Chapter VIII

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 -- Causes of the sterility of first crosses and of hybrids --

Parallelism between the effects of changed conditions of life and crossing

-- Fertility of varieties when crossed and of their mongrel offspring not

universal -- Hybrids and mongrels compared independently of their fertility

-- Summary.

The view generally entertained by naturalists is that species, when

intercrossed, have been specially endowed with the quality of sterility, in

order to prevent the confusion of all organic forms. This view certainly

seems at first probable, for species within the same country could hardly

have kept distinct had they been capable of crossing freely. The

importance of the fact that hybrids are very generally sterile, has, I

think, been much underrated by some late writers. On the theory of natural

selection the case is especially important, inasmuch as the sterility of

hybrids could not possibly be of any advantage to them, and therefore could

not have been acquired by the continued preservation of successive

profitable degrees of sterility. I hope, however, to be able to show that

sterility is not a specially acquired or endowed quality, but is incidental

on other acquired differences.

In treating this subject, two classes of facts, to a large extent

fundamentally different, have generally been confounded together; namely,

the sterility of two 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 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 has probably 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 have

descended from common parents, when intercrossed, and likewise the

fertility of their mongrel offspring, is, on my theory, of equal importance

with the sterility of species; for it seems to make a broad and clear

distinction 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 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 crossed and by their hybrid offspring,

with the average number produced by both pure parent-species in a state of

nature. But a serious cause of error seems to me to be here introduced: 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 experimentised on by

Gartner were potted, and apparently 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 during

several years repeatedly crossed the primrose and cowslip, which we have

such good reason to believe to be varieties, and only once or twice

succeeded in getting fertile seed; as he found the common red and blue

pimpernels (Anagallis arvensis and coerulea), which the best botanists rank

as varieties, absolutely sterile together; and as he came to the same

conclusion in several other analogous cases; it seems to me that we may

well be permitted to doubt whether many other species are really so

sterile, when intercrossed, as Gartner believes.

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, should have

arrived at diametrically opposite conclusions in regard to the very same

species. 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 author, from experiments made during different

years. It can thus be shown that neither sterility nor fertility affords

any clear distinction between species and varieties; but that 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

increased, but generally greatly decreased. I do not doubt that this is

usually the case, and that the fertility often suddenly decreases in the

first few generations. Nevertheless I believe that in all these

experiments the fertility has been diminished by an independent cause,

namely, from close interbreeding. I have collected so large a body of

facts, showing that close interbreeding lessens fertility, and, on the

other hand, that an occasional cross with a distinct individual or variety

increases fertility, that I cannot doubt the correctness of this almost

universal belief amongst breeders. 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

will generally be fertilised during each generation by their own individual

pollen; and I am convinced that this would 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 of manipulation, sometimes decidedly increases, and

goes on increasing. Now, in 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 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 the 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 the increase of fertility in the successive generations of artificially

fertilised hybrids may, I believe, be accounted for by close interbreeding

having been avoided.

Now let us turn to the results arrived at by the 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 experimentised

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 hothouses 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 (he says) I never saw to occur in a case

of its natural fecundation.' So that we here 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 most singular fact, namely,

that there are individual plants, as with certain species of Lobelia, and

with all the species of the genus Hippeastrum, which can be far more easily

fertilised by the pollen of another and distinct species, than by their own

pollen. For these plants have been found to yield seed to the pollen of a

distinct species, though quite sterile with their own pollen,

notwithstanding that their own pollen was found to be perfectly good, for

it fertilised distinct species. So that certain individual plants and all

the individuals of certain species can actually be hybridised much more

readily than they can be self-fertilised! For 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 other and 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.' In a

letter to me, in 1839, Mr. Herbert told me that he had then tried the

experiment during five years, and he continued to try it during several

subsequent years, and always with the same result. This result has, also,

been confirmed by other observers in the case of Hippeastrum with its

sub-genera, and in the case of some other genera, as Lobelia, Passiflora

and Verbascum. Although the plants in these experiments appeared perfectly

healthy, and although both the ovules and pollen of the same flower were

perfectly good with respect to other species, yet as they were functionally

imperfect in their mutual self-action, we must infer that the plants were

in an unnatural state. Nevertheless these facts show on what slight and

mysterious causes the lesser or greater fertility of species when crossed,

in comparison with the same species when self-fertilised, 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, &c., 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, 'reproduced 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,

gone on decreasing in fertility in each successive generation, as Gartner

believes to be the case, the fact would have been notorious to nurserymen.

Horticulturists raise large beds of the same hybrids, and such alone are

fairly treated, for by insect agency the several individuals of the same

hybrid variety are allowed to freely cross 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 separated in the scale

of nature can be more easily crossed than in the case of plants; but the

hybrids themselves are, I think, more sterile. I doubt whether any case of

a perfectly fertile hybrid animal can be considered as thoroughly well

authenticated. 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 other

finches, but as not one of these nine species 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. If we were to act thus, and pair

brothers and sisters in the case of any pure animal, which from any cause

had the least tendency to sterility, the breed would assuredly be lost in a

very few generations.

Although I do not know of any thoroughly well-authenticated cases of

perfectly fertile hybrid animals, I have some reason to believe that the

hybrids from Cervulus vaginalis and Reevesii, and from Phasianus colchicus

with P. torquatus and with P. versicolor are perfectly fertile. 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 Capt. 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

fertile.

A doctrine which originated with Pallas, has been largely accepted by

modern naturalists; namely, that most of our domestic animals have

descended from two or more aboriginal species, since commingled by

intercrossing. On this view, the aboriginal species must either at first

have produced quite fertile hybrids, or the hybrids must have become in

subsequent generations quite fertile under domestication. This latter

alternative seems to me the most probable, and I am inclined to believe in

its truth, although it rests on no direct evidence. I believe, for

instance, that our dogs have descended from several wild stocks; yet, with

perhaps the exception of certain indigenous domestic dogs of South America,

all are quite fertile together; and 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 there is reason to believe

that our European and the humped Indian cattle are quite fertile together;

but from facts communicated to me by Mr. Blyth, I think they must be

considered as distinct species. On this view of the origin of many of our

domestic animals, we must either give up the belief of the almost universal

sterility of distinct species of animals when crossed; or we must look at

sterility, not as an indelible characteristic, but as one capable of being

removed by domestication.

Finally, looking to 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 circumstances and rules governing

the sterility of first crosses and of hybrids. Our chief object will be to

see whether or not the rules indicate that species have specially been

endowed with this quality, in order to prevent their crossing and blending

together in utter confusion. The following rules and conclusions are

chiefly drawn up from Gartner's admirable work on the hybridisation of

plants. I have taken much pains to ascertain how far the rules 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 to exist;

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 of the same

genus applied to the stigma of some one species, 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 will

produce. So in hybrids themselves, there are some which never have

produced, and probably never would produce, even with the pollen of either

pure parent, 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.

Hybrids 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 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 the fertility of pure species.

But the degree of fertility is likewise innately variable; for it is not

always the same when the same two species are crossed under the same

circumstances, but 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 exactly the same conditions.

By the term systematic affinity is meant, the resemblance between species

in structure and in constitution, more especially in the structure of parts

which are of high physiological importance and which differ little in the

allied species. Now the fertility of first crosses between species, 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. Very 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 stallion-horse being first crossed with a female-ass, and then a

male-ass with a mare: 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, or of any recognisable

difference in their whole organisation. On the other hand, these cases

clearly show that the capacity for crossing is connected with

constitutional differences imperceptible by us, and confined to the

reproductive system. This difference in the result of reciprocal crosses

between the same two species was long ago observed by Kolreuter. To give

an instance: Mirabilis jalappa 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. jalappa, 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 forms so closely related (as Matthiola annua and glabra) that many

botanists rank them 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, 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 amongst 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 fertility in the hybrid is

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 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 to be 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, chiefly in the reproductive

systems, of the species which are crossed. The differences being of so

peculiar and limited a nature, that, in reciprocal crosses between 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 so entirely unimportant for its 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,

&c.; 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 trees together

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 very different case from the

difficulty of uniting two pure species, which have their reproductive

organs perfect; yet these two distinct cases run to a certain extent

parallel. Something analogous occurs in grafting; for Thouin found that

three species of Robinia, which seeded freely on their own roots, and which

could be grafted with no great difficulty on another species, when thus

grafted were rendered barren. On the other hand, certain species of

Sorbus, when grafted on other species, yielded twice as much fruit as when

on their own roots. We are reminded by this latter fact of the

extraordinary case of Hippeastrum, Lobelia, &c., which seeded much more

freely when fertilised with the pollen of distinct species, than when

self-fertilised with their own pollen.

We thus see, that although there is a clear and fundamental difference

between the mere adhesion of grafted stocks, and the union of the male and

female elements in the act of reproduction, yet that there is a rude degree

of parallelism in the results of grafting and of crossing distinct species.

And as we must look at the curious and complex laws governing the facility

with which trees can be grafted on each other as incidental on unknown

differences in their vegetative systems, so I believe that the still more

complex laws governing the facility of first crosses, are incidental on

unknown differences, chiefly in their reproductive systems. These

differences, in both cases, follow to a certain extent, as might have been

expected, systematic affinity, by which every kind of resemblance and

dissimilarity between organic beings is attempted to be expressed. The

facts by no means seem to me to indicate that the greater or lesser

difficulty of either grafting or crossing together various species has been

a special endowment; although in the case of crossing, the difficulty is as

important for the endurance and stability of specific forms, as in the case

of grafting it is unimportant for their welfare.

Causes of the Sterility of first Crosses and of Hybrids. -- We may now look

a little closer at the probable causes of the sterility of first crosses

and of hybrids. These two cases are fundamentally different, for, as just

remarked, in the union of two pure species the male and female sexual

elements are perfect, whereas in hybrids they are imperfect. Even in first

crosses, the greater or lesser difficulty in effecting a union apparently

depends on several distinct causes. There must sometimes be a physical

impossibility in the male element reaching the ovule, as would be the case

with a plant having a pistil too long for the pollen-tubes to reach the

ovarium. It has also been observed that when pollen of one species is

placed on the stigma of a distantly allied species, though the pollen-tubes

protrude, they do not penetrate the stigmatic surface. Again, the male

element may reach the female element, but be incapable of causing an embryo

to be developed, as seems to have been the case with some of Thuret's

experiments on Fuci. No explanation can be given of these facts, any more

than why certain trees cannot be grafted on others. Lastly, an embryo may

be developed, and then perish at an early period. This latter alternative

has not been sufficiently attended to; but I believe, from observations

communicated to me by Mr. Hewitt, who has had great experience in

hybridising gallinaceous birds, that the early death of the embryo is a

very frequent cause of sterility in first crosses. I was at first very

unwilling to believe in this view; as hybrids, when once born, are

generally healthy and long-lived, as we see in the case of the common mule.

Hybrids, however, are differently circumstanced before and after birth:

when born and living in a country where their two parents can live, they

are generally placed under suitable conditions of life. But a hybrid

partakes of only half of the nature and constitution of its mother, and

therefore before birth, as long as it is nourished within its mother's womb

or within the egg or seed produced by the mother, it may be exposed to

conditions in some degree unsuitable, and consequently be liable to perish

at an early period; more especially as all very young beings seem eminently

sensitive to injurious or unnatural conditions of life.

In regard to the sterility of hybrids, in which the sexual elements are

imperfectly developed, the case is very different. I have more than once

alluded to a large body of facts, which I have collected, showing that when

animals and plants are removed from their natural conditions, they are

extremely liable to have their reproductive systems seriously affected.

This, in fact, is the great bar to the domestication of animals. Between

the sterility thus superinduced and that of hybrids, there are many points

of similarity. In both cases the sterility is independent of general

health, and is often accompanied by excess of size or great luxuriance. In

both cases, the sterility occurs in various degrees; in both, the male

element is the most liable to be affected; but sometimes the female more

than the male. In both, the tendency goes to a certain extent with

systematic affinity, or whole groups of animals and plants are rendered

impotent by the same unnatural conditions; and whole groups of species tend

to produce sterile hybrids. On the other hand, one species in a group will

sometimes resist great changes of conditions with unimpaired fertility; and

certain species in a group will produce unusually fertile hybrids. No one

can tell, till he tries, whether any particular animal will breed under

confinement or any plant seed freely under culture; nor can he tell, till

he tries, whether any two species of a genus will produce more or less

sterile hybrids. Lastly, when organic beings are placed during several

generations under conditions not natural to them, they are extremely liable

to vary, which is due, as I believe, to their reproductive systems having

been specially affected, though in a lesser degree than when sterility

ensues. So it is with hybrids, for hybrids in successive generations are

eminently liable to vary, as every experimentalist has observed.

Thus we see that when organic beings are placed under new and unnatural

conditions, and when hybrids are produced by the unnatural crossing of two

species, the reproductive system, independently of the general state of

health, is affected by sterility in a very similar manner. In the one

case, the conditions of life have been disturbed, though often in so slight

a degree as to be inappreciable by us; in the other case, or that of

hybrids,the external conditions have remained the same, but the

organisation has been disturbed by two different structures and

constitutions having been blended into one. For it is scarcely possible

that two organisations should be compounded into one, without some

disturbance occurring in the development, or periodical action, or mutual

relation of the different parts and organs one to another, or to the

conditions of life. When hybrids are able to breed inter se, they transmit

to their offspring from generation to generation the same compounded

organisation, and hence we need not be surprised that their sterility,

though in some degree variable, rarely diminishes.

It must, however, be confessed that we cannot understand, excepting on

vague hypotheses, several facts with respect to the sterility of hybrids;

for instance, the unequal fertility of hybrids produced from reciprocal

crosses; or the increased sterility in those hybrids which occasionally and

exceptionally resemble closely either pure parent. Nor do I pretend that

the foregoing remarks go to the root of the matter: no explanation is

offered why an organism, when placed under unnatural conditions, is

rendered sterile. All that I have attempted to show, is that in two cases,

in some respects allied, sterility is the common result,--in the one case

from the conditions of life having been disturbed, in the other case from

the organisation having been disturbed by two organisations having been

compounded into one.

It may seem fanciful, but I suspect that a similar parallelism extends to

an allied yet very different class of facts. It is an old and almost

universal belief, founded, I think, on a considerable body of evidence,

that slight changes in the conditions of life are beneficial to all living

things. We see this acted on by farmers and gardeners in their frequent

exchanges of seed, tubers, &c., from one soil or climate to another, and

back again. During the convalescence of animals, we plainly see that great

benefit is derived from almost any change in the habits of life. Again,

both with plants and animals, there is abundant evidence, that a cross

between very distinct individuals of the same species, that is between

members of different strains or sub-breeds, gives vigour and fertility to

the offspring. I believe, indeed, from the facts alluded to in our fourth

chapter, that a certain amount of crossing is indispensable even with

hermaphrodites; and that close interbreeding continued during several

generations between the nearest relations, especially if these be kept

under the same conditions of life, always induces weakness and sterility in

the progeny.

Hence it seems that, on the one hand, slight changes in the conditions of

life benefit all organic beings, and on the other hand, that slight

crosses, that is crosses between the males and females of the same species

which have varied and become slightly different, give vigour and fertility

to the offspring. But we have seen that greater changes, or changes of a

particular nature, often render organic beings in some degree sterile; and

that greater crosses, that is crosses between males and females which have

become widely or specifically different, produce hybrids which are

generally sterile in some degree. I cannot persuade myself that this

parallelism is an accident or an illusion. Both series of facts seem to be

connected together by some common but unknown bond, which is essentially

related to the principle of life.

Fertility of Varieties when crossed, and of their Mongrel off-spring. -- It

may be urged, as a most forcible argument, that there must be some

essential distinction between species and varieties, and that there must be

some error in all the foregoing remarks, inasmuch as varieties, however

much they may differ from each other in external appearance, cross with

perfect facility, and yield perfectly fertile offspring. I fully admit

that this is almost invariably the case. But if we look to varieties

produced under nature, we are immediately involved in hopeless

difficulties; for if two hitherto reputed varieties be found in any degree

sterile together, they are at once ranked by most naturalists as species.

For instance, the blue and red pimpernel, the primrose and cowslip, which

are considered by many of our best botanists as varieties, are said by

Gartner not to be quite fertile when crossed, and he consequently ranks

them as undoubted species. If we thus argue in a circle, the fertility of

all varieties produced under nature will assuredly have to be granted.

If we turn to varieties, produced, or supposed to have been produced, under

domestication, we are still involved in doubt. For when it is stated, for

instance, that the German Spitz dog unites more easily than other dogs with

foxes, or that certain South American indigenous domestic dogs do not

readily cross with European dogs, the explanation which will occur to

everyone, and probably the true one, is that these dogs have descended from

several aboriginally distinct species. Nevertheless the perfect fertility

of so many domestic varieties, differing widely from each other in

appearance, for instance of the pigeon or of the cabbage, is a remarkable

fact; more especially when we reflect how many species there are, which,

though resembling each other most closely, are utterly sterile when

intercrossed. Several considerations, however, render the fertility of

domestic varieties less remarkable than at first appears. It can, in the

first place, be clearly shown that mere external dissimilarity between two

species does not determine their greater or lesser degree of sterility when

crossed; and we may apply the same rule to domestic varieties. In the

second place, some eminent naturalists believe that a long course of

domestication tends to eliminate sterility in the successive generations of

hybrids, which were at first only slightly sterile; and if this be so, we

surely ought not to expect to find sterility both appearing and

disappearing under nearly the same conditions of life. Lastly, and this

seems to me by far the most important consideration, new races of animals

and plants are produced under domestication by man's methodical and

unconscious power of selection, for his own use and pleasure: he neither

wishes to select, nor could select, slight differences in the reproductive

system, or other constitutional differences correlated with the

reproductive system. He supplies his several varieties with the same food;

treats them in nearly the same manner, and does not wish to alter their

general habits of life. Nature acts uniformly and slowly during vast

periods of time on the whole organisation, in any way which may be for each

creature's own good; and thus she may, either directly, or more probably

indirectly, through correlation, modify the reproductive system in the

several descendants from any one species. Seeing this difference in the

process of selection, as carried on by man and nature, we need not be

surprised at some difference in the result.

I have as yet spoken as if the varieties of the same species were

invariably fertile when intercrossed. But it seems to me impossible to

resist the evidence of the existence of a certain amount of sterility in

the few following cases, which I will briefly abstract. The evidence is at

least as good as that from which we believe in the sterility of a multitude

of species. The evidence is, also, derived from hostile witnesses, who in

all other cases consider fertility and sterility as safe criterions of

specific distinction. Gartner kept during several years a dwarf kind of

maize with yellow seeds, and a tall variety with red seeds, growing near

each other in his garden; and although these plants have separated sexes,

they never naturally crossed. He then fertilised thirteen flowers of the

one with the pollen of the other; but only a single head produced any seed,

and this one head produced only five grains. Manipulation in this case

could not have been injurious, as the plants have separated sexes. No one,

I believe, has suspected that these varieties of maize are distinct

species; and it is important to notice that the hybrid plants thus raised

were themselves perfectly fertile; so that even Gartner did not venture to

consider the two varieties as specifically distinct.

Girou de Buzareingues crossed three varieties of gourd, which like the

maize has separated sexes, and he asserts that their mutual fertilisation

is by so much the less easy as their differences are greater. How far

these experiments may be trusted, I know not; but the forms experimentised

on, are ranked by Sagaret, who mainly founds his classification by the test

of infertility, as varieties.

The following case is far more remarkable, and seems at first quite

incredible; but it is the result of an astonishing number of experiments

made during many years on nine species of Verbascum, by so good an observer

and so hostile a witness, as Gartner: namely, that yellow and white

varieties of the same species of Verbascum when intercrossed produce less

seed, than do either coloured varieties when fertilised with pollen from

their own coloured flowers. Moreover, he asserts that when yellow and

white varieties of one species are crossed with yellow and white varieties

of a distinct species, more seed is produced by the crosses between the

same coloured flowers, than between those which are differently coloured.

Yet these varieties of Verbascum present no other difference besides the

mere colour of the flower; and one variety can sometimes be raised from the

seed of the other.

From observations which I have made on certain varieties of hollyhock, I am

inclined to suspect that they present analogous facts.

Kolreuter, whose accuracy has been confirmed by every subsequent observer,

has proved the remarkable fact, that one variety of the common tobacco is

more fertile, when crossed with a widely distinct species, than are the

other varieties. He experimentised on five forms, which are commonly

reputed to be varieties, and which he tested by the severest trial, namely,

by reciprocal crosses, and he found their mongrel offspring perfectly

fertile. But one of these five varieties, when used either as father or

mother, and crossed with the Nicotiana glutinosa, always yielded hybrids

not so sterile as those which were produced from the four other varieties

when crossed with N. glutinosa. Hence the reproductive system of this one

variety must have been in some manner and in some degree modified.

From these facts; from the great difficulty of ascertaining the infertility

of varieties in a state of nature, for a supposed variety if infertile in

any degree would generally be ranked as species; from man selecting only

external characters in the production of the most distinct domestic

varieties, and from not wishing or being able to produce recondite and

functional differences in the reproductive system; from these several

considerations and facts, I do not think that the very general fertility of

varieties can be proved to be of universal occurrence, or to form a

fundamental distinction between varieties and species. The general

fertility of varieties does not seem to me sufficient to overthrow the view

which I have taken with respect to the very general, but not invariable,

sterility of first crosses and of hybrids, namely, that it is not a special

endowment, but is incidental on slowly acquired modifications, more

especially in the reproductive systems of the forms which are crossed.

Hybrids and Mongrels compared, independently of their fertility. --

Independently of the question of fertility, the offspring of species when

crossed and of varieties when crossed may be compared in several other

respects. Gartner, whose strong wish was to draw a marked line of

distinction between species and varieties, could find very few and, as it

seems to me, quite unimportant differences between the so-called hybrid

offspring of species, and the so-called mongrel offspring of varieties.

And, on the other hand, they agree most closely in very many important

respects.

I shall here discuss this subject with extreme brevity. The most important

distinction is, that in the first generation mongrels are more variable

than hybrids; but Gartner admits that hybrids from species which have long

been cultivated are often variable in the first generation; and I have

myself seen striking instances of this fact. Gartner further admits that

hybrids between very closely allied species are more variable than those

from very distinct species; and this shows that the difference in the

degree of variability graduates away. When mongrels and the more fertile

hybrids are propagated for several generations an extreme amount of

variability in their offspring is notorious; but some few cases both of

hybrids and mongrels long retaining uniformity of character could be given.

The variability, however, in the successive generations of mongrels is,

perhaps, greater than in hybrids.

This greater variability of mongrels than of hybrids does not seem to me at

all surprising. For the parents of mongrels are varieties, and mostly

domestic varieties (very few experiments having been tried on natural

varieties), and this implies in most cases that there has been recent

variability; and therefore we might expect that such variability would

often continue and be super-added to that arising from the mere act of

crossing. The slight degree of variability in hybrids from the first cross

or in the first generation, in contrast with their extreme variability in

the succeeding generations, is a curious fact and deserves attention. For

it bears on and corroborates the view which I have taken on the cause of

ordinary variability; namely, that it is due to the reproductive system

being eminently sensitive to any change in the conditions of life, being

thus often rendered either impotent or at least incapable of its proper

function of producing offspring identical with the parent-form. Now

hybrids in the first generation are descended from species (excluding those

long cultivated) which have not had their reproductive systems in any way

affected, and they are not variable; but hybrids themselves have their

reproductive systems seriously affected, and their descendants are highly

variable.

But to return to our comparison of mongrels and hybrids: Gartner states

that mongrels are more liable than hybrids to revert to either parent-form;

but this, if it be true, is certainly only a difference in degree. Gartner

further insists that when any two species, although most closely allied to

each other, are crossed with a third species, the hybrids are widely

different from each other; whereas if two very distinct varieties of one

species are crossed with another species, the hybrids do not differ much.

But this conclusion, as far as I can make out, is founded on a single

experiment; and seems directly opposed to the results of several

experiments made by Kolreuter.

These alone are the unimportant differences, which Gartner is able to point

out, between hybrid and mongrel plants. On the other hand, the resemblance

in mongrels and in hybrids to their respective parents, more especially in

hybrids produced from nearly related species, follows according to Gartner

the same laws. When two species are crossed, one has sometimes a prepotent

power of impressing its likeness on the hybrid; and so I believe it to be

with varieties of plants. With animals one variety certainly often has

this prepotent power over another variety. Hybrid plants produced from a

reciprocal cross, generally resemble each other closely; and so it is with

mongrels from a reciprocal cross. Both hybrids and mongrels can be reduced

to either pure parent-form, by repeated crosses in successive generations

with either parent.

These several remarks are apparently applicable to animals; but the subject

is here excessively complicated, partly owing to the existence of secondary

sexual characters; but more especially owing to prepotency in transmitting

likeness running more strongly in one sex than in the other, both when one

species is crossed with another, and when one variety is crossed with

another variety. For instance, I think those authors are right, who

maintain that the ass has a prepotent power over the horse, so that both

the mule and the hinny more resemble the ass than the horse; but that the

prepotency runs more strongly in the male-ass than in the female, so that

the mule, which is the offspring of the male-ass and mare, is more like an

ass, than is the hinny, which is the offspring of the female-ass and

stallion.

Much stress has been laid by some authors on the supposed fact, that

mongrel animals alone are born closely like one of their parents; but it

can be shown that this does sometimes occur with hybrids; yet I grant much

less frequently with hybrids than with mongrels. Looking to the cases

which I have collected of cross-bred animals closely resembling one parent,

the resemblances seem chiefly confined to characters almost monstrous in

their nature, and which have suddenly appeared--such as albinism, melanism,

deficiency of tail or horns, or additional fingers and toes; and do not

relate to characters which have been slowly acquired by selection.

Consequently, sudden reversions to the perfect character of either parent

would be more likely to occur with mongrels, which are descended from

varieties often suddenly produced and semi-monstrous in character, than

with hybrids, which are descended from species slowly and naturally

produced. On the whole I entirely agree with Dr. Prosper Lucas, who, after

arranging an enormous body of facts with respect to animals, comes to the

conclusion, that the laws of resemblance of the child to its parents are

the same, whether the two parents differ much or little from each other,

namely in the union of individuals of the same variety, or of different

varieties, or of distinct species.

Laying aside the question of fertility and sterility, in all other respects

there seems to be a general and close similarity in the offspring of

crossed species, and of crossed varieties. If we look at species as having

been specially created, and at varieties as having been produced by

secondary laws, this similarity would be an astonishing fact. But it

harmonises perfectly with the view that there is no essential distinction

between species and varieties.

Summary of Chapter -- First crosses between forms sufficiently distinct to

be ranked as species, and their hybrids, are very generally, but not

universally, sterile. The sterility is of all degrees, and is often so

slight that the two most careful experimentalists who have ever lived, have

come to diametrically opposite conclusions in ranking forms by this test.

The sterility is innately variable in individuals of the same species, and

is eminently susceptible of favourable and unfavourable conditions. The

degree of sterility does not strictly follow systematic affinity, but is

governed by several curious and complex laws. It is generally different,

and sometimes widely different, in reciprocal crosses between the same two

species. It is not always equal in degree in a first cross and in the

hybrid produced from this cross.

In the same manner as in grafting trees, the capacity of one species or

variety to take on another, is incidental on generally unknown differences

in their vegetative systems, so in crossing, the greater or less facility

of one species to unite with another, is incidental on unknown differences

in their reproductive systems. There is no more reason to think that

species have been specially endowed with various degrees of sterility to

prevent them crossing and blending in nature, than to think that trees have

been specially endowed with various and somewhat analogous degrees of

difficulty in being grafted together in order to prevent them becoming

inarched in our forests.

The sterility of first crosses between pure species, which have their

reproductive systems perfect, seems to depend on several circumstances; in

some cases largely on the early death of the embryo. The sterility of

hybrids, which have their reproductive systems imperfect, and which have

had this system and their whole organisation disturbed by being compounded

of two distinct species, seems closely allied to that sterility which so

frequently affects pure species, when their natural conditions of life have

been disturbed. This view is supported by a parallelism of another

kind;--namely, that the crossing of forms only slightly different is

favourable to the vigour and fertility of their offspring; and that slight

changes in the conditions of life are apparently favourable to the vigour

and fertility of all organic beings. It is not surprising that the degree

of difficulty in uniting two species, and the degree of sterility of their

hybrid-offspring should generally correspond, though due to distinct

causes; for both depend on the amount of difference of some kind between

the species which are crossed. Nor is it surprising that the facility of

effecting a first cross, the fertility of the hybrids produced, and the

capacity of being grafted together--though this latter capacity evidently

depends on widely different circumstances--should all run, to a certain

extent, parallel with the systematic affinity of the forms which are

subjected to experiment; for systematic affinity attempts to express all

kinds of resemblance between all species.

First crosses between forms known to be varieties, or sufficiently alike to

be considered as varieties, and their mongrel offspring, are very

generally, but not quite universally, fertile. Nor is this nearly general

and perfect fertility surprising, when we remember how liable we are to

argue in a circle with respect to varieties in a state of nature; and when

we remember that the greater number of varieties have been produced under

domestication by the selection of mere external differences, and not of

differences in the reproductive system. In all other respects, excluding

fertility, there is a close general resemblance between hybrids and

mongrels. Finally, then, the facts briefly given in this chapter do not

seem to me opposed to, but even rather to support the view, that there is

no fundamental distinction between species and varieties.