- 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. |