HYBRIDISM.
ON THE GOOD DERIVED ON THE EFFECTS OF CROSSING—THE INFLUENCE OF
DOMESTICATION ON FERTILITY—CLOSE INTERBREEDING—GOOD AND EVIL RESULTS
FROM CHANGED CONDITIONS OF LIFE—VARIETIES WHEN CROSSED NOT INVARIABLY
FERTILE—ON THE DIFFERENCE IN FERTILITY BETWEEN CROSSED SPECIES AND
VARIETIES—CONCLUSIONS WITH RESPECT TO HYBRIDISM—LIGHT THROWN ON
HYBRIDISM BY THE ILLEGITIMATE PROGENY OF HETEROSTYLED PLANTS—STERILITY
OF CROSSED SPECIES DUE TO DIFFERENCES CONFINED TO THE REPRODUCTIVE
SYSTEM—NOT ACCUMULATED THROUGH NATURAL SELECTION—REASONS WHY DOMESTIC
VARIETIES ARE NOT MUTUALLY STERILE—TOO MUCH STRESS HAS BEEN LAID ON THE
DIFFERENCE IN FERTILITY BETWEEN CROSSED SPECIES AND CROSSED
VARIETIES—CONCLUSION.
It was shown in the fifteenth chapter that when individuals of the same
variety, or even of a distinct variety, are allowed freely to
intercross, uniformity of character is ultimately acquired. Some few
characters, however, are incapable of fusion, but these are
unimportant, as they are often of a semi-monstrous nature, and have
suddenly appeared. Hence, to preserve our domesticated breeds true, or
to improve them by methodical selection, it is obviously necessary that
they should be kept separate. Nevertheless, a whole body of individuals
may be slowly modified, through unconscious selection, as we shall see
in a future chapter, without separating them into distinct lots.
Domestic races have often been intentionally modified by one or two
crosses, made with some allied race, and occasionally even by repeated
crosses with very distinct races; but in almost all such cases,
long-continued and careful selection has been absolutely necessary,
owing to the excessive variability of the crossed offspring, due to the
principle of reversion. In a few instances, however, mongrels have
retained a uniform character from their first production.
When two varieties are allowed to cross freely, and one is much more
numerous than the other, the former will ultimately absorb the latter.
Should both varieties exist in nearly equal numbers, it is probable
that a considerable period would elapse before the acquirement of a
uniform character; and the character ultimately acquired would largely
depend on prepotency of transmission and on the conditions of life; for
the nature of these conditions would generally favour one variety more
than another, so that a kind of natural selection would come into play.
Unless the crossed offspring were slaughtered by man without the least
discrimination, some degree of unmethodical selection would likewise
come into action. From these several considerations we may infer, that
when two or more closely allied species first came into the possession
of the same tribe, their crossing will not have influenced, in so great
a degree as has often been supposed, the character of the offspring in
future times; although in some cases it probably has had a considerable
effect.
Domestication, as a general rule, increases the prolificness of animals
and plants. It eliminates the tendency to sterility which is common to
species when first taken from a state of nature and crossed. On this
latter head we have no direct evidence; but as our races of dogs,
cattle, pigs etc., are almost certainly descended from aboriginally
distinct stocks, and as these races are now fully fertile together, or
at least incomparably more fertile than most species when crossed, we
may with entire confidence accept this conclusion.
Abundant evidence has been given that crossing adds to the size,
vigour, and fertility of the offspring. This holds good when there has
been no previous close interbreeding. It applies to the individuals of
the same variety but belonging to different families, to distinct
varieties, sub-species, and even to species. In the latter case, though
size is gained, fertility is lost; but the increased size, vigour, and
hardiness of many hybrids cannot be accounted for solely on the
principle of compensation from the inaction of the reproductive system.
Certain plants whilst growing under their natural conditions, others
when cultivated, and others of hybrid origin, are completely
self-impotent, though perfectly healthy; and such plants can be
stimulated to fertility only by being crossed with other individuals of
the same or of a distinct species.
On the other hand, long-continued close interbreeding between the
nearest relations diminishes the constitutional vigour, size, and
fertility of the offspring; and occasionally leads to malformations,
but not necessarily to general deterioration of form or structure. This
failure of fertility shows that the evil results of interbreeding are
independent of the augmentation of morbid tendencies common to both
parents, though this augmentation no doubt is often highly injurious.
Our belief that evil follows from close interbreeding rests to a
certain extent on the experience of practical breeders, especially of
those who have reared many animals of quickly propagating kinds; but it
likewise rests on several carefully recorded experiments. With some
animals close interbreeding may be carried on for a long period with
impunity by the selection of the most vigorous and healthy individuals;
but sooner or later evil follows. The evil, however, comes on so slowly
and gradually that it easily escapes observation, but can be recognised
by the almost instantaneous manner in which size, constitutional
vigour, and fertility are regained when animals that have long been
interbred are crossed with a distinct family.
These two great classes of facts, namely, the good derived from
crossing, and the evil from close interbreeding, with the consideration
of the innumerable adaptations throughout nature for compelling, or
favouring, or at least permitting, the occasional union of distinct
individuals, taken together, lead to the conclusion that it is a law of
nature that organic beings shall not fertilise themselves for
perpetuity. This law was first plainly hinted at in 1799, with respect
to plants, by Andrew Knight[1] and, not long afterwards, that sagacious
observer Kölreuter, after showing how well the Malvaceæ are adapted for
crossing, asks, “an id aliquid in recessu habeat, quod hujuscemodi
flores nunquam proprio suo pulvere, sed semper eo aliarum su speciei
impregnentur, merito quæritur? Certe natura nil facit frustra.”
Although we may demur to Kölreuter’s saying that nature does nothing in
vain, seeing how many rudimentary and useless organs there are, yet
undoubtedly the argument from the innumerable contrivances, which
favour crossing, is of the greatest weight. The most important result
of this law is that it leads to uniformity of character in the
individuals of the same species. In the case of certain hermaphrodites,
which probably intercross only at long intervals of time, and with
unisexual animals inhabiting somewhat separated localities, which can
only occasionally come into contact and pair, the greater vigour and
fertility of the crossed offspring will ultimately tend to give
uniformity of character. But when we go beyond the limits of the same
species, free intercrossing is barred by the law of sterility.
In searching for facts which might throw light on the cause of the good
effects from crossing, and of the evil effects from close
interbreeding, we have seen that, on the one hand, it is a widely
prevalent and ancient belief, that animals and plants profit from
slight changes in their condition of life; and it would appear that the
germ, in a somewhat analogous manner, is more effectually stimulated by
the male element, when taken from a distinct individual, and therefore
slightly modified in nature, than when taken from a male having the
same identical constitution. On the other hand, numerous facts have
been given, showing that when animals are first subjected to captivity,
even in their native land, and although allowed much liberty, their
reproductive functions are often greatly impaired or quite annulled.
Some groups of animals are more affected than others, but with
apparently capricious exceptions in every group. Some animals never or
rarely couple under confinement; some couple freely, but never or
rarely conceive. The secondary male characters, the maternal functions
and instincts, are occasionally affected. With plants, when first
subjected to cultivation, analogous facts have been observed. We
probably owe our double flowers, rich seedless fruits, and in some
cases greatly developed tubers, etc., to incipient sterility of the
above nature combined with a copious supply of nutriment. Animals which
have long been domesticated, and plants which have long been
cultivated, can generally withstand, with unimpaired fertility, great
changes in their conditions of life; though both are sometimes slightly
affected. With animals the somewhat rare capacity of breeding freely
under confinement, together with their utility, mainly determine the
kinds which have been domesticated.
We can in no case precisely say what is the cause of the diminished
fertility of an animal when first captured, or of a plant when first
cultivated; we can only infer that it is caused by a change of some
kind in the natural conditions of life. The remarkable susceptibility
of the reproductive system to such changes,—a susceptibility not common
to any other organ,—apparently has an important bearing on Variability,
as we shall see in a future chapter.
It is impossible not to be struck with the double parallelism between
the two classes of facts just alluded to. On the one hand, slight
changes in the conditions of life, and crosses between slightly
modified forms or varieties, are beneficial as far as prolificness and
constitutional vigour are concerned. On the other hand, changes in the
conditions greater in degree, or of a different nature, and crosses
between forms which have been slowly and greatly modified by natural
means,—in other words, between species,—are highly injurious, as far as
the reproductive system is concerned, and in some few instances as far
as constitutional vigour is concerned. Can this parallelism be
accidental? Does it not rather indicate some real bond of connection?
As a fire goes out unless it be stirred up, so the vital forces are
always tending, according to Mr. Herbert Spencer, to a state of
equilibrium, unless disturbed and renovated through the action of other
forces.
In some few cases varieties tend to keep distinct, by breeding at
different seasons, by great difference in size, or by sexual
preference. But the crossing of varieties, far from diminishing,
generally adds to the fertility of the first union and of the mongrel
offspring. Whether all the more widely distinct domestic varieties are
invariably quite fertile when crossed, we do not positively know; much
time and trouble would be requisite for the necessary experiments, and
many difficulties occur, such as the descent of the various races from
aboriginally distinct species, and the doubts whether certain forms
ought to be ranked as species or varieties. Nevertheless, the wide
experience of practical breeders proves that the great majority of
varieties, even if some should hereafter prove not to be indefinitely
fertile _inter se,_ are far more fertile when crossed, than the vast
majority of closely allied natural species. A few remarkable cases
have, however, been given on the authority of excellent observers,
showing that with plants certain forms, which undoubtedly must be
ranked as varieties, yield fewer seeds when crossed than is natural to
the parent-species. Other varieties have had their reproductive powers
so far modified that they are either more or less fertile than their
parents, when crossed with a distinct species.
Nevertheless, the fact remains indisputable that domesticated
varieties, of animals and of plants, which differ greatly from one
another in structure, but which are certainly descended from the same
aboriginal species, such as the races of the fowl, pigeon, many
vegetables, and a host of other productions, are extremely fertile when
crossed; and this seems to make a broad and impassable barrier between
domestic varieties and natural species. But, as I will now attempt to
show, the distinction is not so great and overwhelmingly important as
it at first appears.
_On the Difference in Fertility between Varieties and Species
when crossed._
This work is not the proper place for fully treating the subject of
hybridism, and I have already given in my ‘Origin of Species’ a
moderately full abstract. I will here merely enumerate the general
conclusions which may be relied on, and which bear on our present
point.
_Firstly,_ the laws governing the production of hybrids are identical,
or nearly identical, in the animal and vegetable kingdoms.
_Secondly,_ the sterility of distinct species when first united, and
that of their hybrid offspring, graduate, by an almost infinite number
of steps, from zero, when the ovule is never impregnated and a
seed-capsule is never formed, up to complete fertility. We can only
escape the conclusion that some species are fully fertile when crossed,
by determining to designate as varieties all the forms which are quite
fertile. This high degree of fertility is, however, rare. Nevertheless,
plants, which have been exposed to unnatural conditions, sometimes
become modified in so peculiar a manner, that they are much more
fertile when crossed with a distinct species than when fertilised by
their own pollen. Success in effecting a first union between two
species, and the fertility of their hybrids, depend in an eminent
degree on the conditions of life being favourable. The innate sterility
of hybrids of the same parentage and raised from the same seed-capsule
often differs much in degree.
_Thirdly,_ the degree of sterility of a first cross between two species
does not always run strictly parallel with that of their hybrid
offspring. Many cases are known of species which can be crossed with
ease, but yield hybrids excessively sterile; and conversely some which
can be crossed with great difficulty, but produce fairly fertile
hybrids. This is an inexplicable fact, on the view that species have
been specially endowed with mutual sterility in order to keep them
distinct.
_Fourthly,_ the degree of sterility often differs greatly in two
species when reciprocally crossed; for the first will readily fertilise
the second; but the latter is incapable, after hundreds of trials, of
fertilising the former. Hybrids produced from reciprocal crosses
between the same two species likewise sometimes differ in their degree
of sterility. These cases also are utterly inexplicable on the view of
sterility being a special endowment.
_Fifthly,_ the degree of sterility of first crosses and of hybrids
runs, to a certain extent, parallel with the general or systematic
affinity of the forms which are united. For species belonging to
distinct genera can rarely, and those belonging to distinct families
can never, be crossed. The parallelism, however, is far from complete;
for a multitude of closely allied species will not unite, or unite with
extreme difficulty, whilst other species, widely different from one
another, can be crossed with perfect facility. Nor does the difficulty
depend on ordinary constitutional differences, for annual and perennial
plants, deciduous and evergreen trees, plants flowering at different
seasons, inhabiting different stations, and naturally living under the
most opposite climates, can often be crossed with ease. The difficulty
or facility apparently depends exclusively on the sexual constitution
of the species which are crossed; or on their sexual elective affinity,
_i.e. Wahlverwandtschaft_ of Gärtner. As species rarely or never become
modified in one character, without being at the same time modified in
many characters, and as systematic affinity includes all visible
similarities and dissimilarities, any difference in sexual constitution
between two species would naturally stand in more or less close
relation with their systematic position.
_Sixthly,_ the sterility of species when first crossed, and that of
hybrids, may possibly depend to a certain extent on distinct causes.
With pure species the reproductive organs are in a perfect condition,
whilst with hybrids they are often plainly deteriorated. A hybrid
embryo which partakes of the constitution of its father and mother is
exposed to unnatural conditions, as long as it is nourished within the
womb, or egg, or seed of the mother-form; and as we know that unnatural
conditions often induce sterility, the reproductive organs of the
hybrid might at this early age be permanently affected. But this cause
has no bearing on the infertility of first unions. The diminished
number of the offspring from first unions may often result, as is
certainly sometimes the case, from the premature death of most of the
hybrid embryos. But we shall immediately see that a law of an unknown
nature apparently exists, which leads to the offspring from unions,
which are infertile, being themselves more or less infertile; and this
at present is all that can be said.
_Seventhly,_ hybrids and mongrels present, with the one great exception
of fertility, the most striking accordance in all other respects;
namely, in the laws of their resemblance to their two parents, in their
tendency to reversion, in their variability, and in being absorbed
through repeated crosses by either parent-form.
After arriving at these conclusions, I was led to investigate a subject
which throws considerable light on hybridism, namely, the fertility of
heterostyled or dimorphic and trimorphic plants, when illegitimately
united. I have had occasion several times to allude to these plants,
and I may here give a brief abstract of my observations. Several plants
belonging to distinct orders present two forms, which exist in about
equal numbers, and which differ in no respect except in their
reproductive organs; one form having a long pistil with short stamens,
the other a short pistil with long stamens; both with differently sized
pollen-grains. With trimorphic plants there are three forms likewise
differing in the lengths of their pistils and stamens, in the size and
colour of the pollen-grains, and in some other respects; and as in each
of the three forms there are two sets of stamens, there are altogether
six sets of stamens and three kinds of pistils. These organs are so
proportioned in length to one another that, in any two of the forms,
half the stamens in each stand on a level with the stigma of the third
form. Now I have shown, and the result has been confirmed by other
observers, that, in order to obtain full fertility with these plants,
it is necessary that the stigma of the one form should be fertilised by
pollen taken from the stamens of corresponding height in the other
form. So that with dimorphic species two unions, which may be called
legitimate, are fully fertile, and two, which may be called
illegitimate, are more or less infertile. With trimorphic species six
unions are legitimate, or fully fertile, and twelve are illegitimate,
or more or less infertile.[2]
The infertility which may be observed in various dimorphic and
trimorphic plants, when illegitimately fertilised, that is, by pollen
taken from stamens not corresponding in height with the pistil, differs
much in degree, up to absolute and utter sterility; just in the same
manner as occurs in crossing distinct species. As the degree of
sterility in the latter case depends in an eminent degree on the
conditions of life being more or less favourable, so I have found it
with illegitimate unions. It is well known that if pollen of a distinct
species be placed on the stigma of a flower, and its own pollen be
afterwards, even after a considerable interval of time, placed on the
same stigma, its action is so strongly prepotent that it generally
annihilates the effect of the foreign pollen; so it is with the pollen
of the several forms of the same species, for legitimate pollen is
strongly prepotent over illegitimate pollen, when both are placed on
the same stigma. I ascertained this by fertilising several flowers,
first illegitimately, and twenty-four hours afterwards legitimately,
with pollen taken from a peculiarly coloured variety, and all the
seedlings were similarly coloured; this shows that the legitimate
pollen, though applied twenty-four hours subsequently, had wholly
destroyed or prevented the action of the previously applied
illegitimate pollen. Again, as, in making reciprocal crosses between
the same two species, there is occasionally a great difference in the
result, so the same thing occurs with trimorphic plants; for instance,
the mid-styled form of _Lythrum salicaria_ could be illegitimately
fertilised with the greatest ease by pollen from the longer stamens of
the short-styled form, and yielded many seeds; but the short-styled
form did not yield a single seed when fertilised by the longer stamens
of the mid-styled form.
In all these respects the forms of the same undoubted species, when
illegitimately united, behave in exactly the same manner as do two
distinct species when crossed. This led me carefully to observe during
four years many seedlings, raised from several illegitimate unions. The
chief result is that these illegitimate plants, as they may be called,
are not fully fertile. It is possible to raise from dimorphic species,
both long-styled and short-styled illegitimate plants, and from
trimorphic plants all three illegitimate forms. These can then be
properly united in a legitimate manner. When this is done, there is no
apparent reason why they should not yield as many seeds as did their
parents when legitimately fertilised. But such is not the case; they
are all infertile, but in various degrees; some being so utterly and
incurably sterile that they did not yield during four seasons a single
seed or even seed-capsule. These illegitimate plants, which are so
sterile, although united with each other in a legitimate manner, may be
strictly compared with hybrids when crossed _inter se,_ and it is well
known how sterile these latter generally are. When, on the other hand,
a hybrid is crossed with either pure parent-species, the sterility is
usually much lessened: and so it is when an illegitimate plant is
fertilised by a legitimate plant. In the same manner as the sterility
of hybrids does not always run parallel with the difficulty of making
the first cross between the two parent-species, so the sterility of
certain illegitimate plants was unusually great, whilst the sterility
of the union from which they were derived was by no means great. With
hybrids raised from the same seed-capsule the degree of sterility is
innately variable, so it is in a marked manner with illegitimate
plants. Lastly, many hybrids are profuse and persistent flowerers,
whilst other and more sterile hybrids produce few flowers, and are
weak, miserable dwarfs; exactly similar cases occur with the
illegitimate offspring of various dimorphic and trimorphic plants.
Although there is the closest identity in character and behaviour
between illegitimate plants and hybrids, it is hardly an exaggeration
to maintain that the former are hybrids, but produced within the limits
of the same species by the improper union of certain forms, whilst
ordinary hybrids are produced from an improper union between so-called
distinct species. We have already seen that there is the closest
similarity in all respects between first illegitimate unions, and first
crosses between distinct species. This will perhaps be made more fully
apparent by an illustration:—we may suppose that a botanist found two
well-marked varieties (and such occur) of the long-styled form of the
trimorphic _Lithrum salicaria,_ and that he determined to try by
crossing whether they were specifically distinct. He would find that
they yielded only about one-fifth of the proper number of seed, and
that they behaved in all the other above-specified respects as if they
had been two distinct species. But to make the case sure, he would
raise plants from his supposed hybridised seed, and he would find that
the seedlings were miserably dwarfed and utterly sterile, and that they
behaved in all other respects like ordinary hybrids, he might then
maintain that he had actually proved, in accordance with the common
view, that his two varieties were as good and as distinct species as
any in the world; but he would be completely mistaken.
The facts now given on dimorphic and trimorphic plants are important,
because they show us, first, that the physiological test of lessened
fertility, both in first crosses and in hybrids, is no criterion of
specific distinction; secondly, because we may conclude that there is
some unknown bond which connects the infertility of illegitimate unions
with that of their illegitimate offspring, and we are led to extend the
same view to first crosses and hybrids; thirdly, because we find, and
this seems to me of especial importance, that two or three forms of the
same species may exist and may differ in no respect whatever, either in
structure or in constitution, relatively to external conditions, and
yet be sterile when united in certain ways. For we must remember that
it is the union of the sexual elements of individuals of the same form,
for instance, of two long-styled forms, which results in sterility;
whilst it is the union of the sexual element proper to two distinct
forms which is fertile. Hence the case appears at first sight exactly
the reverse of what occurs in the ordinary unions of the individuals of
the same species, and with crosses between distinct species. It is,
however, doubtful whether this is really so; but I will not enlarge on
this obscure subject.
We may, however, infer as probable from the consideration of dimorphic
and trimorphic plants, that the sterility of distinct species when
crossed, and of their hybrid progeny, depends exclusively on the nature
of their sexual elements, and not on any difference in their structure
or general constitution. We are also led to this same conclusion by
considering reciprocal crosses, in which the male of one species cannot
be united, or only with great difficulty, with the female of a second
species, whilst the converse cross can be effected with perfect
facility. That excellent observer, Gärtner, likewise concluded that
species when crossed are sterile owing to differences confined to their
reproductive systems.
On the principle which makes it necessary for man, whilst he is
selecting and improving his domestic varieties, to keep them separate,
it would clearly be advantageous to varieties in a state of nature,
that is to incipient species, if they could be kept from blending,
either through sexual aversion, or by becoming mutually sterile. Hence
it at one time appeared to me probable, as it has to others, that this
sterility might have been acquired through natural selection. On this
view we must suppose that a shade of lessened fertility first
spontaneously appeared, like any other modification, in certain
individuals of a species when crossed with other individuals of the
same species; and that successive slight degrees of infertility, from
being advantageous, were slowly accumulated. This appears all the more
probable, if we admit that the structural differences between the forms
of dimorphic and trimorphic plants, as the length and curvature of the
pistil, etc., have been co-adapted through natural selection; for if
this be admitted, we can hardly avoid extending the same conclusion to
their mutual infertility. Sterility, moreover, has been acquired
through natural selection for other and widely different purposes, as
with neuter insects in reference to their social economy. In the case
of plants, the flowers on the circumference of the truss in the guelder
rose (_Viburnum opulus_) and those on the summit of the spike in the
feather-hyacinth (_Muscari comosum_) have been rendered conspicuous,
and apparently in consequence sterile, in order that insects might
easily discover and visit the perfect flowers. But when we endeavour to
apply the principle of natural selection to the acquirement by distinct
species of mutual sterility, we meet with great difficulties. In the
first place, it may be remarked that separate regions are often
inhabited by groups of species or by single species, which when brought
together and crossed are found to be more or less sterile; now it could
clearly have been no advantage to such separated species to have been
rendered mutually sterile, and consequently this could not have been
effected through natural selection; but it may perhaps be argued, that,
if a species were rendered sterile with some one compatriot, sterility
with other species would follow as a necessary consequence. In the
second place, it is as much opposed to the theory of natural selection,
as to the theory of special creation, that in reciprocal crosses the
male element of one form should have been rendered utterly impotent on
a second form, whilst at the same time the male element of this second
form is enabled freely to fertilise the first form; for this peculiar
state of the reproductive system could not possibly have been
advantageous to either species.
In considering the probability of natural selection having come into
action in rendering species mutually sterile, one of the greatest
difficulties will be found to lie in the existence of many graduated
steps from slightly lessened fertility to absolute sterility. It may be
admitted, on the principle above explained, that it would profit an
incipient species if it were rendered in some slight degree sterile
when crossed with its parent-form or with some other variety; for thus
fewer bastardised and deteriorated offspring would be produced to
commingle their blood with the new species in process of formation. But
he who will take the trouble to reflect on the steps by which this
first degree of sterility could be increased through natural selection
to that higher degree which is common to so many species, and which is
universal with species which have been differentiated to a generic or
family rank, will find the subject extraordinarily complex. After
mature reflection it seems to me that this could not have been effected
through natural selection. Take the case of any two species which, when
crossed, produce few and sterile offspring; now, what is there which
could favour the survival of those individuals which happened to be
endowed in a slightly higher degree with mutual infertility, and which
thus approached by one small step towards absolute sterility? Yet an
advance of this kind, if the theory of natural selection be brought to
bear, must have incessantly occurred with many species, for a multitude
are mutually quite barren. With sterile neuter insects we have reason
to believe that modifications in their structure and fertility have
been slowly accumulated by natural selection, from an advantage having
been thus indirectly given to the community to which they belonged over
other communities of the same species; but an individual animal not
belonging to a social community, if rendered slightly sterile when
crossed with some other variety, would not thus itself gain any
advantage or indirectly give any advantage to the other individuals of
the same variety, thus leading to their preservation.
But it would be superfluous to discuss this question in detail; for
with plants we have conclusive evidence that the sterility of crossed
species must be due to some principle, quite independent of natural
selection. Both Gärtner and Kolreuter have proved that in general
including numerous species, a series can be formed from species which
when crossed yield fewer and fewer seeds, to species which never
produce a single seed, but yet are affected by the pollen of certain
other species, for the germen swells. It is here manifestly impossible
to select the more sterile individuals, which have already ceased to
yield seeds; so that this acme of sterility, when the germen alone is
affected, cannot have been gained through selection; and from the laws
governing the various grades of sterility being so uniform throughout
the animal and vegetable kingdoms, we may infer that the cause,
whatever it may be, is the same or nearly the same in all cases.
As species have not been rendered mutually infertile through the
accumulative action of natural selection, and as we may safely
conclude, from the previous as well as from other and more general
considerations, that they have not been endowed through an act of
creation with this quality, we must infer that it has arisen
incidentally during their slow formation in connection with other and
unknown changes in their organisation. By a quality arising
incidentally, I refer to such cases as different species of animals and
plants being differently affected by poisons to which they are not
naturally exposed; and this difference in susceptibility is clearly
incidental on other and unknown differences in their organisation. So
again the capacity in different kinds of trees to be grafted on each
other, or on a third species, differs much, and is of no advantage to
these trees, but is incidental on structural or functional differences
in their woody tissues. We need not feel surprise at sterility
incidentally resulting from crosses between distinct species,—the
modified descendants of a common progenitor,—when we bear in mind how
easily the reproductive system is affected by various causes—often by
extremely slight changes in the conditions of life, by too close
interbreeding, and by other agencies. It is well to bear in mind such
cases as that of the _Passiflora alata,_ which recovered its
self-fertility from being grafted on a distinct species—the cases of
plants which normally or abnormally are self-impotent, but can readily
be fertilised by the pollen of a distinct species—and lastly the cases
of individual domesticated animals which evince towards each other
sexual incompatibility.
We now at last come to the immediate point under discussion: how is it
that, with some few exceptions in the case of plants, domesticated
varieties, such as those of the dog, fowl, pigeon, several fruit-trees,
and culinary vegetables, which differ from each other in external
characters more than many species, are perfectly fertile when crossed,
or even fertile in excess, whilst closely allied species are almost
invariably in some degree sterile? We can, to a certain extent, give a
satisfactory answer to this question. Passing over the fact that the
amount of external difference between two species is no sure guide to
their degree of mutual sterility, so that similar differences in the
case of varieties would be no sure guide, we know that with species the
cause lies exclusively in differences in their sexual constitution. Now
the conditions to which domesticated animals and cultivated plants have
been subjected have had so little tendency towards modifying the
reproductive system in a manner leading to mutual sterility, that we
have very good grounds for admitting the directly opposite doctrine of
Pallas, namely, that such conditions generally eliminate this tendency;
so that the domesticated descendants of species, which in their natural
state would have been in some degree sterile when crossed, become
perfectly fertile together. With plants, so far is cultivation from
giving a tendency towards mutual sterility, that in several
well-authenticated cases, already often alluded to, certain species
have been affected in a very different manner, for they have become
self-impotent, whilst still retaining the capacity of fertilising, and
being fertilised by, distinct species. If the Pallasian doctrine of the
elimination of sterility through long-continued domestication be
admitted, and it can hardly be rejected, it becomes in the highest
degree improbable that similar circumstances should commonly both
induce and eliminate the same tendency; though in certain cases, with
species having a peculiar constitution, sterility might occasionally be
thus induced. Thus, as I believe, we can understand why with
domesticated animals varieties have not been produced which are
mutually sterile; and why with plants only a few such cases have been
observed, namely, by Gärtner, with certain varieties of maize and
verbascum, by other experimentalists with varieties of the gourd and
melon, and by Kölreuter with one kind of tobacco.
With respect to varieties which have originated in a state of nature,
it is almost hopeless to expect to prove by direct evidence that they
have been rendered mutually sterile; for if even a trace of sterility
could be detected, such varieties would at once be raised by almost
every naturalist to the rank of distinct species. If, for instance,
Gärtner’s statement were fully confirmed, that the blue and red
flowered forms of the pimpernel (_Anagallis arvensis_) are sterile when
crossed, I presume that all the botanists who now maintain on various
grounds that these two forms are merely fleeting varieties, would at
once admit that they were specifically distinct.
The real difficulty in our present subject is not, as it appears to me,
why domestic varieties have not become mutually infertile when crossed,
but why this has so generally occurred with natural varieties as soon
as they have been modified in a sufficient and permanent degree to take
rank as species. We are far from precisely knowing the cause; but we
can see that the species, owing to their struggle for existence with
numerous competitors, must have been exposed to more uniform conditions
of life during long periods of time than domestic varieties have been,
and this may well make a wide difference in the result. For we know how
commonly wild animals and plants, when taken from their natural
conditions and subjected to captivity, are rendered sterile; and the
reproductive functions of organic beings which have always lived and
been slowly modified under natural conditions would probably in like
manner be eminently sensitive to the influence of an unnatural cross.
Domesticated productions, on the other hand, which, as shown by the
mere fact of their domestication, were not originally highly sensitive
to changes in their conditions of life, and which can now generally
resist with undiminished fertility repeated changes of conditions,
might be expected to produce varieties, which would be little liable to
have their reproductive powers injuriously affected by the act of
crossing with other varieties which had originated in a like manner.
Certain naturalists have recently laid too great stress, as it appears
to me, on the difference in fertility between varieties and species
when crossed. Some allied species of trees cannot be grafted on one
another, whilst all varieties can be so grafted. Some allied animals
are affected in a very different manner by the same poison, but with
varieties no such case until recently was known; whilst now it has been
proved that immunity from certain poisons sometimes stands in
correlation with the colour of the individuals of the same species. The
period of gestation generally differs much in distinct species, but
with varieties until lately no such difference had been observed. Here
we have various physiological differences, and no doubt others could be
added, between one species and another of the same genus, which do not
occur, or occur with extreme rarity, in the case of varieties; and
these differences are apparently wholly or in chief part incidental on
other constitutional differences, just in the same manner as the
sterility of crossed species is incidental on differences confined to
the sexual system. Why, then, should these latter differences, however
serviceable they may indirectly be in keeping the inhabitants of the
same country distinct, be thought of such paramount importance, in
comparison with other incidental and functional differences? No
sufficient answer to this question can be given. Hence the fact that
widely distinct domestic varieties are, with rare exceptions, perfectly
fertile when crossed, and produce fertile offspring, whilst closely
allied species are, with rare exceptions, more or less sterile, is not
nearly so formidable an objection as it appears at first to the theory
of the common descent of allied species.
REFERENCES
[1] ‘Transactions Phil. Soc.,’ 1799, p. 202. For Kölreuter _see_ ‘Mém.
de l’Acad. de St.-Pétersbourg,’ tom. iii. 1809 (published 1811) p.
197. In reading C. K. Sprengel’s remarkable work, ‘Das entdeckte
Geheimniss,’ etc., 1793, it is curious to observe how often this
wonderfully acute observer failed to understand the full meaning of
the structure of the flowers which he has so well described, from not
always having before his mind the key to the problem, namely, the good
derived from the crossing of distinct individual plants.
[2] My observations ‘On the Character and hybrid-like nature of the
offspring from the illegitimate union of Dimorphic and Trimorphic
Plants’ were published in the ‘Journal of the Linnean Soc.,’ vol. x.
p. 393. The abstract here given is nearly the same with that which
appeared in the 6th edition of my ‘Origin of Species.’