On the Origin of Species, 6th Edition
by
Charles Darwin
Part 6 out of 11
into play. I do not pretend that the facts given in this chapter
strengthen in any great degree my theory; but none of the cases of
difficulty, to the best of my judgment, annihilate it. On the other hand,
the fact that instincts are not always absolutely perfect and are liable to
mistakes; that no instinct can be shown to have been produced for the good
of other animals, though animals take advantage of the instincts of others;
that the canon in natural history, of "Natura non facit saltum," is
applicable to instincts as well as to corporeal structure, and is plainly
explicable on the foregoing views, but is otherwise inexplicable--all tend
to corroborate the theory of natural selection.
This theory is also strengthened by some few other facts in regard to
instincts; as by that common case of closely allied, but distinct, species,
when inhabiting distant parts of the world and living under considerably
different conditions of life, yet often retaining nearly the same
instincts. For instance, we can understand, on the principle of
inheritance, how it is that the thrush of tropical South America lines its
nest with mud, in the same peculiar manner as does our British thrush; how
it is that the Hornbills of Africa and India have the same extraordinary
instinct of plastering up and imprisoning the females in a hole in a tree,
with only a small hole left in the plaster through which the males feed
them and their young when hatched; how it is that the male wrens
(Troglodytes) of North America, build "cock-nests," to roost in, like the
males of our Kitty-wrens,--a habit wholly unlike that of any other known
bird. Finally, it may not be a logical deduction, but to my imagination it
is far more satisfactory to look at such instincts as the young cuckoo
ejecting its foster-brothers, ants making slaves, the larvae of
ichneumonidae feeding within the live bodies of caterpillars, not as
specially endowed or created instincts, but as small consequences of one
general law leading to the advancement of all organic beings--namely,
multiply, vary, let the strongest live and the weakest die.
CHAPTER IX.
HYBRIDISM.
Distinction between the sterility of first crosses and of hybrids --
Sterility various in degree, not universal, affected by close
interbreeding, removed by domestication -- Laws governing the sterility of
hybrids -- Sterility not a special endowment, but incidental on other
differences, not accumulated by natural selection -- Causes of the
sterility of first crosses and of hybrids -- Parallelism between the
effects of changed conditions of life and of crossing -- Dimorphism and
trimorphism -- Fertility of varieties when crossed and of their mongrel
offspring not universal -- Hybrids and mongrels compared independently of
their fertility -- Summary.
The view commonly entertained by naturalists is that species, when
intercrossed, have been specially endowed with sterility, in order to
prevent their confusion. This view certainly seems at first highly
probable, for species living together could hardly have been kept distinct
had they been capable of freely crossing. The subject is in many ways
important for us, more especially as the sterility of species when first
crossed, and that of their hybrid offspring, cannot have been acquired, as
I shall show, by the preservation of successive profitable degrees of
sterility. It is an incidental result of differences in the reproductive
systems of the parent-species.
In treating this subject, two classes of facts, to a large extent
fundamentally different, have generally been confounded; namely, the
sterility of species when first crossed, and the sterility of the hybrids
produced from them.
Pure species have of course their organs of reproduction in a perfect
condition, yet when intercrossed they produce either few or no offspring.
Hybrids, on the other hand, have their reproductive organs functionally
impotent, as may be clearly seen in the state of the male element in both
plants and animals; though the formative organs themselves are perfect in
structure, as far as the microscope reveals. In the first case the two
sexual elements which go to form the embryo are perfect; in the second case
they are either not at all developed, or are imperfectly developed. This
distinction is important, when the cause of the sterility, which is common
to the two cases, has to be considered. The distinction probably has been
slurred over, owing to the sterility in both cases being looked on as a
special endowment, beyond the province of our reasoning powers.
The fertility of varieties, that is of the forms known or believed to be
descended from common parents, when crossed, and likewise the fertility of
their mongrel offspring, is, with reference to my theory, of equal
importance with the sterility of species; for it seems to make a broad and
clear distinction between varieties and species.
DEGREES OF STERILITY.
First, for the sterility of species when crossed and of their hybrid
offspring. It is impossible to study the several memoirs and works of
those two conscientious and admirable observers, Kolreuter and Gartner, who
almost devoted their lives to this subject, without being deeply impressed
with the high generality of some degree of sterility. Kolreuter makes the
rule universal; but then he cuts the knot, for in ten cases in which he
found two forms, considered by most authors as distinct species, quite
fertile together, he unhesitatingly ranks them as varieties. Gartner,
also, makes the rule equally universal; and he disputes the entire
fertility of Kolreuter's ten cases. But in these and in many other cases,
Gartner is obliged carefully to count the seeds, in order to show that
there is any degree of sterility. He always compares the maximum number of
seeds produced by two species when first crossed, and the maximum produced
by their hybrid offspring, with the average number produced by both pure
parent-species in a state of nature. But causes of serious error here
intervene: a plant, to be hybridised, must be castrated, and, what is
often more important, must be secluded in order to prevent pollen being
brought to it by insects from other plants. Nearly all the plants
experimented on by Gartner were potted, and were kept in a chamber in his
house. That these processes are often injurious to the fertility of a
plant cannot be doubted; for Gartner gives in his table about a score of
cases of plants which he castrated, and artificially fertilised with their
own pollen, and (excluding all cases such as the Leguminosae, in which
there is an acknowledged difficulty in the manipulation) half of these
twenty plants had their fertility in some degree impaired. Moreover, as
Gartner repeatedly crossed some forms, such as the common red and blue
pimpernels (Anagallis arvensis and coerulea), which the best botanists rank
as varieties, and found them absolutely sterile, we may doubt whether many
species are really so sterile, when intercrossed, as he believed.
It is certain, on the one hand, that the sterility of various species when
crossed is so different in degree and graduates away so insensibly, and, on
the other hand, that the fertility of pure species is so easily affected by
various circumstances, that for all practical purposes it is most difficult
to say where perfect fertility ends and sterility begins. I think no
better evidence of this can be required than that the two most experienced
observers who have ever lived, namely Kolreuter and Gartner, arrived at
diametrically opposite conclusions in regard to some of the very same
forms. It is also most instructive to compare--but I have not space here
to enter on details--the evidence advanced by our best botanists on the
question whether certain doubtful forms should be ranked as species or
varieties, with the evidence from fertility adduced by different
hybridisers, or by the same observer from experiments made during different
years. It can thus be shown that neither sterility nor fertility affords
any certain distinction between species and varieties. The evidence from
this source graduates away, and is doubtful in the same degree as is the
evidence derived from other constitutional and structural differences.
In regard to the sterility of hybrids in successive generations; though
Gartner was enabled to rear some hybrids, carefully guarding them from a
cross with either pure parent, for six or seven, and in one case for ten
generations, yet he asserts positively that their fertility never
increases, but generally decreases greatly and suddenly. With respect to
this decrease, it may first be noticed that when any deviation in structure
or constitution is common to both parents, this is often transmitted in an
augmented degree to the offspring; and both sexual elements in hybrid
plants are already affected in some degree. But I believe that their
fertility has been diminished in nearly all these cases by an independent
cause, namely, by too close interbreeding. I have made so many experiments
and collected so many facts, showing on the one hand that an occasional
cross with a distinct individual or variety increases the vigour and
fertility of the offspring, and on the other hand that very close
interbreeding lessens their vigour and fertility, that I cannot doubt the
correctness of this conclusion. Hybrids are seldom raised by
experimentalists in great numbers; and as the parent-species, or other
allied hybrids, generally grow in the same garden, the visits of insects
must be carefully prevented during the flowering season: hence hybrids, if
left to themselves, will generally be fertilised during each generation by
pollen from the same flower; and this would probably be injurious to their
fertility, already lessened by their hybrid origin. I am strengthened in
this conviction by a remarkable statement repeatedly made by Gartner,
namely, that if even the less fertile hybrids be artificially fertilised
with hybrid pollen of the same kind, their fertility, notwithstanding the
frequent ill effects from manipulation, sometimes decidedly increases, and
goes on increasing. Now, in the process of artificial fertilisation,
pollen is as often taken by chance (as I know from my own experience) from
the anthers of another flower, as from the anthers of the flower itself
which is to be fertilised; so that a cross between two flowers, though
probably often on the same plant, would be thus effected. Moreover,
whenever complicated experiments are in progress, so careful an observer as
Gartner would have castrated his hybrids, and this would have insured in
each generation a cross with pollen from a distinct flower, either from the
same plant or from another plant of the same hybrid nature. And thus, the
strange fact of an increase of fertility in the successive generations of
ARTIFICIALLY FERTILISED hybrids, in contrast with those spontaneously self-
fertilised, may, as I believe, be accounted for by too close interbreeding
having been avoided.
Now let us turn to the results arrived at by a third most experienced
hybridiser, namely, the Hon. and Rev. W. Herbert. He is as emphatic in his
conclusion that some hybrids are perfectly fertile--as fertile as the pure
parent-species--as are Kolreuter and Gartner that some degree of sterility
between distinct species is a universal law of nature. He experimented on
some of the very same species as did Gartner. The difference in their
results may, I think, be in part accounted for by Herbert's great
horticultural skill, and by his having hot-houses at his command. Of his
many important statements I will here give only a single one as an example,
namely, that "every ovule in a pod of Crinum capense fertilised by C.
revolutum produced a plant, which I never saw to occur in a case of its
natural fecundation." So that here we have perfect, or even more than
commonly perfect fertility, in a first cross between two distinct species.
This case of the Crinum leads me to refer to a singular fact, namely, that
individual plants of certain species of Lobelia, Verbascum and Passiflora,
can easily be fertilised by the pollen from a distinct species, but not by
pollen from the same plant, though this pollen can be proved to be
perfectly sound by fertilising other plants or species. In the genus
Hippeastrum, in Corydalis as shown by Professor Hildebrand, in various
orchids as shown by Mr. Scott and Fritz Muller, all the individuals are in
this peculiar condition. So that with some species, certain abnormal
individuals, and in other species all the individuals, can actually be
hybridised much more readily than they can be fertilised by pollen from the
same individual plant! To give one instance, a bulb of Hippeastrum aulicum
produced four flowers; three were fertilised by Herbert with their own
pollen, and the fourth was subsequently fertilised by the pollen of a
compound hybrid descended from three distinct species: the result was that
"the ovaries of the three first flowers soon ceased to grow, and after a
few days perished entirely, whereas the pod impregnated by the pollen of
the hybrid made vigorous growth and rapid progress to maturity, and bore
good seed, which vegetated freely." Mr. Herbert tried similar experiments
during many years, and always with the same result. These cases serve to
show on what slight and mysterious causes the lesser or greater fertility
of a species sometimes depends.
The practical experiments of horticulturists, though not made with
scientific precision, deserve some notice. It is notorious in how
complicated a manner the species of Pelargonium, Fuchsia, Calceolaria,
Petunia, Rhododendron, etc., have been crossed, yet many of these hybrids
seed freely. For instance, Herbert asserts that a hybrid from Calceolaria
integrifolia and plantaginea, species most widely dissimilar in general
habit, "reproduces itself as perfectly as if it had been a natural species
from the mountains of Chile." I have taken some pains to ascertain the
degree of fertility of some of the complex crosses of Rhododendrons, and I
am assured that many of them are perfectly fertile. Mr. C. Noble, for
instance, informs me that he raises stocks for grafting from a hybrid
between Rhod. ponticum and catawbiense, and that this hybrid "seeds as
freely as it is possible to imagine." Had hybrids, when fairly treated,
always gone on decreasing in fertility in each successive generation, as
Gartner believed to be the case, the fact would have been notorious to
nurserymen. Horticulturists raise large beds of the same hybrid, and such
alone are fairly treated, for by insect agency the several individuals are
allowed to cross freely with each other, and the injurious influence of
close interbreeding is thus prevented. Any one may readily convince
himself of the efficiency of insect agency by examining the flowers of the
more sterile kinds of hybrid Rhododendrons, which produce no pollen, for he
will find on their stigmas plenty of pollen brought from other flowers.
In regard to animals, much fewer experiments have been carefully tried than
with plants. If our systematic arrangements can be trusted, that is, if
the genera of animals are as distinct from each other as are the genera of
plants, then we may infer that animals more widely distinct in the scale of
nature can be crossed more easily than in the case of plants; but the
hybrids themselves are, I think, more sterile. It should, however, be
borne in mind that, owing to few animals breeding freely under confinement,
few experiments have been fairly tried: for instance, the canary-bird has
been crossed with nine distinct species of finches, but, as not one of
these breeds freely in confinement, we have no right to expect that the
first crosses between them and the canary, or that their hybrids, should be
perfectly fertile. Again, with respect to the fertility in successive
generations of the more fertile hybrid animals, I hardly know of an
instance in which two families of the same hybrid have been raised at the
same time from different parents, so as to avoid the ill effects of close
interbreeding. On the contrary, brothers and sisters have usually been
crossed in each successive generation, in opposition to the constantly
repeated admonition of every breeder. And in this case, it is not at all
surprising that the inherent sterility in the hybrids should have gone on
increasing.
Although I know of hardly any thoroughly well-authenticated cases of
perfectly fertile hybrid animals, I have reason to believe that the hybrids
from Cervulus vaginalis and Reevesii, and from Phasianus colchicus with P.
torquatus, are perfectly fertile. M. Quatrefages states that the hybrids
from two moths (Bombyx cynthia and arrindia) were proved in Paris to be
fertile inter se for eight generations. It has lately been asserted that
two such distinct species as the hare and rabbit, when they can be got to
breed together, produce offspring, which are highly fertile when crossed
with one of the parent-species. The hybrids from the common and Chinese
geese (A. cygnoides), species which are so different that they are
generally ranked in distinct genera, have often bred in this country with
either pure parent, and in one single instance they have bred inter se.
This was effected by Mr. Eyton, who raised two hybrids from the same
parents, but from different hatches; and from these two birds he raised no
less than eight hybrids (grandchildren of the pure geese) from one nest.
In India, however, these cross-bred geese must be far more fertile; for I
am assured by two eminently capable judges, namely Mr. Blyth and Captain
Hutton, that whole flocks of these crossed geese are kept in various parts
of the country; and as they are kept for profit, where neither pure
parent-species exists, they must certainly be highly or perfectly fertile.
With our domesticated animals, the various races when crossed together are
quite fertile; yet in many cases they are descended from two or more wild
species. From this fact we must conclude either that the aboriginal
parent-species at first produced perfectly fertile hybrids, or that the
hybrids subsequently reared under domestication became quite fertile. This
latter alternative, which was first propounded by Pallas, seems by far the
most probable, and can, indeed, hardly be doubted. It is, for instance,
almost certain that our dogs are descended from several wild stocks; yet,
with perhaps the exception of certain indigenous domestic dogs of South
America, all are quite fertile together; but analogy makes me greatly
doubt, whether the several aboriginal species would at first have freely
bred together and have produced quite fertile hybrids. So again I have
lately acquired decisive evidence that the crossed offspring from the
Indian humped and common cattle are inter se perfectly fertile; and from
the observations by Rutimeyer on their important osteological differences,
as well as from those by Mr. Blyth on their differences in habits, voice,
constitution, etc., these two forms must be regarded as good and distinct
species. The same remarks may be extended to the two chief races of the
pig. We must, therefore, either give up the belief of the universal
sterility of species when crossed; or we must look at this sterility in
animals, not as an indelible characteristic, but as one capable of being
removed by domestication.
Finally, considering all the ascertained facts on the intercrossing of
plants and animals, it may be concluded that some degree of sterility, both
in first crosses and in hybrids, is an extremely general result; but that
it cannot, under our present state of knowledge, be considered as
absolutely universal.
LAWS GOVERNING THE STERILITY OF FIRST CROSSES AND OF HYBRIDS.
We will now consider a little more in detail the laws governing the
sterility of first crosses and of hybrids. Our chief object will be to see
whether or not these laws indicate that species have been specially endowed
with this quality, in order to prevent their crossing and blending together
in utter confusion. The following conclusions are drawn up chiefly from
Gartner's admirable work on the hybridisation of plants. I have taken much
pains to ascertain how far they apply to animals, and, considering how
scanty our knowledge is in regard to hybrid animals, I have been surprised
to find how generally the same rules apply to both kingdoms.
It has been already remarked, that the degree of fertility, both of first
crosses and of hybrids, graduates from zero to perfect fertility. It is
surprising in how many curious ways this gradation can be shown; but only
the barest outline of the facts can here be given. When pollen from a
plant of one family is placed on the stigma of a plant of a distinct
family, it exerts no more influence than so much inorganic dust. From this
absolute zero of fertility, the pollen of different species applied to the
stigma of some one species of the same genus, yields a perfect gradation in
the number of seeds produced, up to nearly complete or even quite complete
fertility; and, as we have seen, in certain abnormal cases, even to an
excess of fertility, beyond that which the plant's own pollen produces. So
in hybrids themselves, there are some which never have produced, and
probably never would produce, even with the pollen of the pure parents, a
single fertile seed: but in some of these cases a first trace of fertility
may be detected, by the pollen of one of the pure parent-species causing
the flower of the hybrid to wither earlier than it otherwise would have
done; and the early withering of the flower is well known to be a sign of
incipient fertilisation. From this extreme degree of sterility we have
self-fertilised hybrids producing a greater and greater number of seeds up
to perfect fertility.
The hybrids raised from two species which are very difficult to cross, and
which rarely produce any offspring, are generally very sterile; but the
parallelism between the difficulty of making a first cross, and the
sterility of the hybrids thus produced--two classes of facts which are
generally confounded together--is by no means strict. There are many
cases, in which two pure species, as in the genus Verbascum, can be united
with unusual facility, and produce numerous hybrid offspring, yet these
hybrids are remarkably sterile. On the other hand, there are species which
can be crossed very rarely, or with extreme difficulty, but the hybrids,
when at last produced, are very fertile. Even within the limits of the
same genus, for instance in Dianthus, these two opposite cases occur.
The fertility, both of first crosses and of hybrids, is more easily
affected by unfavourable conditions, than is that of pure species. But the
fertility of first crosses is likewise innately variable; for it is not
always the same in degree when the same two species are crossed under the
same circumstances; it depends in part upon the constitution of the
individuals which happen to have been chosen for the experiment. So it is
with hybrids, for their degree of fertility is often found to differ
greatly in the several individuals raised from seed out of the same capsule
and exposed to the same conditions.
By the term systematic affinity is meant, the general resemblance between
species in structure and constitution. Now the fertility of first crosses,
and of the hybrids produced from them, is largely governed by their
systematic affinity. This is clearly shown by hybrids never having been
raised between species ranked by systematists in distinct families; and on
the other hand, by very closely allied species generally uniting with
facility. But the correspondence between systematic affinity and the
facility of crossing is by no means strict. A multitude of cases could be
given of very closely allied species which will not unite, or only with
extreme difficulty; and on the other hand of very distinct species which
unite with the utmost facility. In the same family there may be a genus,
as Dianthus, in which very many species can most readily be crossed; and
another genus, as Silene, in which the most persevering efforts have failed
to produce between extremely close species a single hybrid. Even within
the limits of the same genus, we meet with this same difference; for
instance, the many species of Nicotiana have been more largely crossed than
the species of almost any other genus; but Gartner found that N. acuminata,
which is not a particularly distinct species, obstinately failed to
fertilise, or to be fertilised, by no less than eight other species of
Nicotiana. Many analogous facts could be given.
No one has been able to point out what kind or what amount of difference,
in any recognisable character, is sufficient to prevent two species
crossing. It can be shown that plants most widely different in habit and
general appearance, and having strongly marked differences in every part of
the flower, even in the pollen, in the fruit, and in the cotyledons, can be
crossed. Annual and perennial plants, deciduous and evergreen trees,
plants inhabiting different stations and fitted for extremely different
climates, can often be crossed with ease.
By a reciprocal cross between two species, I mean the case, for instance,
of a female-ass being first crossed by a stallion, and then a mare by a
male-ass: these two species may then be said to have been reciprocally
crossed. There is often the widest possible difference in the facility of
making reciprocal crosses. Such cases are highly important, for they prove
that the capacity in any two species to cross is often completely
independent of their systematic affinity, that is of any difference in
their structure or constitution, excepting in their reproductive systems.
The diversity of the result in reciprocal crosses between the same two
species was long ago observed by Kolreuter. To give an instance:
Mirabilis jalapa can easily be fertilised by the pollen of M. longiflora,
and the hybrids thus produced are sufficiently fertile; but Kolreuter tried
more than two hundred times, during eight following years, to fertilise
reciprocally M. longiflora with the pollen of M. jalapa, and utterly
failed. Several other equally striking cases could be given. Thuret has
observed the same fact with certain sea-weeds or Fuci. Gartner, moreover,
found that this difference of facility in making reciprocal crosses is
extremely common in a lesser degree. He has observed it even between
closely related forms (as Matthiola annua and glabra) which many botanists
rank only as varieties. It is also a remarkable fact that hybrids raised
from reciprocal crosses, though of course compounded of the very same two
species, the one species having first been used as the father and then as
the mother, though they rarely differ in external characters, yet generally
differ in fertility in a small, and occasionally in a high degree.
Several other singular rules could be given from Gartner: for instance,
some species have a remarkable power of crossing with other species; other
species of the same genus have a remarkable power of impressing their
likeness on their hybrid offspring; but these two powers do not at all
necessarily go together. There are certain hybrids which, instead of
having, as is usual, an intermediate character between their two parents,
always closely resemble one of them; and such hybrids, though externally so
like one of their pure parent-species, are with rare exceptions extremely
sterile. So again among hybrids which are usually intermediate in
structure between their parents, exceptional and abnormal individuals
sometimes are born, which closely resemble one of their pure parents; and
these hybrids are almost always utterly sterile, even when the other
hybrids raised from seed from the same capsule have a considerable degree
of fertility. These facts show how completely the fertility of a hybrid
may be independent of its external resemblance to either pure parent.
Considering the several rules now given, which govern the fertility of
first crosses and of hybrids, we see that when forms, which must be
considered as good and distinct species, are united, their fertility
graduates from zero to perfect fertility, or even to fertility under
certain conditions in excess; that their fertility, besides being eminently
susceptible to favourable and unfavourable conditions, is innately
variable; that it is by no means always the same in degree in the first
cross and in the hybrids produced from this cross; that the fertility of
hybrids is not related to the degree in which they resemble in external
appearance either parent; and lastly, that the facility of making a first
cross between any two species is not always governed by their systematic
affinity or degree of resemblance to each other. This latter statement is
clearly proved by the difference in the result of reciprocal crosses
between the same two species, for, according as the one species or the
other is used as the father or the mother, there is generally some
difference, and occasionally the widest possible difference, in the
facility of effecting an union. The hybrids, moreover, produced from
reciprocal crosses often differ in fertility.
Now do these complex and singular rules indicate that species have been
endowed with sterility simply to prevent their becoming confounded in
nature? I think not. For why should the sterility be so extremely
different in degree, when various species are crossed, all of which we must
suppose it would be equally important to keep from blending together? Why
should the degree of sterility be innately variable in the individuals of
the same species? Why should some species cross with facility and yet
produce very sterile hybrids; and other species cross with extreme
difficulty, and yet produce fairly fertile hybrids? Why should there often
be so great a difference in the result of a reciprocal cross between the
same two species? Why, it may even be asked, has the production of hybrids
been permitted? To grant to species the special power of producing
hybrids, and then to stop their further propagation by different degrees of
sterility, not strictly related to the facility of the first union between
their parents, seems a strange arrangement.
The foregoing rules and facts, on the other hand, appear to me clearly to
indicate that the sterility, both of first crosses and of hybrids, is
simply incidental or dependent on unknown differences in their reproductive
systems; the differences being of so peculiar and limited a nature, that,
in reciprocal crosses between the same two species, the male sexual element
of the one will often freely act on the female sexual element of the other,
but not in a reversed direction. It will be advisable to explain a little
more fully, by an example, what I mean by sterility being incidental on
other differences, and not a specially endowed quality. As the capacity of
one plant to be grafted or budded on another is unimportant for their
welfare in a state of nature, I presume that no one will suppose that this
capacity is a SPECIALLY endowed quality, but will admit that it is
incidental on differences in the laws of growth of the two plants. We can
sometimes see the reason why one tree will not take on another from
differences in their rate of growth, in the hardness of their wood, in the
period of the flow or nature of their sap, etc.; but in a multitude of
cases we can assign no reason whatever. Great diversity in the size of two
plants, one being woody and the other herbaceous, one being evergreen and
the other deciduous, and adaptation to widely different climates, does not
always prevent the two grafting together. As in hybridisation, so with
grafting, the capacity is limited by systematic affinity, for no one has
been able to graft together trees belonging to quite distinct families;
and, on the other hand, closely allied species and varieties of the same
species, can usually, but not invariably, be grafted with ease. But this
capacity, as in hybridisation, is by no means absolutely governed by
systematic affinity. Although many distinct genera within the same family
have been grafted together, in other cases species of the same genus will
not take on each other. The pear can be grafted far more readily on the
quince, which is ranked as a distinct genus, than on the apple, which is a
member of the same genus. Even different varieties of the pear take with
different degrees of facility on the quince; so do different varieties of
the apricot and peach on certain varieties of the plum.
As 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 different case from the difficulty
of uniting two pure species, which have their reproductive organs perfect;
yet these two distinct classes of cases run to a large 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 a fourth 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 cases of
Hippeastrum, Passiflora, etc., which seed much more freely when fertilised
with the pollen of a distinct species than when fertilised with pollen from
the same plant.
We thus see that, although there is a clear and great 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 in their reproductive systems. These differences in
both cases follow, to a certain extent, as might have been expected,
systematic affinity, by which term every kind of resemblance and
dissimilarity between organic beings is attempted to be expressed. The
facts by no means seem to indicate that the greater or lesser difficulty of
either grafting or crossing 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.
ORIGIN AND CAUSES OF THE STERILITY OF FIRST CROSSES AND OF HYBRIDS.
At one time it appeared to me probable, as it has to others, that the
sterility of first crosses and of hybrids might have been slowly acquired
through the natural selection of slightly lessened degrees of fertility,
which, like any other variation, spontaneously appeared in certain
individuals of one variety when crossed with those of another variety. For
it would clearly be advantageous to two varieties or incipient species if
they could be kept from blending, on the same principle that, when man is
selecting at the same time two varieties, it is necessary that he should
keep them separate. In the first place, it may be remarked that species
inhabiting distinct regions are often sterile when crossed; now it could
clearly have been of 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 was rendered sterile with some one compatriot, sterility with
other species would follow as a necessary contingency. In the second
place, it is almost as much opposed to the theory of natural selection as
to that of special creation, that in reciprocal crosses the male element of
one form should have been rendered utterly impotent on a second form, while
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 hardly have been advantageous to either species.
In considering the probability of natural selection having come into
action, in rendering species mutually sterile, the greatest difficulty will
be found to lie in the existence of many graduated steps, from slightly
lessened fertility to absolute sterility. It may be admitted 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
high degree which is common with 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, produced
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 genera 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 effected, 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.
We will now look a little closer at the probable nature of the differences
between species which induce sterility in first crosses and in hybrids. In
the case of first crosses, the greater or less difficulty in effecting a
union and in obtaining offspring 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 the 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 pheasants and
fowls, that the early death of the embryo is a very frequent cause of
sterility in first crosses. Mr. Salter has recently given the results of
an examination of about 500 eggs produced from various crosses between
three species of Gallus and their hybrids; the majority of these eggs had
been fertilised; and in the majority of the fertilised eggs, the embryos
had either been partially developed and had then perished, or had become
nearly mature, but the young chickens had been unable to break through the
shell. Of the chickens which were born, more than four-fifths died within
the first few days, or at latest weeks, "without any obvious cause,
apparently from mere inability to live;" so that from the 500 eggs only
twelve chickens were reared. With plants, hybridized embryos probably
often perish in a like manner; at least it is known that hybrids raised
from very distinct species are sometimes weak and dwarfed, and perish at an
early age; of which fact Max Wichura has recently given some striking cases
with hybrid willows. It may be here worth noticing that in some cases of
parthenogenesis, the embryos within the eggs of silk moths which had not
been fertilised, pass through their early stages of development and then
perish like the embryos produced by a cross between distinct species.
Until becoming acquainted with these facts, I was unwilling to believe in
the frequent early death of hybrid embryos; for 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 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; it may
therefore, before birth, as long as it is nourished within its mother's
womb, or within the egg or seed produced by the mother, 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 are eminently
sensitive to injurious or unnatural conditions of life. But after all, the
cause more probably lies in some imperfection in the original act of
impregnation, causing the embryo to be imperfectly developed, rather than
in the conditions to which it is subsequently exposed.
In regard to the sterility of hybrids, in which the sexual elements are
imperfectly developed, the case is somewhat different. I have more than
once alluded to a large body of facts 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, for
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
exotic 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 seems to be partly due to their reproductive systems having been
specially affected, though in a lesser degree than when sterility ensues.
So it is with hybrids, for their offspring 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 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 distinct structures and constitutions, including of course
the reproductive systems, 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
relations 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, does not diminish; it is even apt to
increase, this being generally the result, as before explained, of too
close interbreeding. The above view of the sterility of hybrids being
caused by two constitutions being compounded into one has been strongly
maintained by Max Wichura.
It must, however, be owned that we cannot understand, on the above or any
other view, 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 being
compounded into one.
A similar parallelism holds good with an allied yet very different class of
facts. It is an old and almost universal belief, founded on a considerable
body of evidence, which I have elsewhere given, 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,
etc., from one soil or climate to another, and back again. During the
convalescence of animals, great benefit is derived from almost any change
in their habits of life. Again, both with plants and animals, there is the
clearest evidence that a cross between individuals of the same species,
which differ to a certain extent, gives vigour and fertility to the
offspring; and that close interbreeding continued during several
generations between the nearest relations, if these be kept under the same
conditions of life, almost always leads to decreased size, weakness, or
sterility.
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 been subjected to slightly different conditions, or
which have slightly varied, give vigour and fertility to the offspring.
But, as we have seen, organic beings long habituated to certain uniform
conditions under a state of nature, when subjected, as under confinement,
to a considerable change in their conditions, very frequently are rendered
more or less sterile; and we know that a cross between two forms that have
become widely or specifically different, produce hybrids which are almost
always in some degree sterile. I am fully persuaded that this double
parallelism is by no means an accident or an illusion. He who is able to
explain why the elephant, and a multitude of other animals, are incapable
of breeding when kept under only partial confinement in their native
country, will be able to explain the primary cause of hybrids being so
generally sterile. He will at the same time be able to explain how it is
that the races of some of our domesticated animals, which have often been
subjected to new and not uniform conditions, are quite fertile together,
although they are descended from distinct species, which would probably
have been sterile if aboriginally crossed. The above two parallel series
of facts seem to be connected together by some common but unknown bond,
which is essentially related to the principle of life; this principle,
according to Mr. Herbert Spencer, being that life depends on, or consists
in, the incessant action and reaction of various forces, which, as
throughout nature, are always tending towards an equilibrium; and when this
tendency is slightly disturbed by any change, the vital forces gain in
power.
RECIPROCAL DIMORPHISM AND TRIMORPHISM.
This subject may be here briefly discussed, and will be found to throw some
light on hybridism. 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; the two
having 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, the three
forms possess altogether six sets of stamens and three kinds of pistils.
These organs are so proportioned in length to each other that half the
stamens in two of the forms 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 another 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.
The infertility which may be observed in various dimorphic and trimorphic
plants, when they are 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 was illegitimately fertilised with the greatest ease by pollen
from the longer stamens of the short-styled form, and yielded many seeds;
but the latter form did not yield a single seed when fertilised by the
longer stamens of the mid-styled form.
In all these respects, and in others which might be added, 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, 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. The sterility of these illegitimate plants, when united with
each other in a legitimate manner, may be strictly compared with that of
hybrids when crossed inter se. If, 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 that sterility of certain illegitimate plants was
unusually great, while 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, while 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.
Altogether there is the closest identity in character and behaviour between
illegitimate plants and hybrids. It is hardly an exaggeration to maintain
that illegitimate plants are hybrids, produced within the limits of the
same species by the improper union of certain forms, while ordinary hybrids
are produced from an improper union between so-called distinct species. We
have also 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 Lythrum 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 safe 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; while it is the union of the
sexual elements 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
can be united with great difficulty, with the female of a second species,
while 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.
FERTILITY OF VARIETIES WHEN CROSSED, AND OF THEIR MONGREL OFFSPRING, NOT
UNIVERSAL.
It may be urged as an overwhelming argument that there must be some
essential distinction between species and varieties inasmuch as the latter,
however much they may differ from each other in external appearance, cross
with perfect facility, and yield perfectly fertile offspring. With some
exceptions, presently to be given, I fully admit that this is the rule.
But the subject is surrounded by difficulties, for, looking to varieties
produced under nature, if two forms hitherto reputed to be 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, which
are considered by most botanists as varieties, are said by Gartner to be
quite sterile 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 some doubt. For when it is stated,
for instance, that certain South American indigenous domestic dogs do not
readily unite with European dogs, the explanation which will occur to
everyone, and probably the true one, is that they are descended from
aboriginally distinct species. Nevertheless the perfect fertility of so
many domestic races, differing widely from each other in appearance, for
instance, those 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. In the first place, it may be observed 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. It is certain that with species the cause lies
exclusively in differences in their sexual constitution. Now the varying
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 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 probably would have
been in some degree sterile when crossed, become perfectly fertile
together. With plants, so far is cultivation from giving a tendency
towards sterility between distinct species, that in several well-
authenticated cases already alluded to, certain plants have been affected
in an opposite manner, for they have become self-impotent, while still
retaining the capacity of fertilising, and being fertilised by, other
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 conditions long-
continued should likewise induce this tendency; though in certain cases,
with species having a peculiar constitution, sterility might occasionally
be thus caused. 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, immediately to be
given, have been observed.
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 permanently modified in a sufficient degree to take rank as species.
We are far from precisely knowing the cause; nor is this surprising, seeing
how profoundly ignorant we are in regard to the normal and abnormal action
of the reproductive system. But we can see that species, owing to their
struggle for existence with numerous competitors, will have been exposed
during long periods of time to more uniform conditions, than have domestic
varieties; 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 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.
I have as yet spoken as if the varieties of the same species were
invariably fertile when intercrossed. But it is 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
kind with 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 experimented on
are ranked by Sagaret, who mainly founds his classification by the test of
infertility, as varieties, and Naudin has come to the same conclusion.
The following case is far more remarkable, and seems at first 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 the yellow and white varieties
when crossed produce less seed than the similarly coloured varieties of the
same species. 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 similarly
coloured flowers, than between those which are differently coloured. Mr.
Scott also has experimented on the species and varieties of Verbascum; and
although unable to confirm Gartner's results on the crossing of the
distinct species, he finds that the dissimilarly coloured varieties of the
same species yield fewer seeds, in the proportion of eighty-six to 100,
than the similarly coloured varieties. Yet these varieties differ in no
respect, except in the colour of their flowers; and one variety can
sometimes be raised from the seed of another.
Kolreuter, whose accuracy has been confirmed by every subsequent observer,
has proved the remarkable fact that one particular variety of the common
tobacco was more fertile than the other varieties, when crossed with a
widely distinct species. He experimented 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 the 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 it can no longer be maintained that varieties when crossed
are invariably quite fertile. From the great difficulty of ascertaining
the infertility of varieties in a state of nature, for a supposed variety,
if proved to be infertile in any degree, would almost universally be ranked
as a species; from man attending only to external characters in his
domestic varieties, and from such varieties not having been exposed for
very long periods to uniform conditions of life; from these several
considerations we may conclude that fertility does not constitute a
fundamental distinction between varieties and species when crossed. The
general sterility of crossed species may safely be looked at, not as a
special acquirement or endowment, but as incidental on changes of an
unknown nature in their sexual elements.
HYBRIDS AND MONGRELS COMPARED, INDEPENDENTLY OF THEIR FERTILITY.
Independently of the question of fertility, the offspring of species and of
varieties when crossed may be compared in several other respects. Gartner,
whose strong wish it was to draw a distinct line 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 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 the offspring in both cases is notorious; but some few
instances of both hybrids and mongrels long retaining a uniform character
could be given. The variability, however, in the successive generations of
mongrels is, perhaps, greater than in hybrids.
This greater variability in mongrels than in hybrids does not seem 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 that there has been recent variability; which would often
continue and would augment that arising from the act of crossing. The
slight variability of hybrids in the first generation, in contrast with
that in the succeeding generations, is a curious fact and deserves
attention. For it bears on the view which I have taken of one of the
causes of ordinary variability; namely, that the reproductive system, from
being eminently sensitive to changed conditions of life, fails under these
circumstances to perform its proper function of producing offspring closely
similar in all respects to 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.
Moreover, Gartner expressly states that the hybrids from long cultivated
plants are more subject to reversion than hybrids from species in their
natural state; and this probably explains the singular difference in the
results arrived at by different observers. Thus Max Wichura doubts whether
hybrids ever revert to their parent forms, and he experimented on
uncultivated species of willows, while Naudin, on the other hand, insists
in the strongest terms on the almost universal tendency to reversion in
hybrids, and he experimented chiefly on cultivated plants. Gartner further
states 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.
Such alone are the unimportant differences which Gartner is able to point
out between hybrid and mongrel plants. On the other hand, the degrees and
kinds of resemblance in mongrels and in hybrids to their respective
parents, more especially in hybrids produced from nearly related species,
follow, according to Gartner the same laws. When two species are crossed,
one has sometimes a prepotent power of impressing its likeness on the
hybrid. So I believe it to be with varieties of plants; and 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 mongrel plants 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 much 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 resemble more closely the ass than the horse; but
that the prepotency runs more strongly in the male than in the female ass,
so that the mule, which is an 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 it is
only with mongrels that the offspring are not intermediate in character,
but closely resemble one of their parents; but this does sometimes occur
with hybrids, yet I grant much less frequently 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
through selection. A tendency to sudden reversions to the perfect
character of either parent would, also, be much 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
little or much from each other, namely, in the union of individuals of the
same variety, or of different varieties, or of distinct species.
Independently of 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 most careful
experimentalists have arrived at diametrically opposite conclusions in
ranking forms by this test. The sterility is innately variable in
individuals of the same species, and is eminently susceptible to action 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 hybrids produced from this cross.
In the same manner as in grafting trees, the capacity in one species or
variety to take on another, is incidental on differences, generally of an
unknown nature, 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 their 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 their
inarching in our forests.
The sterility of first crosses and of their hybrid progeny has not been
acquired through natural selection. In the case of first crosses it seems
to depend on several circumstances; in some instances in chief part on the
early death of the embryo. In the case of hybrids, it apparently depends
on their whole organisation having been disturbed by being compounded from
two distinct forms; the sterility being closely allied to that which so
frequently affects pure species, when exposed to new and unnatural
conditions of life. He who will explain these latter cases will be able to
explain the sterility of hybrids. This view is strongly supported by a
parallelism of another kind: namely, that, firstly, slight changes in the
conditions of life add to the vigour and fertility of all organic beings;
and secondly, that the crossing of forms, which have been exposed to
slightly different conditions of life, or which have varied, favours the
size, vigour and fertility of their offspring. The facts given on the
sterility of the illegitimate unions of dimorphic and trimorphic plants and
of their illegitimate progeny, perhaps render it probable that some unknown
bond in all cases connects the degree of fertility of first unions with
that of their offspring. The consideration of these facts on dimorphism,
as well as of the results of reciprocal crosses, clearly leads to the
conclusion that the primary cause of the sterility of crossed species is
confined to differences in their sexual elements. But why, in the case of
distinct species, the sexual elements should so generally have become more
or less modified, leading to their mutual infertility, we do not know; but
it seems to stand in some close relation to species having been exposed for
long periods of time to nearly uniform conditions of life.
It is not surprising that the difficulty in crossing any two species, and
the sterility of their hybrid offspring, should in most cases correspond,
even if due to distinct causes: for both depend on the amount of
difference between the species which are crossed. Nor is it surprising
that the facility of effecting a first cross, and the fertility of the
hybrids thus 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 subjected to experiment; for systematic
affinity includes resemblances of all kinds.
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, as is so often stated, invariably fertile. Nor is this
almost universal and perfect fertility surprising, when it is remembered
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 that they have not been long exposed to uniform conditions
of life. It should also be especially kept in mind, that long-continued
domestication tends to eliminate sterility, and is therefore little likely
to induce this same quality. Independently of the question of fertility,
in all other respects there is the closest general resemblance between
hybrids and mongrels, in their variability, in their power of absorbing
each other by repeated crosses, and in their inheritance of characters from
both parent-forms. Finally, then, although we are as ignorant of the
precise cause of the sterility of first crosses and of hybrids as we are
why animals and plants removed from their natural conditions become
sterile, yet the facts given in this chapter do not seem to me opposed to
the belief that species aboriginally existed as varieties.
CHAPTER X.
ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.
On the absence of intermediate varieties at the present day -- On the
nature of extinct intermediate varieties; on their number -- On the lapse
of time, as inferred from the rate of denudation and of deposition number
-- On the lapse of time as estimated by years -- On the poorness of our
palaeontological collections -- On the intermittence of geological
formations -- On the denudation of granitic areas -- On the absence of
intermediate varieties in any one formation -- On the sudden appearance of
groups of species -- On their sudden appearance in the lowest known
fossiliferous strata -- Antiquity of the habitable earth.
In the sixth chapter I enumerated the chief objections which might be
justly urged against the views maintained in this volume. Most of them
have now been discussed. One, namely, the distinctness of specific forms
and their not being blended together by innumerable transitional links, is
a very obvious difficulty. I assigned reasons why such links do not
commonly occur at the present day under the circumstances apparently most
favourable for their presence, namely, on an extensive and continuous area
with graduated physical conditions. I endeavoured to show, that the life
of each species depends in a more important manner on the presence of other
already defined organic forms, than on climate, and, therefore, that the
really governing conditions of life do not graduate away quite insensibly
like heat or moisture. I endeavoured, also, to show that intermediate
varieties, from existing in lesser numbers than the forms which they
connect, will generally be beaten out and exterminated during the course of
further modification and improvement. The main cause, however, of
innumerable intermediate links not now occurring everywhere throughout
nature depends, on the very process of natural selection, through which new
varieties continually take the places of and supplant their parent-forms.
But just in proportion as this process of extermination has acted on an
enormous scale, so must the number of intermediate varieties, which have
formerly existed, be truly enormous. Why then is not every geological
formation and every stratum full of such intermediate links? Geology
assuredly does not reveal any such finely graduated organic chain; and
this, perhaps, is the most obvious and serious objection which can be urged
against my theory. The explanation lies, as I believe, in the extreme
imperfection of the geological record.
In the first place, it should always be borne in mind what sort of
intermediate forms must, on the theory, have formerly existed. I have
found it difficult, when looking at any two species, to avoid picturing to
myself forms DIRECTLY intermediate between them. But this is a wholly
false view; we should always look for forms intermediate between each
species and a common but unknown progenitor; and the progenitor will
generally have differed in some respects from all its modified descendants.
To give a simple illustration: the fantail and pouter pigeons are both
descended from the rock-pigeon; if we possessed all the intermediate
varieties which have ever existed, we should have an extremely close series
between both and the rock-pigeon; but we should have no varieties directly
intermediate between the fantail and pouter; none, for instance, combining
a tail somewhat expanded with a crop somewhat enlarged, the characteristic
features of these two breeds. These two breeds, moreover, have become so
much modified, that, if we had no historical or indirect evidence regarding
their origin, it would not have been possible to have determined from a
mere comparison of their structure with that of the rock-pigeon, C. livia,
whether they had descended from this species or from some other allied
species, such as C. oenas.
So with natural species, if we look to forms very distinct, for instance to
the horse and tapir, we have no reason to suppose that links directly
intermediate between them ever existed, but between each and an unknown
common parent. The common parent will have had in its whole organisation
much general resemblance to the tapir and to the horse; but in some points
of structure may have differed considerably from both, even perhaps more
than they differ from each other. Hence, in all such cases, we should be
unable to recognise the parent-form of any two or more species, even if we
closely compared the structure of the parent with that of its modified
descendants, unless at the same time we had a nearly perfect chain of the
intermediate links.
It is just possible, by the theory, that one of two living forms might have
descended from the other; for instance, a horse from a tapir; and in this
case DIRECT intermediate links will have existed between them. But such a
case would imply that one form had remained for a very long period
unaltered, whilst its descendants had undergone a vast amount of change;
and the principle of competition between organism and organism, between
child and parent, will render this a very rare event; for in all cases the
new and improved forms of life tend to supplant the old and unimproved
forms.
By the theory of natural selection all living species have been connected
with the parent-species of each genus, by differences not greater than we
see between the natural and domestic varieties of the same species at the
present day; and these parent-species, now generally extinct, have in their
turn been similarly connected with more ancient forms; and so on backwards,
always converging to the common ancestor of each great class. So that the
number of intermediate and transitional links, between all living and
extinct species, must have been inconceivably great. But assuredly, if
this theory be true, such have lived upon the earth.
ON THE LAPSE OF TIME, AS INFERRED FROM THE RATE OF DEPOSITION AND EXTENT OF
DENUDATION.
Independently of our not finding fossil remains of such infinitely numerous
connecting links, it may be objected that time cannot have sufficed for so
great an amount of organic change, all changes having been effected slowly.
It is hardly possible for me to recall to the reader who is not a practical
geologist, the facts leading the mind feebly to comprehend the lapse of
time. He who can read Sir Charles Lyell's grand work on the Principles of
Geology, which the future historian will recognise as having produced a
revolution in natural science, and yet does not admit how vast have been
the past periods of time, may at once close this volume. Not that it
suffices to study the Principles of Geology, or to read special treatises
by different observers on separate formations, and to mark how each author
attempts to give an inadequate idea of the duration of each formation, or
even of each stratum. We can best gain some idea of past time by knowing
the agencies at work; and learning how deeply the surface of the land has
been denuded, and how much sediment has been deposited. As Lyell has well
remarked, the extent and thickness of our sedimentary formations are the
result and the measure of the denudation which the earth's crust has
elsewhere undergone. Therefore a man should examine for himself the great
piles of superimposed strata, and watch the rivulets bringing down mud, and
the waves wearing away the sea-cliffs, in order to comprehend something
about the duration of past time, the monuments of which we see all around
us.
It is good to wander along the coast, when formed of moderately hard rocks,
and mark the process of degradation. The tides in most cases reach the
cliffs only for a short time twice a day, and the waves eat into them only
when they are charged with sand or pebbles; for there is good evidence that
pure water effects nothing in wearing away rock. At last the base of the
cliff is undermined, huge fragments fall down, and these remaining fixed,
have to be worn away atom by atom, until after being reduced in size they
can be rolled about by the waves, and then they are more quickly ground
into pebbles, sand, or mud. But how often do we see along the bases of
retreating cliffs rounded boulders, all thickly clothed by marine
productions, showing how little they are abraded and how seldom they are
rolled about! Moreover, if we follow for a few miles any line of rocky
cliff, which is undergoing degradation, we find that it is only here and
there, along a short length or round a promontory, that the cliffs are at
the present time suffering. The appearance of the surface and the
vegetation show that elsewhere years have elapsed since the waters washed
their base.
We have, however, recently learned from the observations of Ramsay, in the
van of many excellent observers--of Jukes, Geikie, Croll and others, that
subaerial degradation is a much more important agency than coast-action, or
the power of the waves. The whole surface of the land is exposed to the
chemical action of the air and of the rainwater, with its dissolved
carbonic acid, and in colder countries to frost; the disintegrated matter
is carried down even gentle slopes during heavy rain, and to a greater
extent than might be supposed, especially in arid districts, by the wind;
it is then transported by the streams and rivers, which, when rapid deepen
their channels, and triturate the fragments. On a rainy day, even in a
gently undulating country, we see the effects of subaerial degradation in
the muddy rills which flow down every slope. Messrs. Ramsay and Whitaker
have shown, and the observation is a most striking one, that the great
lines of escarpment in the Wealden district and those ranging across
England, which formerly were looked at as ancient sea-coasts, cannot have
been thus formed, for each line is composed of one and the same formation,
while our sea-cliffs are everywhere formed by the intersection of various
formations. This being the case, we are compelled to admit that the
escarpments owe their origin in chief part to the rocks of which they are
composed, having resisted subaerial denudation better than the surrounding
surface; this surface consequently has been gradually lowered, with the
lines of harder rock left projecting. Nothing impresses the mind with the
vast duration of time, according to our ideas of time, more forcibly than
the conviction thus gained that subaerial agencies, which apparently have
so little power, and which seem to work so slowly, have produced great
results.
When thus impressed with the slow rate at which the land is worn away
through subaerial and littoral action, it is good, in order to appreciate
the past duration of time, to consider, on the one hand, the masses of rock
which have been removed over many extensive areas, and on the other hand
the thickness of our sedimentary formations. I remember having been much
struck when viewing volcanic islands, which have been worn by the waves and
pared all round into perpendicular cliffs of one or two thousand feet in
height; for the gentle slope of the lava-streams, due to their formerly
liquid state, showed at a glance how far the hard, rocky beds had once
extended into the open ocean. The same story is told still more plainly by
faults--those great cracks along which the strata have been upheaved on one
side, or thrown down on the other, to the height or depth of thousands of
feet; for since the crust cracked, and it makes no great difference whether
the upheaval was sudden, or, as most geologists now believe, was slow and
effected by many starts, the surface of the land has been so completely
planed down that no trace of these vast dislocations is externally visible.
The Craven fault, for instance, extends for upward of thirty miles, and
along this line the vertical displacement of the strata varies from 600 to
3,000 feet. Professor Ramsay has published an account of a downthrow in
Anglesea of 2,300 feet; and he informs me that he fully believes that there
is one in Merionethshire of 12,000 feet; yet in these cases there is
nothing on the surface of the land to show such prodigious movements; the
pile of rocks on either side of the crack having been smoothly swept away.
On the other hand, in all parts of the world the piles of sedimentary
strata are of wonderful thickness. In the Cordillera, I estimated one mass
of conglomerate at ten thousand feet; and although conglomerates have
probably been accumulated at a quicker rate than finer sediments, yet from
being formed of worn and rounded pebbles, each of which bears the stamp of
time, they are good to show how slowly the mass must have been heaped
together. Professor Ramsay has given me the maximum thickness, from actual
measurement in most cases, of the successive formations in DIFFERENT parts
of Great Britain; and this is the result:
Feet
Palaeozoic strata (not including igneous beds)..57,154
Secondary strata................................13,190
Tertiary strata..................................2,240
--making altogether 72,584 feet; that is, very nearly thirteen and
three-quarters British miles. Some of these formations, which are
represented in England by thin beds, are thousands of feet in thickness on
the Continent. Moreover, between each successive formation we have, in the
opinion of most geologists, blank periods of enormous length. So that the
lofty pile of sedimentary rocks in Britain gives but an inadequate idea of
the time which has elapsed during their accumulation. The consideration of
these various facts impresses the mind almost in the same manner as does
the vain endeavour to grapple with the idea of eternity.
Nevertheless this impression is partly false. Mr. Croll, in an interesting
paper, remarks that we do not err "in forming too great a conception of the
length of geological periods," but in estimating them by years. When
geologists look at large and complicated phenomena, and then at the figures
representing several million years, the two produce a totally different
effect on the mind, and the figures are at once pronounced too small. In
regard to subaerial denudation, Mr. Croll shows, by calculating the known
amount of sediment annually brought down by certain rivers, relatively to
their areas of drainage, that 1,000 feet of solid rock, as it became
gradually disintegrated, would thus be removed from the mean level of the
whole area in the course of six million years. This seems an astonishing
result, and some considerations lead to the suspicion that it may be too
large, but if halved or quartered it is still very surprising. Few of us,
however, know what a million really means: Mr. Croll gives the following
illustration: Take a narrow strip of paper, eighty-three feet four inches
in length, and stretch it along the wall of a large hall; then mark off at
one end the tenth of an inch. This tenth of an inch will represent one
hundred years, and the entire strip a million years. But let it be borne
in mind, in relation to the subject of this work, what a hundred years
implies, represented as it is by a measure utterly insignificant in a hall
of the above dimensions. Several eminent breeders, during a single
lifetime, have so largely modified some of the higher animals, which
propagate their kind much more slowly than most of the lower animals, that
they have formed what well deserves to be called a new sub-breed. Few men
have attended with due care to any one strain for more than half a century,
so that a hundred years represents the work of two breeders in succession.
It is not to be supposed that species in a state of nature ever change so
quickly as domestic animals under the guidance of methodical selection.
The comparison would be in every way fairer with the effects which follow
from unconscious selection, that is, the preservation of the most useful or
beautiful animals, with no intention of modifying the breed; but by this
process of unconscious selection, various breeds have been sensibly changed
in the course of two or three centuries.
Species, however, probably change much more slowly, and within the same
country only a few change at the same time. This slowness follows from all
the inhabitants of the same country being already so well adapted to each
other, that new places in the polity of nature do not occur until after
long intervals, due to the occurrence of physical changes of some kind, or
through the immigration of new forms. Moreover, variations or individual
differences of the right nature, by which some of the inhabitants might be
better fitted to their new places under the altered circumstance, would not
always occur at once. Unfortunately we have no means of determining,
according to the standard of years, how long a period it takes to modify a
species; but to the subject of time we must return.
ON THE POORNESS OF PALAEONTOLOGICAL COLLECTIONS.
Now let us turn to our richest museums, and what a paltry display we
behold! That our collections are imperfect is admitted by every one. The
remark of that admirable palaeontologist, Edward Forbes, should never be
forgotten, namely, that very many fossil species are known and named from
single and often broken specimens, or from a few specimens collected on
some one spot. Only a small portion of the surface of the earth has been
geologically explored, and no part with sufficient care, as the important
discoveries made every year in Europe prove. No organism wholly soft can
be preserved. Shells and bones decay and disappear when left on the bottom
of the sea, where sediment is not accumulating. We probably take a quite
erroneous view, when we assume that sediment is being deposited over nearly
the whole bed of the sea, at a rate sufficiently quick to embed and
preserve fossil remains. Throughout an enormously large proportion of the
ocean, the bright blue tint of the water bespeaks its purity. The many
cases on record of a formation conformably covered, after an immense
interval of time, by another and later formation, without the underlying
bed having suffered in the interval any wear and tear, seem explicable only
on the view of the bottom of the sea not rarely lying for ages in an
unaltered condition. The remains which do become embedded, if in sand or
gravel, will, when the beds are upraised, generally be dissolved by the
percolation of rain water charged with carbonic acid. Some of the many
kinds of animals which live on the beach between high and low water mark
seem to be rarely preserved. For instance, the several species of the
Chthamalinae (a sub-family of sessile cirripedes) coat the rocks all over
the world in infinite numbers: they are all strictly littoral, with the
exception of a single Mediterranean species, which inhabits deep water and
this has been found fossil in Sicily, whereas not one other species has
hitherto been found in any tertiary formation: yet it is known that the
genus Chthamalus existed during the Chalk period. Lastly, many great
deposits, requiring a vast length of time for their accumulation, are
entirely destitute of organic remains, without our being able to assign any
reason: one of the most striking instances is that of the Flysch
formation, which consists of shale and sandstone, several thousand,
occasionally even six thousand feet in thickness, and extending for at
least 300 miles from Vienna to Switzerland; and although this great mass
has been most carefully searched, no fossils, except a few vegetable
remains, have been found.
With respect to the terrestrial productions which lived during the
Secondary and Palaeozoic periods, it is superfluous to state that our
evidence is fragmentary in an extreme degree. For instance, until recently
not a land-shell was known belonging to either of these vast periods, with
the exception of one species discovered by Sir C. Lyell and Dr. Dawson in
the carboniferous strata of North America; but now land-shells have been
found in the lias. In regard to mammiferous remains, a glance at the
historical table published in Lyell's Manual, will bring home the truth,
how accidental and rare is their preservation, far better than pages of
detail. Nor is their rarity surprising, when we remember how large a
proportion of the bones of tertiary mammals have been discovered either in
caves or in lacustrine deposits; and that not a cave or true lacustrine bed
is known belonging to the age of our secondary or palaeozoic formations.
But the imperfection in the geological record largely results from another
and more important cause than any of the foregoing; namely, from the
several formations being separated from each other by wide intervals of
time. This doctrine has been emphatically admitted by many geologists and
palaeontologists, who, like E. Forbes, entirely disbelieve in the change of
species. When we see the formations tabulated in written works, or when we
follow them in nature, it is difficult to avoid believing that they are
closely consecutive. But we know, for instance, from Sir R. Murchison's
great work on Russia, what wide gaps there are in that country between the
superimposed formations; so it is in North America, and in many other parts
of the world. The most skilful geologist, if his attention had been
confined exclusively to these large territories, would never have suspected
that during the periods which were blank and barren in his own country,
great piles of sediment, charged with new and peculiar forms of life, had
elsewhere been accumulated. And if, in every separate territory, hardly
any idea can be formed of the length of time which has elapsed between the
consecutive formations, we may infer that this could nowhere be
ascertained. The frequent and great changes in the mineralogical
composition of consecutive formations, generally implying great changes in
the geography of the surrounding lands, whence the sediment was derived,
accord with the belief of vast intervals of time having elapsed between
each formation.
We can, I think, see why the geological formations of each region are
almost invariably intermittent; that is, have not followed each other in
close sequence. Scarcely any fact struck me more when examining many
hundred miles of the South American coasts, which have been upraised
several hundred feet within the recent period, than the absence of any
recent deposits sufficiently extensive to last for even a short geological
period. Along the whole west coast, which is inhabited by a peculiar
marine fauna, tertiary beds are so poorly developed that no record of
several successive and peculiar marine faunas will probably be preserved to
a distant age. A little reflection will explain why, along the rising
coast of the western side of South America, no extensive formations with
recent or tertiary remains can anywhere be found, though the supply of
sediment must for ages have been great, from the enormous degradation of
the coast rocks and from the muddy streams entering the sea. The
explanation, no doubt, is that the littoral and sub-littoral deposits are
continually worn away, as soon as they are brought up by the slow and
gradual rising of the land within the grinding action of the coast-waves.
We may, I think, conclude that sediment must be accumulated in extremely
thick, solid, or extensive masses, in order to withstand the incessant
action of the waves, when first upraised and during subsequent oscillations
of level, as well as the subsequent subaerial degradation. Such thick and
extensive accumulations of sediment may be formed in two ways; either in
profound depths of the sea, in which case the bottom will not be inhabited
by so many and such varied forms of life as the more shallow seas; and the
mass when upraised will give an imperfect record of the organisms which
existed in the neighbourhood during the period of its accumulation. Or
sediment may be deposited to any thickness and extent over a shallow
bottom, if it continue slowly to subside. In this latter case, as long as
the rate of subsidence and supply of sediment nearly balance each other,
the sea will remain shallow and favourable for many and varied forms, and
thus a rich fossiliferous formation, thick enough, when upraised, to resist
a large amount of denudation, may be formed.
I am convinced that nearly all our ancient formations, which are throughout
the greater part of their thickness RICH IN FOSSILS, have thus been formed
during subsidence. Since publishing my views on this subject in 1845, I
have watched the progress of geology, and have been surprised to note how
author after author, in treating of this or that great formation, has come
to the conclusion that it was accumulated during subsidence. I may add,
that the only ancient tertiary formation on the west coast of South
America, which has been bulky enough to resist such degradation as it has
as yet suffered, but which will hardly last to a distant geological age,
was deposited during a downward oscillation of level, and thus gained
considerable thickness.
All geological facts tell us plainly that each area has undergone numerous
slow oscillations of level, and apparently these oscillations have affected
wide spaces. Consequently, formations rich in fossils and sufficiently
thick and extensive to resist subsequent degradation, will have been formed
over wide spaces during periods of subsidence, but only where the supply of
sediment was sufficient to keep the sea shallow and to embed and preserve
the remains before they had time to decay. On the other hand, as long as
the bed of the sea remained stationary, THICK deposits cannot have been
accumulated in the shallow parts, which are the most favourable to life.
Still less can this have happened during the alternate periods of
elevation; or, to speak more accurately, the beds which were then
accumulated will generally have been destroyed by being upraised and
brought within the limits of the coast-action.
These remarks apply chiefly to littoral and sublittoral deposits. In the
case of an extensive and shallow sea, such as that within a large part of
the Malay Archipelago, where the depth varies from thirty or forty to sixty
fathoms, a widely extended formation might be formed during a period of
elevation, and yet not suffer excessively from denudation during its slow
upheaval; but the thickness of the formation could not be great, for owing
to the elevatory movement it would be less than the depth in which it was
formed; nor would the deposit be much consolidated, nor be capped by
overlying formations, so that it would run a good chance of being worn away
by atmospheric degradation and by the action of the sea during subsequent
oscillations of level. It has, however, been suggested by Mr. Hopkins,
that if one part of the area, after rising and before being denuded,
subsided, the deposit formed during the rising movement, though not thick,
might afterwards become protected by fresh accumulations, and thus be
preserved for a long period.
Mr. Hopkins also expresses his belief that sedimentary beds of considerable
horizontal extent have rarely been completely destroyed. But all
geologists, excepting the few who believe that our present metamorphic
schists and plutonic rocks once formed the primordial nucleus of the globe,
will admit that these latter rocks have been stripped of their covering to
an enormous extent. For it is scarcely possible that such rocks could have
been solidified and crystallised while uncovered; but if the metamorphic
action occurred at profound depths of the ocean, the former protecting
mantle of rock may not have been very thick. Admitting then that gneiss,
mica-schist, granite, diorite, etc., were once necessarily covered up, how
can we account for the naked and extensive areas of such rocks in many
parts of the world, except on the belief that they have subsequently been
completely denuded of all overlying strata? That such extensive areas do
exist cannot be doubted: the granitic region of Parime is described by
Humboldt as being at least nineteen times as large as Switzerland. South
of the Amazon, Boue colours an area composed of rocks of this nature as
equal to that of Spain, France, Italy, part of Germany, and the British
Islands, all conjoined. This region has not been carefully explored, but
from the concurrent testimony of travellers, the granitic area is very
large: thus Von Eschwege gives a detailed section of these rocks,
stretching from Rio de Janeiro for 260 geographical miles inland in a
straight line; and I travelled for 150 miles in another direction, and saw
nothing but granitic rocks. Numerous specimens, collected along the whole
coast, from near Rio de Janeiro to the mouth of the Plata, a distance of
1,100 geographical miles, were examined by me, and they all belonged to
this class. Inland, along the whole northern bank of the Plata, I saw,
besides modern tertiary beds, only one small patch of slightly
metamorphosed rock, which alone could have formed a part of the original
capping of the granitic series. Turning to a well-known region, namely, to
the United States and Canada, as shown in Professor H.D. Rogers' beautiful
map, I have estimated the areas by cutting out and weighing the paper, and
I find that the metamorphic (excluding the "semi-metamorphic") and granite
rocks exceed, in the proportion of 19 to 12.5, the whole of the newer
Palaeozoic formations. In many regions the metamorphic and granite rocks
would be found much more widely extended than they appear to be, if all the
sedimentary beds were removed which rest unconformably on them, and which
could not have formed part of the original mantle under which they were
crystallised. Hence, it is probable that in some parts of the world whole
formations have been completely denuded, with not a wreck left behind.
One remark is here worth a passing notice. During periods of elevation the
area of the land and of the adjoining shoal parts of the sea will be
increased and new stations will often be formed--all circumstances
favourable, as previously explained, for the formation of new varieties and
species; but during such periods there will generally be a blank in the
geological record. On the other hand, during subsidence, the inhabited
area and number of inhabitants will decrease (excepting on the shores of a
continent when first broken up into an archipelago), and consequently
during subsidence, though there will be much extinction, few new varieties
or species will be formed; and it is during these very periods of
subsidence that the deposits which are richest in fossils have been
accumulated.
ON THE ABSENCE OF NUMEROUS INTERMEDIATE VARIETIES IN ANY SINGLE FORMATION.
>From these several considerations it cannot be doubted that the geological
record, viewed as a whole, is extremely imperfect; but if we confine our
attention to any one formation, it becomes much more difficult to
understand why we do not therein find closely graduated varieties between
the allied species which lived at its commencement and at its close.
Several cases are on record of the same species presenting varieties in the
upper and lower parts of the same formation. Thus Trautschold gives a
number of instances with Ammonites, and Hilgendorf has described a most
curious case of ten graduated forms of Planorbis multiformis in the
successive beds of a fresh-water formation in Switzerland. Although each
formation has indisputably required a vast number of years for its
deposition, several reasons can be given why each should not commonly
include a graduated series of links between the species which lived at its
commencement and close, but I cannot assign due proportional weight to the
following considerations.
Although each formation may mark a very long lapse of years, each probably
is short compared with the period requisite to change one species into
another. I am aware that two palaeontologists, whose opinions are worthy
of much deference, namely Bronn and Woodward, have concluded that the
average duration of each formation is twice or thrice as long as the
average duration of specific forms. But insuperable difficulties, as it
seems to me, prevent us from coming to any just conclusion on this head.
When we see a species first appearing in the middle of any formation, it
would be rash in the extreme to infer that it had not elsewhere previously
existed. So again, when we find a species disappearing before the last
layers have been deposited, it would be equally rash to suppose that it
then became extinct. We forget how small the area of Europe is compared
with the rest of the world; nor have the several stages of the same
formation throughout Europe been correlated with perfect accuracy.
We may safely infer that with marine animals of all kinds there has been a
large amount of migration due to climatal and other changes; and when we
see a species first appearing in any formation, the probability is that it
only then first immigrated into that area. It is well known, for instance,
that several species appear somewhat earlier in the palaeozoic beds of
North America than in those of Europe; time having apparently been required
for their migration from the American to the European seas. In examining
the latest deposits, in various quarters of the world, it has everywhere
been noted, that some few still existing species are common in the deposit,
but have become extinct in the immediately surrounding sea; or, conversely,
that some are now abundant in the neighbouring sea, but are rare or absent
in this particular deposit. It is an excellent lesson to reflect on the
ascertained amount of migration of the inhabitants of Europe during the
glacial epoch, which forms only a part of one whole geological period; and
likewise to reflect on the changes of level, on the extreme change of
climate, and on the great lapse of time, all included within this same
glacial period. Yet it may be doubted whether, in any quarter of the
world, sedimentary deposits, INCLUDING FOSSIL REMAINS, have gone on
accumulating within the same area during the whole of this period. It is
not, for instance, probable that sediment was deposited during the whole of
the glacial period near the mouth of the Mississippi, within that limit of
depth at which marine animals can best flourish: for we know that great
geographical changes occurred in other parts of America during this space
of time. When such beds as were deposited in shallow water near the mouth
of the Mississippi during some part of the glacial period shall have been
upraised, organic remains will probably first appear and disappear at
different levels, owing to the migrations of species and to geographical
changes. And in the distant future, a geologist, examining these beds,
would be tempted to conclude that the average duration of life of the
embedded fossils had been less than that of the glacial period, instead of
having been really far greater, that is, extending from before the glacial
epoch to the present day.
In order to get a perfect gradation between two forms in the upper and
lower parts of the same formation, the deposit must have gone on
continuously accumulating during a long period, sufficient for the slow
process of modification; hence, the deposit must be a very thick one; and
the species undergoing change must have lived in the same district
throughout the whole time. But we have seen that a thick formation,
fossiliferous throughout its entire thickness, can accumulate only during a
period of subsidence; and to keep the depth approximately the same, which
is necessary that the same marine species may live on the same space, the
supply of sediment must nearly counterbalance the amount of subsidence.
But this same movement of subsidence will tend to submerge the area whence
the sediment is derived, and thus diminish the supply, whilst the downward
movement continues. In fact, this nearly exact balancing between the
supply of sediment and the amount of subsidence is probably a rare
contingency; for it has been observed by more than one palaeontologist that
very thick deposits are usually barren of organic remains, except near
their upper or lower limits.
It would seem that each separate formation, like the whole pile of
formations in any country, has generally been intermittent in its
accumulation. When we see, as is so often the case, a formation composed
of beds of widely different mineralogical composition, we may reasonably
suspect that the process of deposition has been more or less interrupted.
Nor will the closest inspection of a formation give us any idea of the
length of time which its deposition may have consumed. Many instances
could be given of beds, only a few feet in thickness, representing
formations which are elsewhere thousands of feet in thickness, and which
must have required an enormous period for their accumulation; yet no one
ignorant of this fact would have even suspected the vast lapse of time
represented by the thinner formation. Many cases could be given of the
lower beds of a formation having been upraised, denuded, submerged, and
then re-covered by the upper beds of the same formation--facts, showing
what wide, yet easily overlooked, intervals have occurred in its
accumulation. In other cases we have the plainest evidence in great
fossilised trees, still standing upright as they grew, of many long
intervals of time and changes of level during the process of deposition,
which would not have been suspected, had not the trees been preserved:
thus Sir C. Lyell and Dr. Dawson found carboniferous beds 1,400 feet thick
in Nova Scotia, with ancient root-bearing strata, one above the other, at
no less than sixty-eight different levels. Hence, when the same species
occurs at the bottom, middle, and top of a formation, the probability is
that it has not lived on the same spot during the whole period of
deposition, but has disappeared and reappeared, perhaps many times, during
the same geological period. Consequently if it were to undergo a
considerable amount of modification during the deposition of any one
geological formation, a section would not include all the fine intermediate
gradations which must on our theory have existed, but abrupt, though
perhaps slight, changes of form.
It is all-important to remember that naturalists have no golden rule by
which to distinguish species and varieties; they grant some little
variability to each species, but when they meet with a somewhat greater
amount of difference between any two forms, they rank both as species,
unless they are enabled to connect them together by the closest
intermediate gradations; and this, from the reasons just assigned, we can
seldom hope to effect in any one geological section. Supposing B and C to
be two species, and a third, A, to be found in an older and underlying bed;
even if A were strictly intermediate between B and C, it would simply be
ranked as a third and distinct species, unless at the same time it could be
closely connected by intermediate varieties with either one or both forms.
Nor should it be forgotten, as before explained, that A might be the actual
progenitor of B and C, and yet would not necessarily be strictly
intermediate between them in all respects. So that we might obtain the
parent-species and its several modified descendants from the lower and
upper beds of the same formation, and unless we obtained numerous
transitional gradations, we should not recognise their blood-relationship,
and should consequently rank them as distinct species.
It is notorious on what excessively slight differences many
palaeontologists have founded their species; and they do this the more
readily if the specimens come from different sub-stages of the same
formation. Some experienced conchologists are now sinking many of the very
fine species of D'Orbigny and others into the rank of varieties; and on
this view we do find the kind of evidence of change which on the theory we
ought to find. Look again at the later tertiary deposits, which include
many shells believed by the majority of naturalists to be identical with
existing species; but some excellent naturalists, as Agassiz and Pictet,
maintain that all these tertiary species are specifically distinct, though
the distinction is admitted to be very slight; so that here, unless we
believe that these eminent naturalists have been misled by their
imaginations, and that these late tertiary species really present no
difference whatever from their living representatives, or unless we admit,
in opposition to the judgment of most naturalists, that these tertiary
species are all truly distinct from the recent, we have evidence of the
frequent occurrence of slight modifications of the kind required. If we
look to rather wider intervals of time, namely, to distinct but consecutive
stages of the same great formation, we find that the embedded fossils,
though universally ranked as specifically different, yet are far more
closely related to each other than are the species found in more widely
separated formations; so that here again we have undoubted evidence of
change in the direction required by the theory; but to this latter subject
I shall return in the following chapter.
With animals and plants that propagate rapidly and do not wander much,
there is reason to suspect, as we have formerly seen, that their varieties
are generally at first local; and that such local varieties do not spread
widely and supplant their parent-form until they have been modified and
perfected in some considerable degree. According to this view, the chance
of discovering in a formation in any one country all the early stages of
transition between any two forms, is small, for the successive changes are
supposed to have been local or confined to some one spot. Most marine
animals have a wide range; and we have seen that with plants it is those
which have the widest range, that oftenest present varieties, so that, with
shells and other marine animals, it is probable that those which had the
widest range, far exceeding the limits of the known geological formations
in Europe, have oftenest given rise, first to local varieties and
ultimately to new species; and this again would greatly lessen the chance
of our being able to trace the stages of transition in any one geological
formation.
It is a more important consideration, leading to the same result, as lately
insisted on by Dr. Falconer, namely, that the period during which each
species underwent modification, though long as measured by years, was
probably short in comparison with that during which it remained without
undergoing any change.
It should not be forgotten, that at the present day, with perfect specimens
for examination, two forms can seldom be connected by intermediate
varieties, and thus proved to be the same species, until many specimens are
collected from many places; and with fossil species this can rarely be
done. We shall, perhaps, best perceive the improbability of our being
enabled to connect species by numerous, fine, intermediate, fossil links,
by asking ourselves whether, for instance, geologists at some future period
will be able to prove that our different breeds of cattle, sheep, horses,
and dogs are descended from a single stock or from several aboriginal
stocks; or, again, whether certain sea-shells inhabiting the shores of
North America, which are ranked by some conchologists as distinct species
from their European representatives, and by other conchologists as only
varieties, are really varieties, or are, as it is called, specifically
distinct. This could be effected by the future geologist only by his
discovering in a fossil state numerous intermediate gradations; and such
success is improbable in the highest degree.
It has been asserted over and over again, by writers who believe in the
immutability of species, that geology yields no linking forms. This
assertion, as we shall see in the next chapter, is certainly erroneous. As
Sir J. Lubbock has remarked, "Every species is a link between other allied
forms." If we take a genus having a score of species, recent and extinct,
and destroy four-fifths of them, no one doubts that the remainder will
stand much more distinct from each other. If the extreme forms in the
genus happen to have been thus destroyed, the genus itself will stand more
distinct from other allied genera. What geological research has not
revealed, is the former existence of infinitely numerous gradations, as
fine as existing varieties, connecting together nearly all existing and
extinct species. But this ought not to be expected; yet this has been
repeatedly advanced as a most serious objection against my views.
It may be worth while to sum up the foregoing remarks on the causes of the
imperfection of the geological record under an imaginary illustration. The
Malay Archipelago is about the size of Europe from the North Cape to the
Mediterranean, and from Britain to Russia; and therefore equals all the
geological formations which have been examined with any accuracy, excepting
those of the United States of America. I fully agree with Mr. Godwin-
Austen, that the present condition of the Malay Archipelago, with its
numerous large islands separated by wide and shallow seas, probably
represents the former state of Europe, while most of our formations were
accumulating. The Malay Archipelago is one of the richest regions in
organic beings; yet if all the species were to be collected which have ever
lived there, how imperfectly would they represent the natural history of
the world!
But we have every reason to believe that the terrestrial productions of the
archipelago would be preserved in an extremely imperfect manner in the
formations which we suppose to be there accumulating. Not many of the
strictly littoral animals, or of those which lived on naked submarine
rocks, would be embedded; and those embedded in gravel or sand would not
endure to a distant epoch. Wherever sediment did not accumulate on the bed
of the sea, or where it did not accumulate at a sufficient rate to protect
organic bodies from decay, no remains could be preserved.
Formations rich in fossils of many kinds, and of thickness sufficient to
last to an age as distant in futurity as the secondary formations lie in
the past, would generally be formed in the archipelago only during periods
of subsidence. These periods of subsidence would be separated from each
other by immense intervals of time, during which the area would be either
stationary or rising; whilst rising, the fossiliferous formations on the
steeper shores would be destroyed, almost as soon as accumulated, by the
incessant coast-action, as we now see on the shores of South America. Even
throughout the extensive and shallow seas within the archipelago,
sedimentary beds could hardly be accumulated of great thickness during the
periods of elevation, or become capped and protected by subsequent
deposits, so as to have a good chance of enduring to a very distant future.
During the periods of subsidence, there would probably be much extinction
of life; during the periods of elevation, there would be much variation,
but the geological record would then be less perfect.
It may be doubted whether the duration of any one great period of
subsidence over the whole or part of the archipelago, together with a
contemporaneous accumulation of sediment, would EXCEED the average duration
of the same specific forms; and these contingencies are indispensable for
the preservation of all the transitional gradations between any two or more
species. If such gradations were not all fully preserved, transitional
varieties would merely appear as so many new, though closely allied
species. It is also probable that each great period of subsidence would be
interrupted by oscillations of level, and that slight climatical changes
would intervene during such lengthy periods; and in these cases the
inhabitants of the archipelago would migrate, and no closely consecutive
record of their modifications could be preserved in any one formation.
Very many of the marine inhabitants of the archipelago now range thousands
of miles beyond its confines; and analogy plainly leads to the belief that
it would be chiefly these far-ranging species, though only some of them,
which would oftenest produce new varieties; and the varieties would at
first be local or confined to one place, but if possessed of any decided
advantage, or when further modified and improved, they would slowly spread
and supplant their parent-forms. When such varieties returned to their
ancient homes, as they would differ from their former state in a nearly
uniform, though perhaps extremely slight degree, and as they would be found
embedded in slightly different sub-stages of the same formation, they
would, according to the principles followed by many palaeontologists, be
ranked as new and distinct species.
If then there be some degree of truth in these remarks, we have no right to
expect to find, in our geological formations, an infinite number of those
fine transitional forms, which, on our theory, have connected all the past
and present species of the same group into one long and branching chain of
life. We ought only to look for a few links, and such assuredly we do
find--some more distantly, some more closely, related to each other; and
these links, let them be ever so close, if found in different stages of the
same formation, would, by many palaeontologists, be ranked as distinct
species. But I do not pretend that I should ever have suspected how poor
was the record in the best preserved geological sections, had not the
absence of innumerable transitional links between the species which lived
at the commencement and close of each formation, pressed so hardly on my
theory.
ON THE SUDDEN APPEARANCE OF WHOLE GROUPS OF ALLIED SPECIES.
The abrupt manner in which whole groups of species suddenly appear in
certain formations, has been urged by several palaeontologists--for
instance, by Agassiz, Pictet, and Sedgwick, as a fatal objection to the
belief in the transmutation of species. If numerous species, belonging to
the same genera or families, have really started into life at once, the
fact would be fatal to the theory of evolution through natural selection.
For the development by this means of a group of forms, all of which are
descended from some one progenitor, must have been an extremely slow
process; and the progenitors must have lived long before their modified
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