Charles Darwin.

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affect the males and fertile females, which alone leave descendants. I ai
surprised that no one has hitherto advanced this demonstrative case ol
neuter insects, against the well-known doctrine of inherited habit, as ad-
vanced by Lamarck.



I have endeavored in this chapter briefly to show that the mental quahties
of our domestic animals vary, and that the variations are inherited. Still
more briefly I have attempted to show that instincts vary slightly in a state
of nature. No one will dispute that instincts are of the highest importance
to each animal. Therefore, there is no real difficulty, under changing con-
ditions of life, in natural selection accumulating to any extent slight modi-
fications of instinct which are in any way useful. In many cases habit or
use and disuse have probably come 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 Hable 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 inexplicalDle —
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 considerable
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 horn-
bills 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 mak-
ing 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 weakest die.



Distinction between the Sterility of First Crosses and of Hybrids — Sterility Various
in Degree, not Universal, affected by Close Interbreeding, removed by Domesti-
cation — Laws governing the Sterility of Hybrids — Sterility not a Special Endow-
ment, 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 inter-
crossed, have been specially endowed with sterility, in order to prevent their
confusion. This view certainly seems at first highly probable, for species liv-
ing 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 lunda-
mentally different, have generally been confounded; namely, the sterility of
species when first crossed, and the sterility of the hybrids produced from

Pure species have of course their organs of reproduction in a perfect con-
dition, yet when intercrossed they produce either few or no offspring. Hy-
brids, on the other hand, have their reproductive organs functionally impo-
tent, as may by clearly seen in the state of the male element in both plants
and animals; though the formative organs themselves are perfect in struc-
ture, 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 im-
portant, 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 distinc-
tion between varieties and species.


First, for the sterility of species when crossed and of their hybrid offspring.
It is impossible to study the several memoirs and works of those two con-



scientious and admirable observers, Kolreuter and Gartner, who almost de-
voted their lives to this subject, without being deeply impressed with the
high generality of some degree of sterility. Kolreuter makes the rule univer-
sal ; 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 un-
hesitatingly 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 seri-
ous error here intervene; a plant, to be hybridized, must be castrated, and,

I 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 ex-
perimented 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, can-
not be doubted ; for Gartner gives in his table about a score of cases of plants
which he castrated, and artificially fertilized with their own pollen, and (ex-
cluding all cases such as the Leguminosse, 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
ccerulea) , which the best botanists rank as varieties, and found them abso-
lutely 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

jperfect 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 con-
clusions 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 hybridizers, 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 genera-
tions, 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 off-
spring; 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 vigor and fertility of the offspring, and on the other hand that
very close interbreeding lessens their vigor and fertility, that I cannot doubt
the correctness of this conclusion. Hybrids are seldom raised by experimental-
ists in great numbers; and as the parent-species, or other allied hybrids,
igenerally 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 fertilized 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 re-
markable statement repeatedly made by Gartner, namely, that if even the
less fertile hybrids be artificially fertilized with hybrid pollen of the same
kind, their fertility, notwithstanding the frequent ill effects from manipula-
tion, sometimes decidedly increases, and goes on increasing. Now, in the
process of artificial fertilization, 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 fertilized; 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 fertilized hybrids, in contrast with those spontaneously self-
fertilized, 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
hybridizer, 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 diff'erence 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 state-
ments I will here give only a single one as an example, namely, that "every
ovule in a pod of Crinum capense fertilized 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 Lobeha, Verbascum, and Passiflora,
can easily be fertilized 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 fertilizing 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 Miiller, 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 hybridized much more readily than they can be
fertilized by pollen from the same individual plant! To give one instance, a
bulb of Hippeastrum auHcum produced four flowers; three were fertilized by
Herbert with their own pollen, and the fourth was subsequently fertilized 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, Rho-
dodendron, 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
Chili." 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 most 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 them-


selves 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 con-
finement, 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 to-
gether, 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 subse-
quently 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 stock; yet, with per-
haps 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 evi-


dence that the crossed offspring from the Indian humped and common cattle
are inter se perfectly fertile; and from the observations by Riitimeyer 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 ex-
tended 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.



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 hybridization 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 surpris-
ing 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, be-
yond that which the plant's own pollen produces. So in hybrids themselves,
there are some which never have produced, and probably never would pro-
duce, 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 fertilization. From this ex-
treme degree of sterility we have self-fertilized 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 parallel-
ism 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 unfavorable 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

Online LibraryCharles DarwinThe origin of species → online text (page 27 of 50)