SUMMARY OF THE FOUR LAST CHAPTERS, WITH REMARKS ON HYBRIDISM.
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 (19/1. 'Transactions Phil.
Soc.' 1799 page 202. For Kolreuter see 'Mem. de l'Acad. de St.-Petersbourg'
tome 3 1809 published 1811 page 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.) and, not long afterwards, that
sagacious observer Kolreuter, after showing how well the Malvaceae 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 quaritur? Certe natura nil facit frustra." Although we may demur to
Kolreuter'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
Gartner. 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. (19/2. 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.' volume 10 page 393. The
abstract here given is nearly the same with that which appeared in the 6th
edition of my 'Origin of Species.')
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,
Gartner, 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 Gartner
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 Gartner, with certain varieties of maize and
verbascum, by other experimentalists with varieties of the gourd and melon,
and by Kolreuter 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, Gartner'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.