, namely, that good is derived from a cross between individuals, which either innately, or from exposure to dissimilar conditions, have come to differ in sexual constitution.

Exotic animals confined in menageries are sometimes in nearly the same state as the above-described self-impotent plants; for, as we shall see in the following chapter, certain monkeys, the larger carnivora, several finches, geese, and pheasants, cross together, quite as freely as, or even more freely than, the individuals of the same species breed together. Cases will, {142} also, be given of sexual incompatibility between certain male and female domesticated animals, which, nevertheless, are fertile when matched with any other individual of the same kind.

In the early part of this chapter it was shown that the crossing of distinct forms, whether closely or distantly allied, gives increased size and constitutional vigour, and, except in the case of crossed species, increased fertility, to the offspring. The evidence rests on the universal testimony of breeders (for it should be observed that I am not here speaking of the evil results of close interbreeding), and is practically exemplified in the higher value of cross-bred animals for immediate consumption. The good results of crossing have also been demonstrated, in the case of some animals and of numerous plants, by actual weight and measurement. Although animals of pure blood will obviously be deteriorated by crossing, as far as their characteristic qualities are concerned, there seems to be no exception to the rule that advantages of the kind just mentioned are thus gained, even when there has not been any previous close interbreeding. The rule applies to all animals, even to cattle and sheep, which can long resist breeding in-and-in between the nearest blood-relations. It applies to individuals of the same sub-variety but of distinct families, to varieties or races, to sub-species, as well as to quite distinct species.

In this latter case, however, whilst size, vigour, precocity, and hardiness are, with rare exceptions, gained, fertility, in a greater or less degree, is lost; but the gain cannot be exclusively attributed to the principle of compensation; for there is no close parallelism between the increased size and vigour of the offspring and their sterility. Moreover it has been clearly proved that mongrels which are perfectly fertile gain these same advantages as well as sterile hybrids.

The evil consequences of long-continued close interbreeding are not so easily recognised as the good effects from crossing, for the deterioration is gradual. Nevertheless it is the general opinion of those who have had most experience, especially with animals which propagate quickly, that evil does inevitably follow sooner or later, but at different rates with different animals. No doubt a false belief may widely prevail like a superstition; yet it is difficult to suppose that so many acute and original {143} observers have all been deceived at the expense of much cost and trouble. A male animal may sometimes be paired with his daughter, granddaughter, and so on, even for seven generations, without any manifest bad result; but the experiment has never been tried of matching brothers and sisters, which is considered the closest form of interbreeding, for an equal number of generations. There is good reason to believe that by keeping the members of the same family in distinct bodies, especially if exposed to somewhat different conditions of life, and by occasionally crossing these families, the evil results may be much diminished, or quite eliminated. These results are loss of constitutional vigour, size, and fertility; but there is no necessary deterioration in the general form of the body, or in other good qualities. We have seen that with pigs first-rate animals have been produced after long-continued close interbreeding, though they had become extremely infertile when paired with their near relations. The loss of fertility, when it occurs, seems never to be absolute, but only relative to animals of the same blood; so that this sterility is to a certain extent analogous with that of self-impotent plants which cannot be fertilised by their own pollen, but are perfectly fertile with pollen of any other plant of the same species. The fact of infertility of this peculiar nature being one of the results of long-continued interbreeding, shows that interbreeding does not act merely by combining and augmenting various morbid tendencies common to both parents; for animals with such tendencies, if not at the time actually ill, can generally propagate their kind. Although offspring descended from the nearest blood-relations are not necessarily deteriorated in structure, yet some authors[322] believe that they are eminently liable to malformations; and this is not improbable, as everything which lessens the vital powers acts in this manner. Instances of this kind have been recorded in the case of pigs, bloodhounds, and some other animals.

Finally, when we consider the various facts now given which plainly show that good follows from crossing, and less plainly {144} that evil follows from close interbreeding, and when we bear in mind that throughout the whole organic world elaborate provision has been made for the occasional union of distinct individuals, the existence of a great law of nature is, if not proved, at least rendered in the highest degree probable; namely, that the crossing of animals and plants which are not closely related to beach other is highly beneficial or even necessary, and that interbreeding prolonged during many generations is highly injurious.

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