 Chapter
8 - Hybridism
*
Distinction between the sterility of first
crosses and of hybrids *
Sterility various in degree, not universal, affected by
close interbreeding, removed by domestication *
Laws governing the sterility of hybrids *
Sterility not a special endowment, but incidental on other
differences *
Causes of the sterility of first crosses and of hybrids
*
Parallelism between the effects of changed conditions of
life and crossing *
Fertility of varieties when crossed and of their mongrel
offspring not universal *
Hybrids and mongrels compared independently of their fertility
*
Summary
THE view generally entertained by naturalists is that
species, when intercrossed, have been specially endowed
with the quality of sterility, in order to prevent the confusion
of all organic forms. This view certainly seems at first
probable, for species within the same country could hardly
have kept distinct had they been capable of crossing freely.
The importance of the fact that hybrids are very generally
sterile, has, I think, been much underrated by some late
writers. On the theory of natural selection the case is
especially important, inasmuch as the sterility of hybrids
could not possibly be of any advantage to them, and therefore
could not have been acquired by the continued preservation
of successive profitable degrees of sterility. I hope, however,
to be able to show that sterility is not a specially acquired
or endowed quality, but is incidental on other acquired
differences.
In treating this subject, two classes of facts, to a large
extent fundamentally different, have generally been confounded
together; namely, the sterility of two species when first
crossed, and the sterility of the hybrids produced from
them.
Pure species have of course their organs of reproduction
in a perfect condition, yet when intercrossed they produce
either few or no offspring. Hybrids, on the other hand,
have their reproductive organs functionally impotent, as
may be clearly seen in the state of the male element in
both plants and animals; though the organs themselves are
perfect in structure, as far as the microscope reveals.
In the first case the two sexual elements which go to form
the embryo are perfect; in the second case they are either
not at all developed, or are imperfectly developed. This
distinction is important, when the cause of the sterility,
which is common to the two cases, has to be considered.
The distinction has probably been slurred over, owing to
the sterility in both cases being looked on as a special
endowment, beyond the province of our reasoning powers.
The fertility of varieties, that is of the forms known
or believed to have descended from common parents, when
intercrossed, and likewise the fertility of their mongrel
offspring, is, on my theory, of equal importance with the
sterility of species; for it seems to make a broad and clear
distinction between varieties and species.
First, for the sterility of species when crossed and of
their hybrid offspring. It is impossible to study the several
memoirs and works of those two conscientious and admirable
observers, Kölreuter and Gärtner, who almost devoted
their lives to this subject, without being deeply impressed
with the high generality of some degree of sterility. Kölreuter
makes the rule universal; but then he cuts the knot, for
in ten cases in which he found two forms, considered by
most authors as distinct species, quite fertile together,
he unhesitatingly ranks them as varieties. Gärtner,
also, makes the rule equally universal; and he disputes
the entire fertility of Kölreuter's ten cases. But
in these and in many other cases, Gärtner is obliged
carefully to count the seeds, in order to show that there
is any degree of sterility. He always compares the maximum
number of seeds produced by two species when crossed and
by their hybrid offspring, with the average number produced
by both pure parent-species in a state of nature. But a
serious cause of error seems to me to be here introduced:
a plant to be hybridised must be castrated, and, what is
often more important, must be secluded in order to prevent
pollen being brought to it by insects from other plants.
Nearly all the plants experimentised on by Gärtner
were potted, and apparently were kept in a chamber in his
house. That these processes are often injurious to the fertility
of a plant cannot be doubted; for Gärtner gives in
his table about a score of cases of plants which he castrated,
and artificially fertilised with their own pollen, and (excluding
all cases such as the Leguminosae, in which there is an
acknowledged difficulty in the manipulation) half of these
twenty plants had their fertility in some degree impaired.
Moreover, as Gärtner during several years repeatedly
crossed the primrose and cowslip, which we have such good
reason to believe to be varieties, and only once or twice
succeeded in getting fertile seed; as he found the common
red and blue pimpernels (Anagallis arvensis and coerulea),
which the best botanists rank as varieties, absolutely sterile
together; and as he came to the same conclusion in several
other analogous cases; it seems to me that we may well be
permitted to doubt whether many other species are really
so sterile, when intercrossed, as Gärtner believes.
It is certain, on the one hand, that the sterility of
various species when crossed is so different in degree and
graduates away so insensibly, and, on the other hand, that
the fertility of pure species is so easily affected by various
circumstances, that for all practical purposes it is most
difficult to say where perfect fertility ends and sterility
begins. I think no better evidence of this can be required
than that the two most experienced observers who have ever
lived, namely, Kölreuter and Gärtner, should have
arrived at diametrically opposite conclusions in regard
to the very same species. It is also most instructive to
compare but I have not space here to enter on details the
evidence advanced by our best botanists on the question
whether certain doubtful forms should be ranked as species
or varieties, with the evidence from fertility adduced by
different hybridisers, or by the same author, from experiments
made during different years. It can thus be shown that neither
sterility nor fertility affords any clear distinction between
species and varieties; but that the evidence from this source
graduates away, and is doubtful in the same degree as is
the evidence derived from other constitutional and structural
differences.
In regard to the sterility of hybrids in successive generations;
though Gärtner was enabled to rear some hybrids, carefully
guarding them from a cross with either pure parent, for
six or seven, and in one case for ten generations, yet he
asserts positively that their fertility never increased,
but generally greatly decreased. I do not doubt that this
is usually the case, and that the fertility often suddenly
decreases in the first few generations. Nevertheless I believe
that in all these experiments the fertility has been diminished
by an independent cause, namely, from close interbreeding.
I have collected so large a body of facts, showing that
close interbreeding lessens fertility, and, on the other
hand, that an occasional cross with a distinct individual
or variety increases fertility, that I cannot doubt the
correctness of this almost universal belief amongst breeders.
Hybrids are seldom raised by experimentalists in great numbers;
and as the parent-species, or other allied hybrids, generally
grow in the same garden, the visits of insects must be carefully
prevented during the flowering season: hence hybrids will
generally be fertilised during each generation by their
own individual pollen; and I am convinced that this would
be injurious to their fertility, already lessened by their
hybrid origin. I am strengthened in this conviction by a
remarkable statement repeatedly made by Gärtner, namely,
that if even the less fertile hybrids be artificially fertilised
with hybrid pollen of the same kind, their fertility, notwithstanding
the frequent ill effects of manipulation, sometimes decidedly
increases, and goes on increasing. Now, in artificial fertilisation
pollen is as often taken by chance (as I know from my own
experience) from the anthers of another flower, as from
the anthers of the flower itself which is to be fertilised;
so that a cross between two flowers, though probably on
the same plant, would be thus effected. Moreover, whenever
complicated experiments are in progress, so careful an observer
as Gärtner would have castrated his hybrids, and this
would have insured in each generation a cross with the pollen
from a distinct flower, either from the same plant or from
another plant of the same hybrid nature. And thus, the strange
fact of the increase of fertility in the successive generations
of artificially fertilised hybrids may, I believe,
be accounted for by close interbreeding having been avoided.
Now let us turn to the results arrived at by the third
most experienced hybridiser, namely, the Hon. and Rev. W.
Herbert. He is as emphatic in his conclusion that some hybrids
are perfectly fertile as fertile as the pure parent-species
as are Kölreuter and Gärtner that some degree
of sterility between distinct species is a universal law
of nature. He experimentised on some of the very same species
as did Gärtner. The difference in their results may,
I think, be in part accounted for by Herbert's great horticultural
skill, and by his having hothouses at his command. Of his
many important statements I will here give only a single
one as an example, namely, that 'every ovule in a pod of
Crinum capense fertilised by C. revolutum produced a plant,
which (he says) I never saw to occur in a case of its natural
fecundation.' So that we here have perfect, or even more
than commonly perfect, fertility in a first cross between
two distinct species.
This case of the Crinum leads me to refer to a most singular
fact, namely, that there are individual plants, as with
certain species of Lobelia, and with all the species of
the genus Hippeastrum, which can be far more easily fertilised
by the pollen of another and distinct species, than by their
own pollen. For these plants have been found to yield seed
to the pollen of a distinct species, though quite sterile
with their own pollen, notwithstanding that their own pollen
was found to be perfectly good, for it fertilised distinct
species. So that certain individual plants and all the individuals
of certain species can actually be hybridised much more
readily than they can be self-fertilised! For instance,
a bulb of Hippeastrum aulicum produced four flowers; three
were fertilised by Herbert with their own pollen, and the
fourth was subsequently fertilised by the pollen of a compound
hybrid descended from three other and distinct species:
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 pollen of the hybrid
made vigorous growth and rapid progress to maturity, and
bore good seed, which vegetated freely.' In a letter to
me, in 1839, Mr Herbert told me that he had then tried the
experiment during five years, and he continued to try it
during several subsequent years, and always with the same
result. This result has, also, been confirmed by other observers
in the case of Hippeastrum with its sub-genera, and in the
case of some other genera, as Lobelia, Passiflora and Verbascum.
Although the plants in these experiments appeared perfectly
healthy, and although both the ovules and pollen of the
same flower were perfectly good with respect to other species,
yet as they were functionally imperfect in their mutual
self-action, we must infer that the plants were in an unnatural
state. Nevertheless these facts show on what slight and
mysterious causes the lesser or greater fertility of species
when crossed, in comparison with the same species when self-fertilised,
sometimes depends.
The practical experiments of horticulturists, though not
made with scientific precision, deserve some notice. It
is notorious in how complicated a manner the species of
Pelargonium, Fuchsia, Calceolaria, Petunia, Rhododendron,
&c., have been crossed, yet many of these hybrids seed
freely. For instance, Herbert asserts that a hybrid from
Calceolaria integrifolia and plantaginea, species most widely
dissimilar in general habit, 'reproduced itself as perfectly
as if it had been a natural species from the mountains of
Chile.' I have taken some pains to ascertain the degree
of fertility of some of the complex crosses of Rhododendrons,
and I am assured that many of them are perfectly fertile.
Mr C. Noble, for instance, informs me that he raises stocks
for grafting from a hybrid between Rhod. Ponticum and Catawbiense,
and that this hybrid 'seeds as freely as it is possible
to imagine.' Had hybrids, when fairly treated, gone on decreasing
in fertility in each successive generation, as Gärtner
believes to be the case, the fact would have been notorious
to nurserymen. Horticulturists raise large beds of the same
hybrids, and such alone are fairly treated, for by insect
agency the several individuals of the same hybrid variety
are allowed to freely cross with each other, and the injurious
influence of close interbreeding is thus prevented. Any
one may readily convince himself of the efficiency of insect-agency
by examining the flowers of the more sterile kinds of hybrid
rhododendrons, which produce no pollen, for he will find
on their stigmas plenty of pollen brought from other flowers.
In regard to animals, much fewer experiments have been
carefully tried than with plants. If our systematic arrangements
can be trusted, that is if the genera of animals are as
distinct from each other, as are the genera of plants, then
we may infer that animals more widely separated in the scale
of nature can be more easily crossed than in the case of
plants; but the hybrids themselves are, I think, more sterile.
I doubt whether any case of a perfectly fertile hybrid animal
can be considered as thoroughly well authenticated. It should,
however, be borne in mind that, owing to few animals breeding
freely under confinement, few experiments have been fairly
tried: for instance, the canary-bird has been crossed with
nine other finches, but as not one of these nine species
breeds freely in confinement, we have no right to expect
that the first crosses between them and the canary, or that
their hybrids, should be perfectly fertile. Again, with
respect to the fertility in successive generations of the
more fertile hybrid animals, I hardly know of an instance
in which two families of the same hybrid have been raised
at the same time from different parents, so as to avoid
the ill effects of close interbreeding. On the contrary,
brothers and sisters have usually been crossed in each successive
generation, in opposition to the constantly repeated admonition
of every breeder. And in this case, it is not at all surprising
that the inherent sterility in the hybrids should have gone
on increasing. If we were to act thus, and pair brothers
and sisters in the case of any pure animal, which from any
cause had the least tendency to sterility, the breed would
assuredly be lost in a very few generations.
Although I do not know of any thoroughly well-authenticated
cases of perfectly fertile hybrid animals, I have some reason
to believe that the hybrids from Cervulus vaginalis and
Reevesii, and from Phasianus colchicus with p. torquatus
and with p. versicolor are perfectly fertile. The hybrids
from the common and Chinese geese (A. cygnoides), species
which are so different that they are generally ranked in
distinct genera, have often bred in this country with either
pure parent, and in one single instance they have bred inter
se. This was effected by Mr Eyton, who raised two hybrids
from the same parents but from different hatches; and from
these two birds he raised no less than eight hybrids (grandchildren
of the pure geese) from one nest. In India, however, these
cross-bred geese must be far more fertile; for I am assured
by two eminently capable judges, namely Mr Blyth and Capt.
Hutton, that whole flocks of these crossed geese are kept
in various parts of the country; and as they are kept for
profit, where neither pure parent-species exists, they must
certainly be highly fertile.
A doctrine which originated with Pallas, has been largely
accepted by modern naturalists; namely, that most of our
domestic animals have descended from two or more aboriginal
species, since commingled by intercrossing. On this view,
the aboriginal species must either at first have produced
quite fertile hybrids, or the hybrids must have become in
subsequent generations quite fertile under domestication.
This latter alternative seems to me the most probable, and
I am inclined to believe in its truth, although its rests
on no direct evidence. I believe, for instance, that our
dogs have descended from several wild stocks; yet, with
perhaps the exception of certain indigenous domestic dogs
of South America, all are quite fertile together; and analogy
makes me greatly doubt, whether the several aboriginal species
would at first have freely bred together and have produced
quite fertile hybrids. So again there is reason to believe
that our European and the humped Indian cattle are quite
fertile together; but from facts communicated to me by Mr
Blyth, I think they must be considered as distinct species.
On this view of the origin of many of our domestic animals,
we must either give up the belief of the almost universal
sterility of distinct species of animals when crossed; or
we must look at sterility, not as an indelible characteristic,
but as one capable of being removed by domestication.
Finally, looking to all the ascertained facts on the intercrossing
of plants and animals, it may be concluded that some degree
of sterility, both in first crosses and in hybrids, is an
extremely general result; but that it cannot, under our
present state of knowledge, be considered as absolutely
universal.
Laws governing the Sterility of first Crosses and of
Hybrids. We will now consider a little more in detail
the circumstances and rules governing the sterility of first
crosses and of hybrids. Our chief object will be to see
whether or not the rules indicate that species have specially
been endowed with this quality, in order to prevent their
crossing and blending together in utter confusion. The following
rules and conclusions are chiefly drawn up from Gärtner's
admirable work on the hybridisation of plants. I have taken
much pains to ascertain how far the rules apply to animals,
and considering how scanty our knowledge is in regard to
hybrid animals, I have been surprised to find how generally
the same rules apply to both kingdoms.
It has been already remarked, that the degree of fertility,
both of first crosses and of hybrids, graduates from zero
to perfect fertility. It is surprising in how many curious
ways this gradation can be shown to exist; but only the
barest outline of the facts can here be given. When pollen
from a plant of one family is placed on the stigma of a
plant of a distinct family, it exerts no more influence
than so much inorganic dust. From this absolute zero of
fertility, the pollen of different species of the same genus
applied to the stigma of some one species, yields a perfect
gradation in the number of seeds produced, up to nearly
complete or even quite complete fertility; and, as we have
seen, in certain abnormal cases, even to an excess of fertility,
beyond that which the plant's own pollen will produce. So
in hybrids themselves, there are some which never have produced,
and probably never would produce, even with the pollen of
either pure parent, a single fertile seed: but in some of
these cases a first trace of fertility may be detected,
by the pollen of one of the pure parent-species causing
the flower of the hybrid to wither earlier than it otherwise
would have done; and the early withering of the flower is
well known to be a sign of incipient fertilisation. From
this extreme degree of sterility we have self-fertilised
hybrids producing a greater and greater number of seeds
up to perfect fertility.
Hybrids from two species which are very difficult to cross,
and which rarely produce any offspring, are generally very
sterile; but the parallelism between the difficulty of making
a first cross, and the sterility of the hybrids thus produced
two classes of facts which are generally confounded together
is by no means strict. There are many cases, in which two
pure species can be united with unusual facility, and produce
numerous hybrid-offspring, yet these hybrids are remarkably
sterile. On the other hand, there are species which can
be crossed very rarely, or with extreme difficulty, but
the hybrids, when at last produced, are very fertile. Even
within the limits of the same genus, for instance in Dianthus,
these two opposite cases occur.
The fertility, both of first crosses and of hybrids, is
more easily affected by unfavourable conditions, than is
the fertility of pure species. But the degree of fertility
is likewise innately variable; for it is not always the
same when the same two species are crossed under the same
circumstances, but depends in part upon the constitution
of the individuals which happen to have been chosen for
the experiment. So it is with hybrids, for their degree
of fertility is often found to differ greatly in the several
individuals raised from seed out of the same capsule and
exposed to exactly the same conditions.
By the term systematic affinity is meant, the resemblance
between species in structure and in constitution, more especially
in the structure of parts which are of high physiological
importance and which differ little in the allied species.
Now the fertility of first crosses between species, and
of the hybrids produced from them, is largely governed by
their systematic affinity. This is clearly shown by hybrids
never having been raised between species ranked by systematists
in distinct families; and on the other hand, by very closely
allied species generally uniting with facility. But the
correspondence between systematic affinity and the facility
of crossing is by no means strict. A multitude of cases
could be given of very closely allied species which will
not unite, or only with extreme difficulty; and on the other
hand of very distinct species which unite with the utmost
facility. In the same family there may be a genus, as Dianthus,
in which very many species can most readily be crossed;
and another genus, as Silene, in which the most persevering
efforts have failed to produce between extremely close species
a single hybrid. Even within the limits of the same genus,
we meet with this same difference; for instance, the many
species of Nicotiana have been more largely crossed than
the species of almost any other genus; but Gärtner
found that N. acuminata, which is not a particularly distinct
species, obstinately failed to fertilise, or to be fertilised
by, no less than eight other species of Nicotiana. Very
many analogous facts could be given.
No one has been able to point out what kind, or what amount,
of difference in any recognisable character is sufficient
to prevent two species crossing. It can be shown that plants
most widely different in habit and general appearance, and
having strongly marked differences in every part of the
flower, even in the pollen, in the fruit, and in the cotyledons,
can be crossed. Annual and perennial plants, deciduous and
evergreen trees, plants inhabiting different stations and
fitted for extremely different climates, can often be crossed
with ease.
By a reciprocal cross between two species, I mean the
case, for instance, of a stallion-horse being first crossed
with a female-ass, and then a male-ass with a mare: these
two species may then be said to have been reciprocally crossed.
There is often the widest possible difference in the facility
of making reciprocal crosses. Such cases are highly important,
for they prove that the capacity in any two species to cross
is often completely independent of their systematic affinity,
or of any recognisable difference in their whole organisation.
On the other hand, these cases clearly show that the capacity
for crossing is connected with constitutional differences
imperceptible by us, and confined to the reproductive system.
This difference in the result of reciprocal crosses between
the same two species was long ago observed by Kölreuter.
To give an instance: Mirabilis jalappa can easily be fertilised
by the pollen of M. longiflora, and the hybrids thus produced
are sufficiently fertile; but Kölreuter tried more
than two hundred times, during eight following years, to
fertilise reciprocally M. longiflora with the pollen of
M. jalappa, and utterly failed. Several other equally striking
cases could be given. Thuret has observed the same fact
with certain sea-weeds or Fuci. Gärtner, moreover,
found that this difference of facility in making reciprocal
crosses is extremely common in a lesser degree. He has observed
it even between forms so closely related (as Matthiola annua
and glabra) that many botanists rank them only as varieties.
It is also a remarkable fact, that hybrids raised from reciprocal
crosses, though of course compounded of the very same two
species, the one species having first been used as the father
and then as the mother, generally differ in fertility in
a small, and occasionally in a high degree.
Several other singular rules could be given from Gärtner:
for instance, some species have a remarkable power of crossing
with other species; other species of the same genus have
a remarkable power of impressing their likeness on their
hybrid offspring; but these two powers do not at all necessarily
go together. There are certain hybrids which instead of
having, as is usual, an intermediate character between their
two parents, always closely resemble one of them; and such
hybrids, though externally so like one of their pure parent-species,
are with rare exceptions extremely sterile. So again amongst
hybrids which are usually intermediate in structure between
their parents, exceptional and abnormal individuals sometimes
are born, which closely resemble one of their pure parents;
and these hybrids are almost always utterly sterile, even
when the other hybrids raised from seed from the same capsule
have a considerable degree of fertility. These facts show
how completely fertility in the hybrid is independent of
its external resemblance to either pure parent.
Considering the several rules now given, which govern
the fertility of first crosses and of hybrids, we see that
when forms, which must be considered as good and distinct
species, are united, their fertility graduates from zero
to perfect fertility, or even to fertility under certain
conditions in excess. That their fertility, besides being
eminently susceptible to favourable and unfavourable conditions,
is innately variable. That it is by no means always the
same in degree in the first cross and in the hybrids produced
from this cross. That the fertility of hybrids is not related
to the degree in which they resemble in external appearance
either parent. And lastly, that the facility of making a
first cross between any two species is not always governed
by their systematic affinity or degree of resemblance to
each other. This latter statement is clearly proved by reciprocal
crosses between the same two species, for according as the
one species or the other is used as the father or the mother,
there is generally some difference, and occasionally the
widest possible difference, in the facility of effecting
an union. The hybrids, moreover, produced from reciprocal
crosses often differ in fertility.
Now do these complex and singular rules indicate that
species have been endowed with sterility simply to prevent
their becoming confounded in nature? I think not. For why
should the sterility be so extremely different in degree,
when various species are crossed, all of which we must suppose
it would be equally important to keep from blending together?
Why should the degree of sterility be innately variable
in the individuals of the same species? Why should some
species cross with facility, and yet produce very sterile
hybrids; and other species cross with extreme difficulty,
and yet produce fairly fertile hybrids? Why should there
often be so great a difference in the result of a reciprocal
cross between the same two species? Why, it may even be
asked, has the production of hybrids been permitted? To
grant to species the special power of producing hybrids,
and then to stop their further propagation by different
degrees of sterility, not strictly related to the facility
of the first union between their parents, seems to be a
strange arrangement.
The foregoing rules and facts, on the other hand, appear
to me clearly to indicate that the sterility both of first
crosses and of hybrids is simply incidental or dependent
on unknown differences, chiefly in the reproductive systems,
of the species which are crossed. The differences being
of so peculiar and limited a nature, that, in reciprocal
crosses between two species the male sexual element of the
one will often freely act on the female sexual element of
the other, but not in a reversed direction. It will be advisable
to explain a little more fully by an example what I mean
by sterility being incidental on other differences, and
not a specially endowed quality. As the capacity of one
plant to be grafted or budded on another is so entirely
unimportant for its welfare in a state of nature, I presume
that no one will suppose that this capacity is a specially
endowed quality, but will admit that it is incidental on
differences in the laws of growth of the two plants. We
can sometimes see the reason why one tree will not take
on another, from differences in their rate of growth, in
the hardness of their wood, in the period of the flow or
nature of their sap, &c.; but in a multitude of cases
we can assign no reason whatever. Great diversity in the
size of two plants, one being woody and the other herbaceous,
one being evergreen and the other deciduous, and adaptation
to widely different climates, does not always prevent the
two grafting together. As in hybridisation, so with grafting,
the capacity is limited by systematic affinity, for no one
has been able to graft trees together belonging to quite
distinct families; and, on the other hand, closely allied
species, and varieties of the same species, can usually,
but not invariably, be grafted with ease. But this capacity,
as in hybridisation, is by no means absolutely governed
by systematic affinity. Although many distinct genera within
the same family have been grafted together, in other cases
species of the same genus will not take on each other. The
pear can be grafted far more readily on the quince, which
is ranked as a distinct genus, than on the apple, which
is a member of the same genus. Even different varieties
of the pear take with different degrees of facility on the
quince; so do different varieties of the apricot and peach
on certain varieties of the plum.
As Gärtner found that there was sometimes an innate
difference in different individuals of the same two
species in crossing; so Sagaret believes this to be the
case with different individuals of the same two species
in being grafted together. As in reciprocal crosses, the
facility of effecting an union is often very far from equal,
so it sometimes is in grafting; the common gooseberry, for
instance, cannot be grafted on the currant, whereas the
currant will take, though with difficulty, on the gooseberry.
We have seen that the sterility of hybrids, which have
their reproductive organs in an imperfect condition, is
a very different case from the difficulty of uniting two
pure species, which have their reproductive organs perfect;
yet these two distinct cases run to a certain extent parallel.
Something analogous occurs in grafting; for Thouin found
that three species of Robinia, which seeded freely on their
own roots, and which could be grafted with no great difficulty
on another species, when thus grafted were rendered barren.
On the other hand, certain species of Sorbus, when grafted
on other species, yielded twice as much fruit as when on
their own roots. We are reminded by this latter fact of
the extraordinary case of Hippeastrum, Lobelia, &c.,
which seeded much more freely when fertilised with the pollen
of distinct species, than when self-fertilised with their
own pollen.
We thus see, that although there is a clear and fundamental
difference between the mere adhesion of grafted stocks,
and the union of the male and female elements in the act
of reproduction, yet that there is a rude degree of parallelism
in the results of grafting and of crossing distinct species.
And as we must look at the curious and complex laws governing
the facility with which trees can be grafted on each other
as incidental on unknown differences in their vegetative
systems, so I believe that the still more complex laws governing
the facility of first crosses, are incidental on unknown
differences, chiefly in their reproductive systems. These
differences, in both cases, follow to a certain extent,
as might have been expected, systematic affinity, by which
every kind of resemblance and dissimilarity between organic
beings is attempted to be expressed. The facts by no means
seem to me to indicate that the greater or lesser difficulty
of either grafting or crossing together various species
has been a special endowment; although in the case of crossing,
the difficulty is as important for the endurance and stability
of specific forms, as in the case of grafting it is unimportant
for their welfare.
Causes of the Sterility of first Crosses and of Hybrids.
We may now look a little closer at the probable causes of
the sterility of first crosses and of hybrids. These two
cases are fundamentally different, for, as just remarked,
in the union of two pure species the male and female sexual
elements are perfect, whereas in hybrids they are imperfect.
Even in first crosses, the greater or lesser difficulty
in effecting a union apparently depends on several distinct
causes. There must sometimes be a physical impossibility
in the male element reaching the ovule, as would be the
case with a plant having a pistil too long for the pollen-tubes
to reach the ovarium. It has also been observed that when
pollen of one species is placed on the stigma of a distantly
allied species, though the pollen-tubes protrude, they do
not penetrate the stigmatic surface. Again, the male element
may reach the female element, but be incapable of causing
an embryo to be developed, as seems to have been the case
with some of Thuret's experiments on Fuci. No explanation
can be given of these facts, any more than why certain trees
cannot be grafted on others. Lastly, an embryo may be developed,
and then perish at an early period. This latter alternative
has not been sufficiently attended to; but I believe, from
observations communicated to me by Mr. Hewitt, who has had
great experience in hybridising gallinaceous birds, that
the early death of the embryo is a very frequent cause of
sterility in first crosses. I was at first very unwilling
to believe in this view; as hybrids, when once born, are
generally healthy and long-lived, as we see in the case
of the common mule. Hybrids, however, are differently circumstanced
before and after birth: when born and living in a country
where their two parents can live, they are generally placed
under suitable conditions of life. But a hybrid partakes
of only half of the nature and constitution of its mother,
and therefore before birth, as long as it is nourished within
its mother's womb or within the egg or seed produced by
the mother, it may be exposed to conditions in some degree
unsuitable, and consequently be liable to perish at an early
period; more especially as all very young beings seem eminently
sensitive to injurious or unnatural conditions of life.
In regard to the sterility of hybrids, in which the sexual
elements are imperfectly developed, the case is very different.
I have more than once alluded to a large body of facts,
which I have collected, showing that when animals and plants
are removed from their natural conditions, they are extremely
liable to have their reproductive systems seriously affected.
This, in fact, is the great bar to the domestication of
animals. Between the sterility thus superinduced and that
of hybrids, there are many points of similarity. In both
cases the sterility is independent of general health, and
is often accompanied by excess of size or great luxuriance.
In both cases, the sterility occurs in various degrees;
in both, the male element is the most liable to be affected;
but sometimes the female more than the male. In both, the
tendency goes to a certain extent with systematic affinity,
or whole groups of animals and plants are rendered impotent
by the same unnatural conditions; and whole groups of species
tend to produce sterile hybrids. On the other hand, one
species in a group will sometimes resist great changes of
conditions with unimpaired fertility; and certain species
in a group will produce unusually fertile hybrids. No one
can tell, till he tries, whether any particular animal will
breed under confinement or any plant seed freely under culture;
nor can he tell, till he tries, whether any two species
of a genus will produce more or less sterile hybrids. Lastly,
when organic beings are placed during several generations
under conditions not natural to them, they are extremely
liable to vary, which is due, as I believe, to their reproductive
systems having been specially affected, though in a lesser
degree than when sterility ensues. So it is with hybrids,
for hybrids in successive generations are eminently liable
to vary, as every experimentalist has observed.
Thus we see that when organic beings are placed under
new and unnatural conditions, and when hybrids are produced
by the unnatural crossing of two species, the reproductive
system, independently of the general state of health, is
affected by sterility in a very similar manner. In the one
case, the conditions of life have been disturbed, though
often in so slight a degree as to be inappreciable by us;
in the other case, or that of hybrids,the external conditions
have remained the same, but the organisation has been disturbed
by two different structures and constitutions having been
blended into one. For it is scarcely possible that two organisations
should be compounded into one, without some disturbance
occurring in the development, or periodical action, or mutual
relation of the different parts and organs one to another,
or to the conditions of life. When hybrids are able to breed
inter se, they transmit to their offspring from generation
to generation the same compounded organisation, and hence
we need not be surprised that their sterility, though in
some degree variable, rarely diminishes.
It must, however, be confessed that we cannot understand,
excepting on vague hypotheses, several facts with respect
to the sterility of hybrids; for instance, the unequal fertility
of hybrids produced from reciprocal crosses; or the increased
sterility in those hybrids which occasionally and exceptionally
resemble closely either pure parent. Nor do I pretend that
the foregoing remarks go to the root of the matter: no explanation
is offered why an organism, when placed under unnatural
conditions, is rendered sterile. All that I have attempted
to show, is that in two cases, in some respects allied,
sterility is the common result, in the one case from the
conditions of life having been disturbed, in the other case
from the organisation having been disturbed by two organisations
having been compounded into one.
It may seem fanciful, but I suspect that a similar parallelism
extends to an allied yet very different class of facts.
It is an old and almost universal belief, founded, I think,
on a considerable body of evidence, that slight changes
in the conditions of life are beneficial to all living things.
We see this acted on by farmers and gardeners in their frequent
exchanges of seed, tubers, &c., from one soil or climate
to another, and back again. During the convalescence of
animals, we plainly see that great benefit is derived from
almost any change in the habits of life. Again, both with
plants and animals, there is abundant evidence, that a cross
between very distinct individuals of the same species, that
is between members of different strains or sub-breeds, gives
vigour and fertility to the offspring. I believe, indeed,
from the facts alluded to in our fourth chapter, that a
certain amount of crossing is indispensable even with hermaphrodites;
and that close interbreeding continued during several generations
between the nearest relations, especially if these be kept
under the same conditions of life, always induces weakness
and sterility in the progeny.
Hence it seems that, on the one hand, slight changes in
the conditions of life benefit all organic beings, and on
the other hand, that slight crosses, that is crosses between
the males and females of the same species which have varied
and become slightly different, give vigour and fertility
to the offspring. But we have seen that greater changes,
or changes of a particular nature, often render organic
beings in some degree sterile; and that greater crosses,
that is crosses between males and females which have become
widely or specifically different, produce hybrids which
are generally sterile in some degree. I cannot persuade
myself that this parallelism is an accident or an illusion.
Both series of facts seem to be connected together by some
common but unknown bond, which is essentially related to
the principle of life.
Fertility of Varieties when crossed, and of their Mongrel
off-spring. It may be urged, as a most forcible argument,
that there must be some essential distinction between species
and varieties, and that there must be some error in all
the foregoing remarks, inasmuch as varieties, however much
they may differ from each other in external appearance,
cross with perfect facility, and yield perfectly fertile
offspring. I fully admit that this is almost invariably
the case. But if we look to varieties produced under nature,
we are immediately involved in hopeless difficulties; for
if two hitherto reputed varieties be found in any degree
sterile together, they are at once ranked by most naturalists
as species. For instance, the blue and red pimpernel, the
primrose and cowslip, which are considered by many of our
best botanists as varieties, are said by Gärtner not
to be quite fertile when crossed, and he consequently ranks
them as undoubted species. If we thus argue in a circle,
the fertility of all varieties produced under nature will
assuredly have to be granted.
If we turn to varieties, produced, or supposed to have
been produced, under domestication, we are still involved
in doubt. For when it is stated, for instance, that the
German Spitz dog unites more easily than other dogs with
foxes, or that certain South American indigenous domestic
dogs do not readily cross with European dogs, the explanation
which will occur to everyone, and probably the true one,
is that these dogs have descended from several aboriginally
distinct species. Nevertheless the perfect fertility of
so many domestic varieties, differing widely from each other
in appearance, for instance of the pigeon or of the cabbage,
is a remarkable fact; more especially when we reflect how
many species there are, which, though resembling each other
most closely, are utterly sterile when intercrossed. Several
considerations, however, render the fertility of domestic
varieties less remarkable than at first appears. It can,
in the first place, be clearly shown that mere external
dissimilarity between two species does not determine their
greater or lesser degree of sterility when crossed; and
we may apply the same rule to domestic varieties. In the
second place, some eminent naturalists believe that a long
course of domestication tends to eliminate sterility in
the successive generations of hybrids, which were at first
only slightly sterile; and if this be so, we surely ought
not to expect to find sterility both appearing and disappearing
under nearly the same conditions of life. Lastly, and this
seems to me by far the most important consideration, new
races of animals and plants are produced under domestication
by man's methodical and unconscious power of selection,
for his own use and pleasure: he neither wishes to select,
nor could select, slight differences in the reproductive
system, or other constitutional difference correlated with
the reproductive system. He supplies his several varieties
with the same food; treats them in nearly the same manner,
and does not wish to alter their general habits of life.
Nature acts uniformly and slowly during vast periods of
time on the whole organization, in any way which may be
for each creature's own good; and thus she may, either directly,
or more probably indirectly, through correlation, modify
the reproductive system in the several descendants from
any one species. Seeing this difference in the process of
selection, as carried on by man and nature, we need not
be surprised at some difference in the result.
I have as yet spoken as if the varieties of the same species
were invariably fertile when intercrossed. But it seems
to me impossible to resist the evidence of the existence
of a certain amount of sterility in the few following cases,
which I will briefly abstract. The evidence is at least
as good as that from which we believe in the sterility of
a multitude of species. The evidence is, also, derived from
hostile witnesses, who in all other cases consider fertility
and sterility as safe criterions of specific distinction.
Gärtner kept during several years a dwarf kind of maize
with yellow seeds, and a tall variety with red seeds, growing
near each other in his garden; and although these plants
have separated sexes, they never naturally crossed. He then
fertilized thirteen flowers of the one with the pollen of
the other; but only a single head produced any seed, and
this one head produced only five grains. Manipulation in
this case could not have been injurious, as the plants have
separated sexes. No one, I believe, has suspected that these
varieties of maize are distinct species; and it is important
to notice that the hybrid plants thus raised were themselves
perfectly fertile; so that even Gärtner did
not venture to consider the two varieties as specifically
distinct.
Girou de Buzareingues crossed three varieties of gourd,
which like the maize has separated sexes, and he asserts
that their mutual fertilization is by so much the less easy
as their differences are greater. How far these experiments
may be trusted, I know not; but the forms experimentised
on, are ranked by Sagaret, who mainly founds his classification
by the test of infertility, as varieties.
The following case is far more remarkable, and seems at
first quite incredible; but it is the result of an astonishing
number of experiments made during many years on nine species
of Verbascum, by so good an observer and so hostile a witness,
as Gärtner: namely, that yellow and white varieties
of the same species of Verbascum when intercrossed produce
less seed, than do either coloured varieties when fertilized
with pollen from their own coloured flowers. Moreover, he
asserts that when yellow and white varieties of one species
are crossed with yellow and white varieties of a distinct
species, more seed is produced by the crosses between the
same coloured flowers, than between those which are differently
coloured. Yet these varieties of Verbascum present no other
difference besides the mere colour of the flower; and one
variety can sometimes be raised from the seed of the other.
From observations which I have made on certain varieties
of hollyhock, I am inclined to suspect that they present
analogous facts.
Kölreuter, whose accuracy has been confirmed by every
subsequent observer, has proved the remarkable fact, that
one variety of the common tobacco is more fertile, when
crossed with a widely distinct species, than are the other
varieties. He experimentised on five forms, which are commonly
reputed to be varieties, and which he tested by the severest
trial, namely, by reciprocal crosses, and he found their
mongrel offspring perfectly fertile. But one of these five
varieties, when used either as father or mother, and crossed
with the Nicotiana glutinosa, always yielded hybrids not
so sterile as those which were produced from the four other
varieties when crossed with N. glutinosa. Hence the reproductive
system of this one variety must have been in some manner
and in some degree modified.
From these facts; from the great difficulty of ascertaining
the infertility of varieties in a state of nature, for a
supposed variety if infertile in any degree would generally
be ranked as species; from man selecting only external characters
in the production of the most distinct domestic varieties,
and from not wishing or being able to produce recondite
and functional differences in the reproductive system; from
these several considerations and facts, I do not think that
the very general fertility of varieties can be proved to
be of universal occurrence, or to form a fundamental distinction
between varieties and species. The general fertility of
varieties does not seem to me sufficient to overthrow the
view which I have taken with respect to the very general,
but not invariable, sterility of first crosses and of hybrids,
namely, that it is not a special endowment, but is incidental
on slowly acquired modifications, more especially in the
reproductive systems of the forms which are crossed.
Hybrids and Mongrels compared, independently of their
fertility. Independently of the question of fertility,
the offspring of species when crossed and of varieties when
crossed may be compared in several other respects. Gärtner,
whose strong wish was to draw a marked line of distinction
between species and varieties, could find very few and,
as it seems to me, quite unimportant differences between
the so-called hybrid offspring of species, and the so-called
mongrel offspring of varieties. And, on the other hand,
they agree most closely in very many important respects.
I shall here discuss this subject with extreme brevity.
The most important distinction is, that in the first generation
mongrels are more variable than hybrids; but Gärtner
admits that hybrids from species which have long been cultivated
are often variable in the first generation; and I have myself
seen striking instances of this fact. Gärtner further
admits that hybrids between very closely allied species
are more variable than those from very distinct species;
and this shows that the difference in the degree of variability
graduates away. When mongrels and the more fertile hybrids
are propagated for several generations an extreme amount
of variability in their offspring is notorious; but some
few cases both of hybrids and mongrels long retaining uniformity
of character could be given. The variability, however, in
the successive generations of mongrels is, perhaps, greater
than in hybrids.
This greater variability of mongrels than of hybrids does
not seem to me at all surprising. For the parents of mongrels
are varieties, and mostly domestic varieties (very few experiments
having been tried on natural varieties), and this implies
in most cases that there has been recent variability; and
therefore we might expect that such variability would often
continue and be super-added to that arising from the mere
act of crossing. The slight degree of variability in hybrids
from the first cross or in the first generation, in contrast
with their extreme variability in the succeeding generations,
is a curious fact and deserves attention. For it bears on
and corroborates the view which I have taken on the cause
of ordinary variability; namely, that it is due to the reproductive
system being eminently sensitive to any change in the conditions
of life, being thus often rendered either impotent or at
least incapable of its proper function of producing offspring
identical with the parent-form. Now hybrids in the first
generation are descended from species (excluding those long
cultivated) which have not had their reproductive systems
in any way affected, and they are not variable; but hybrids
themselves have their reproductive systems seriously affected,
and their descendants are highly variable.
But to return to our comparison of mongrels and hybrids:
Gärtner states that mongrels are more liable than hybrids
to revert to either parent-form; but this, if it be true,
is certainly only a difference in degree. Gärtner further
insists that when any two species, although most closely
allied to each other, are crossed with a third species,
the hybrids are widely different from each other; whereas
if two very distinct varieties of one species are crossed
with another species, the hybrids do not differ much. But
this conclusion, as far as I can make out, is founded on
a single experiment; and seems directly opposed to the results
of several experiments made by Kölreuter.
These alone are the unimportant differences, which Gärtner
is able to point out, between hybrid and mongrel plants.
On the other hand, the resemblance in mongrels and in hybrids
to their respective parents, more especially in hybrids
produced from nearly related species, follows according
to Gärtner the same laws. When two species are crossed,
one has sometimes a prepotent power of impressing its likeness
on the hybrid; and so I believe it to be with varieties
of plants. With animals one variety certainly often has
this prepotent power over another variety. Hybrid plants
produced from a reciprocal cross, generally resemble each
other closely; and so it is with mongrels from a reciprocal
cross. Both hybrids and mongrels can be reduced to either
pure parent-form, by repeated crosses in successive generations
with either parent.
These several remarks are apparently applicable to animals;
but the subject is here excessively complicated, partly
owing to the existence of secondary sexual characters; but
more especially owing to prepotency in transmitting likeness
running more strongly in one sex than in the other, both
when one species is crossed with another, and when one variety
is crossed with another variety. For instance, I think those
authors are right, who maintain that the ass has a prepotent
power over the horse, so that both the mule and the hinny
more resemble the ass than the horse; but that the prepotency
runs more strongly in the male-ass than in the female, so
that the mule, which is the offspring of the male-ass and
mare, is more like an ass, than is the hinny, which is the
offspring of the female-ass and stallion.
Much stress has been laid by some authors on the supposed
fact, that mongrel animals alone are born closely like one
of their parents; but it can be shown that this does sometimes
occur with hybrids; yet I grant much less frequently with
hybrids than with mongrels. Looking to the cases which I
have collected of cross-bred animals closely resembling
one parent, the resemblances seem chiefly confined to characters
almost monstrous in their nature, and which have suddenly
appeared such as albinism, melanism, deficiency of tail
or horns, or additional fingers and toes; and do not relate
to characters which have been slowly acquired by selection.
Consequently, sudden reversions to the perfect character
of either parent would be more likely to occur with mongrels,
which are descended from varieties often suddenly produced
and semi-monstrous in character, than with hybrids, which
are descended from species slowly and naturally produced.
On the whole I entirely agree with Dr Prosper Lucas, who,
after arranging an enormous body of facts with respect to
animals, comes to the conclusion, that the laws of resemblance
of the child to its parents are the same, whether the two
parents differ much or little from each other, namely in
the union of individuals of the same variety, or of different
varieties, or of distinct species.
Laying aside the question of fertility and sterility,
in all other respects there seems to be a general and close
similarity in the offspring of crossed species, and of crossed
varieties. If we look at species as having been specially
created, and at varieties as having been produced by secondary
laws, this similarity would be an astonishing fact. But
it harmonizes perfectly with the view that there is no essential
distinction between species and varieties.
Summary of Chapter. First crosses between forms
sufficiently distinct to be ranked as species, and their
hybrids, are very generally, but not universally, sterile.
The sterility is of all degrees, and is often so slight
that the two most careful experimentalists who have ever
lived, have come to diametrically opposite conclusions in
ranking forms by this test. The sterility is innately variable
in individuals of the same species, and is eminently susceptible
of favourable and unfavourable conditions. The degree of
sterility does not strictly follow systematic affinity,
but is governed by several curious and complex laws. It
is generally different, and sometimes widely different,
in reciprocal crosses between the same two species. It is
not always equal in degree in a first cross and in the hybrid
produced from this cross.
In the same manner as in grafting trees, the capacity
of one species or variety to take on another, is incidental
on generally unknown differences in their vegetative systems,
so in crossing, the greater or less facility of one species
to unite with another, is incidental on unknown differences
in their reproductive systems. There is no more reason to
think that species have been specially endowed with various
degrees of sterility to prevent them crossing and blending
in nature, than to think that trees have been specially
endowed with various and somewhat analogous degrees of difficulty
in being grafted together in order to prevent them becoming
inarched in our forests.
The sterility of first crosses between pure species, which
have their reproductive systems perfect, seems to depend
on several circumstances; in some cases largely on the early
death of the embryo. The sterility of hybrids, which have
their reproductive systems imperfect, and which have had
this system and their whole organisation disturbed by being
compounded of two distinct species, seems closely allied
to that sterility which so frequently affects pure species,
when their natural conditions of life have been disturbed.
This view is supported by a parallelism of another kind;
namely, that the crossing of forms only slightly different
is favourable to the vigour and fertility of their offspring;
and that slight changes in the conditions of life are apparently
favourable to the vigour and fertility of all organic beings.
It is not surprising that the degree of difficulty in uniting
two species, and the degree of sterility of their hybrid-offspring
should generally correspond, though due to distinct causes;
for both depend on the amount of difference of some kind
between the species which are crossed. Nor is it surprising
that the facility of effecting a first cross, the fertility
of the hybrids produced, and the capacity of being grafted
together though this latter capacity evidently depends on
widely different circumstances should all run, to a certain
extent, parallel with the systematic affinity of the forms
which are subjected to experiment; for systematic affinity
attempts to express all kinds of resemblance between all
species.
First crosses between forms known to be varieties, or
sufficiently alike to be considered as varieties, and their
mongrel offspring, are very generally, but not quite universally,
fertile. Nor is this nearly general and perfect fertility
surprising, when we remember how liable we are to argue
in a circle with respect to varieties in a state of nature;
and when we remember that the greater number of varieties
have been produced under domestication by the selection
of mere external differences, and not of differences in
the reproductive system. In all other respects, excluding
fertility, there is a close general resemblance between
hybrids and mongrels. Finally, then, the facts briefly given
in this chapter do not seem to me opposed to, but even rather
to support the view, that there is no fundamental distinction
between species and varieties.
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