On the Origin of Species by Means of Natural Selection
back :
forward
As Gartner 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
Gartner 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 differences 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 organisation, 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. Gartner 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 fertilised 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 Gartner 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 fertilisation
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 Gartner: namely, that yellow and white
varieties of the same species of Verbascum when intercrossed produce less
seed, than do either coloured varieties when fertilised 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.
Kolreuter, 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. Gartner, 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 Gartner 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. Gartner 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: Gartner 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. Gartner
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 Kolreuter.
These alone are the unimportant differences, which Gartner 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 Gartner
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
harmonises 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.
back : forward