CHAPTER VII

ON THE INFERTILITY OF CROSSES BETWEEN DISTINCT SPECIES AND THE USUAL

STERILITY OF THEIR HYBRID OFFSPRING

Statement of the problem--Extreme susceptibility of the

reproductive functions--Reciprocal crosses--Individual

differences in respect to cross-fertilisation--Dimorphism and

trimorphism among plants--Cases of the fertility of hybrids and

of the infertility of mongrels--The effects of close

interbreeding--Mr. Huth's objections--Fertile hybrids among

animals--Fertility of hybrids among plants--Cases of sterility

of mongrels--Parallelism between crossing and change of

conditions--Remarks on the facts of hybridity--Sterility due to

changed conditions and usually correlated with other

characters--Correlation of colour with constitutional

peculiarities--The isolation of varieties by selective

association--The influence of natural selection upon sterility

and fertility--Physiological selection--Summary and concluding

remarks.

One of the greatest, or perhaps we may say the greatest, of all the

difficulties in the way of accepting the theory of natural selection as

a complete explanation of the origin of species, has been the remarkable

difference between varieties and species in respect of fertility when

crossed. Generally speaking, it may be said that the varieties of any

one species, however different they may be in external appearance, are

perfectly fertile when crossed, and their mongrel offspring are equally

fertile when bred among themselves; while distinct species, on the other

hand, however closely they may resemble each other externally, are

usually infertile when crossed, and their hybrid offspring absolutely

sterile. This used to be considered a fixed law of nature, constituting

the absolute test and criterion of a _species_ as distinct from a

_variety_; and so long as it was believed that species were separate

creations, or at all events had an origin quite distinct from that of

varieties, this law could have no exceptions, because, if any two

species had been found to be fertile when crossed and their hybrid

offspring to be also fertile, this fact would have been held to prove

them to be not _species_ but _varieties_. On the other hand, if two

varieties had been found to be infertile, or their mongrel offspring to

be sterile, then it would have been said: These are not varieties but

true species. Thus the old theory led to inevitable reasoning in a

circle; and what might be only a rather common fact was elevated into a

law which had no exceptions.

The elaborate and careful examination of the whole subject by Mr.

Darwin, who has brought together a vast mass of evidence from the

experience of agriculturists and horticulturists, as well as from

scientific experimenters, has demonstrated that there is no such fixed

law in nature as was formerly supposed. He shows us that crosses between

some varieties are infertile or even sterile, while crosses between some

species are quite fertile; and that there are besides a number of

curious phenomena connected with the subject which render it impossible

to believe that sterility is anything more than an incidental property

of species, due to the extreme delicacy and susceptibility of the

reproductive powers, and dependent on physiological causes we have not

yet been able to trace. Nevertheless, the fact remains that most species

which have hitherto been crossed produce sterile hybrids, as in the

well-known case of the mule; while almost all domestic varieties, when

crossed, produce offspring which are perfectly fertile among themselves.

I will now endeavour to give such a sketch of the subject as may enable

the reader to see something of the complexity of the problem, referring

him to Mr. Darwin's works for fuller details.

_Extreme Susceptibility of the Reproductive Functions._

One of the most interesting facts, as showing how susceptible to changed

conditions or to slight constitutional changes are the reproductive

powers of animals, is the very general difficulty of getting those which

are kept in confinement to breed; and this is frequently the only bar to

domesticating wild species. Thus, elephants, bears, foxes, and numbers

of species of rodents, very rarely breed in confinement; while other

species do so more or less freely. Hawks, vultures, and owls hardly ever

breed in confinement; neither did the falcons kept for hawking ever

breed. Of the numerous small seed-eating birds kept in aviaries, hardly

any breed, neither do parrots. Gallinaceous birds usually breed freely

in confinement, but some do not; and even the guans and curassows, kept

tame by the South American Indians, never breed. This shows that change

of climate has nothing to do with the phenomenon; and, in fact, the same

species that refuse to breed in Europe do so, in almost every case, when

tamed or confined in their native countries. This inability to reproduce

is not due to ill-health, since many of these creatures are perfectly

vigorous and live very long.

With our true domestic animals, on the other hand, fertility is perfect,

and is very little affected by changed conditions. Thus, we see the

common fowl, a native of tropical India, living and multiplying in

almost every part of the world; and the same is the case with our

cattle, sheep, and goats, our dogs and horses, and especially with

domestic pigeons. It therefore seems probable, that this facility for

breeding under changed conditions was an original property of the

species which man has domesticated--a property which, more than any

other, enabled him to domesticate them. Yet, even with these, there is

evidence that great changes of conditions affect the fertility. In the

hot valleys of the Andes sheep are less fertile; while geese taken to

the high plateau of Bogota were at first almost sterile, but after some

generations recovered their fertility. These and many other facts seem

to show that, with the majority of animals, even a slight change of

conditions may produce infertility or sterility; and also that after a

time, when the animal has become thoroughly acclimatised, as it were, to

the new conditions, the infertility is in some cases diminished or

altogether ceases. It is stated by Bechstein that the canary was long

infertile, and it is only of late years that good breeding birds have

become common; but in this case no doubt selection has aided the change.

As showing that these phenomena depend on deep-seated causes and are of

a very general nature, it is interesting to note that they occur also

in the vegetable kingdom. Allowing for all the circumstances which are

known to prevent the production of seed, such as too great luxuriance of

foliage, too little or too much heat, or the absence of insects to

cross-fertilise the flowers, Mr. Darwin shows that many species which

grow and flower with us, apparently in perfect health, yet never produce

seed. Other plants are affected by very slight changes of conditions,

producing seed freely in one soil and not in another, though apparently

growing equally well in both; while, in some cases, a difference of

position even in the same garden produces a similar result.[51]

_Reciprocal Crosses._

Another indication of the extreme delicacy of the adjustment between the

sexes, which is necessary to produce fertility, is afforded by the

behaviour of many species and varieties when reciprocally crossed. This

will be best illustrated by a few of the examples furnished us by Mr.

Darwin. The two distinct species of plants, Mirabilis jalapa and M.

longiflora, can be easily crossed, and will produce healthy and fertile

hybrids when the pollen of the latter is applied to the stigma of the

former plant. But the same experimenter, Kölreuter, tried in vain, more

than two hundred times during eight years, to cross them by applying the

pollen of M. jalapa to the stigma of M. longiflora. In other cases two

plants are so closely allied that some botanists class them as varieties

(as with Matthiola annua and M. glabra), and yet there is the same great

difference in the result when they are reciprocally crossed.

_Individual Differences in respect to Cross-Fertilisation._

A still more remarkable illustration of the delicate balance of

organisation needful for reproduction, is afforded by the individual

differences of animals and plants, as regards both their power of

intercrossing with other individuals or other species, and the fertility

of the offspring thus produced. Among domestic animals, Darwin states

that it is by no means rare to find certain males and females which will

not breed together, though both are known to be perfectly fertile with

other males and females. Cases of this kind have occurred among horses,

cattle, pigs, dogs, and pigeons; and the experiment has been tried so

frequently that there can be no doubt of the fact. Professor G.J.

Romanes states that he has a number of additional cases of this

individual incompatibility, or of absolute sterility, between two

individuals, each of which is perfectly fertile with other individuals.

During the numerous experiments that have been made on the hybridisation

of plants similar peculiarities have been noticed, some individuals

being capable, others incapable, of being crossed with a distinct

species. The same individual peculiarities are found in varieties,

species, and genera. Kölreuter crossed five varieties of the common

tobacco (Nicotiana tabacum) with a distinct species, Nicotiana

glutinosa, and they all yielded very sterile hybrids; but those raised

from one variety were less sterile, in all the experiments, than the

hybrids from the four other varieties. Again, most of the species of the

genus Nicotiana have been crossed, and freely produce hybrids; but one

species, N. acuminata, not particularly distinct from the others, could

neither fertilise, nor be fertilised by, any of the eight other species

experimented on. Among genera we find some--such as Hippeastrum, Crinum,

Calceolaria, Dianthus--almost all the species of which will fertilise

other species and produce hybrid offspring; while other allied genera,

as Zephyranthes and Silene, notwithstanding the most persevering

efforts, have not produced a single hybrid even between the most closely

allied species.

_Dimorphism and Trimorphism._

Peculiarities in the reproductive system affecting individuals of the

same species reach their maximum in what are called heterostyled, or

dimorphic and trimorphic flowers, the phenomena presented by which form

one of the most remarkable of Mr. Darwin's many discoveries. Our common

cowslip and primrose, as well as many other species of the genus

Primula, have two kinds of flowers in about equal proportions. In one

kind the stamens are short, being situated about the middle of the tube

of the corolla, while the style is long, the globular stigma appearing

just in the centre of the open flower. In the other kind the stamens are

long, appearing in the centre or throat of the flower, while the style

is short, the stigma being situated halfway down the tube at the same

level as the stamens in the other form. These two forms have long been

known to florists as the "pin-eyed" and the "thrum-eyed," but they are

called by Darwin the long-styled and short-styled forms (see woodcut).

[Illustration: FIG. 17.--Primula veris (Cowslip).]

The meaning and use of these different forms was quite unknown till

Darwin discovered, first, that cowslips and primroses are absolutely

barren if insects are prevented from visiting them, and then, what is

still more extraordinary, that each form is almost sterile when

fertilised by its own pollen, and comparatively infertile when crossed

with any other plant of its own form, but is perfectly fertile when the

pollen of a long-styled is carried to the stigma of a short-styled

plant, or _vice versâ_. It will be seen, by the figures, that the

arrangement is such that a bee visiting the flowers will carry the

pollen from the long anthers of the short-styled form to the stigma of

the long-styled form, while it would never reach the stigma of another

plant of the short-styled form. But an insect visiting, first, a

long-styled plant, would deposit the pollen on the stigma of another

plant of the same kind if it were next visited; and this is probably the

reason why the wild short-styled plants were found to be almost always

most productive of seed, since they must be all fertilised by the other

form, whereas the long-styled plants might often be fertilised by their

own form. The whole arrangement, however, ensures cross-fertilisation;

and this, as Mr. Darwin has shown by copious experiments, adds both to

the vigour and fertility of almost all plants as well as animals.

Besides the primrose family, many other plants of several distinct

natural orders present similar phenomena, one or two of the most curious

of which must be referred to. The beautiful crimson flax (Linum

grandiflorum) has also two forms, the styles only differing in length;

and in this case Mr. Darwin found by numerous experiments, which have

since been repeated and confirmed by other observers, that each form is

absolutely sterile with pollen from another plant of its own form, but

abundantly fertile when crossed with any plant of the other form. In

this case the pollen of the two forms cannot be distinguished under the

microscope (whereas that of the two forms of Primula differs in size and

shape), yet it has the remarkable property of being absolutely powerless

on the stigmas of half the plants of its own species. The crosses

between the opposite forms, which are fertile, are termed by Mr. Darwin

"legitimate," and those between similar forms, which are sterile,

"illegitimate"; and he remarks that we have here, within the limits of

the same species, a degree of sterility which rarely occurs except

between plants or animals not only of different _species_ but of

different _genera_.

But there is another set of plants, the trimorphic, in which the styles

and stamens have each three forms--long, medium, and short, and in these

it is possible to have eighteen different crosses. By an elaborate

series of experiments it was shown that the six legitimate unions--that

is, when a plant was fertilised by pollen from stamens of length

corresponding to that of its style in the two other forms--were all

abundantly fertile; while the twelve illegitimate unions, when a plant

was fertilised by pollen from stamens of a different length from its

own style, in any of the three forms, were either comparatively or

wholly sterile.[52]

We have here a wonderful amount of constitutional difference of the

reproductive organs within a single species, greater than usually occurs

within the numerous distinct species of a genus or group of genera; and

all this diversity appears to have arisen for a purpose which has been

obtained by many other, and apparently simpler, changes of structure or

of function, in other plants. This seems to show us, in the first place,

that variations in the mutual relations of the reproductive organs of

different individuals must be as frequent as structural variations have

been shown to be; and, also, that sterility in itself can be no test of

specific distinctness. But this point will be better considered when we

have further illustrated and discussed the complex phenomena of

hybridity.

_Cases of the Fertility of Hybrids, and of the Infertility of Mongrels._

I now propose to adduce a few cases in which it has been proved, by

experiment, that hybrids between two distinct species are fertile _inter

se_; and then to consider why it is that such cases are so few in

number.

The common domestic goose (Anser ferns) and the Chinese goose (A.

cygnoides) are very distinct species, so distinct that some naturalists

have placed them in different genera; yet they have bred together, and

Mr. Eyton raised from a pair of these hybrids a brood of eight. This

fact was confirmed by Mr. Darwin himself, who raised several fine birds

from a pair of hybrids which were sent him.[53] In India, according to

Mr. Blyth and Captain Hutton, whole flocks of these hybrid geese are

kept in various parts of the country where neither of the pure parent

species exists, and as they are kept for profit they must certainly be

fully fertile.

Another equally striking case is that of the Indian humped and the

common cattle, species which differ osteologically, and also in habits,

form, voice, and constitution, so that they are by no means closely

allied; yet Mr. Darwin assures us that he has received decisive

evidence that the hybrids between these are perfectly fertile _inter

se_.

Dogs have been frequently crossed with wolves and with jackals, and

their hybrid offspring have been found to be fertile _inter se_ to the

third or fourth generation, and then usually to show some signs of

sterility or of deterioration. The wolf and dog may be originally the

same species, but the jackal is certainly distinct; and the appearance

of infertility or of weakness is probably due to the fact that, in

almost all these experiments, the offspring of a single pair--themselves

usually from the same litter--- were bred in-and-in, and this alone

sometimes produces the most deleterious effects. Thus, Mr. Low in his

great work on the _Domesticated Animals of Great Britain_, says: "If we

shall breed a pair of dogs from the same litter, and unite again the

offspring of this pair, we shall produce at once a feeble race of

creatures; and the process being repeated for one or two generations

more, the family will die out, or be incapable of propagating their

race. A gentleman of Scotland made the experiment on a large scale with

certain foxhounds, and he found that the race actually became monstrous

and perished utterly." The same writer tells us that hogs have been made

the subject of similar experiments: "After a few generations the victims

manifest the change induced in the system. They become of diminished

size; the bristles are changed into hairs; the limbs become feeble and

short; the litters diminish in frequency, and in the number of the young

produced; the mother becomes unable to nourish them, and, if the

experiment be carried as far as the case will allow, the feeble, and

frequently monstrous offspring, will be incapable of being reared up,

and the miserable race will utterly perish."[54]

These precise statements, by one of the greatest authorities on our

domesticated animals, are sufficient to show that the fact of

infertility or degeneracy appearing in the offspring of hybrids after a

few generations need not be imputed to the fact of the first parents

being distinct species, since exactly the same phenomena appear when

individuals of the same species are bred under similar adverse

conditions. But in almost all the experiments that have hitherto been

made in crossing distinct species, no care has been taken to avoid close

interbreeding by securing several hybrids from quite distinct stocks to

start with, and by having two or more sets of experiments carried on at

once, so that crosses between the hybrids produced may be occasionally

made. Till this is done no experiments, such as those hitherto tried,

can be held to prove that hybrids are in all cases infertile _inter se_.

It has, however, been denied by Mr. A.H. Huth, in his interesting work

on _The Marriage of Near Kin_, that any amount of breeding in-and-in is

in itself hurtful; and he quotes the evidence of numerous breeders whose

choicest stocks have always been so bred, as well as cases like the

Porto Santo rabbits, the goats of Juan Fernandez, and other cases in

which animals allowed to run wild have increased prodigiously and

continued in perfect health and vigour, although all derived from a

single pair. But in all these cases there has been rigid selection by

which the weak or the infertile have been eliminated, and with such

selection there is no doubt that the ill effects of close interbreeding

can be prevented for a long time; but this by no means proves that no

ill effects are produced. Mr. Huth himself quotes M. Allié, M. Aubé,

Stephens, Giblett, Sir John Sebright, Youatt, Druce, Lord Weston, and

other eminent breeders, as finding from experience that close

interbreeding _does_ produce bad effects; and it cannot be supposed that

there would be such a consensus of opinion on this point if the evil

were altogether imaginary. Mr. Huth argues, that the evil results which

do occur do not depend on the close interbreeding itself, but on the

tendency it has to perpetuate any constitutional weakness or other

hereditary taints; and he attempts to prove this by the argument that

"if crosses act by virtue of being a cross, and not by virtue of

removing an hereditary taint, then the greater the difference between

the two animals crossed the more beneficial will that act be." He then

shows that, the wider the difference the less is the benefit, and

concludes that a cross, as such, has no beneficial effect. A parallel

argument would be, that change of air, as from inland to the sea-coast,

or from a low to an elevated site, is not beneficial in itself, because,

if so, a change to the tropics or to the polar regions should be more

beneficial. In both these cases it may well be that no benefit would

accrue to a person in perfect health; but then there is no such thing

as "perfect health" in man, and probably no such thing as absolute

freedom from constitutional taint in animals. The experiments of Mr.

Darwin, showing the great and immediate good effects of a cross between

distinct strains in plants, cannot be explained away; neither can the

innumerable arrangements to secure cross-fertilisation by insects, the

real use and purport of which will be discussed in our eleventh chapter.

On the whole, then, the evidence at our command proves that, whatever

may be its ultimate cause, close interbreeding _does_ usually produce

bad results; and it is only by the most rigid selection, whether natural

or artificial, that the danger can be altogether obviated.

_Fertile Hybrids among Animals._

One or two more cases of fertile hybrids may be given before we pass on

to the corresponding experiments in plants. Professor Alfred Newton

received from a friend a pair of hybrid ducks, bred from a common duck

(Anas boschas), and a pintail (Dafila acuta). From these he obtained

four ducklings, but these latter, when grown up, proved infertile, and

did not breed again. In this case we have the results of close

interbreeding, with too great a difference between the original species,

combining to produce infertility, yet the fact of a hybrid from such a

pair producing healthy offspring is itself noteworthy.

Still more extraordinary is the following statement of Mr. Low: "It has

been long known to shepherds, though questioned by naturalists, that the

progeny of the cross between the sheep and goat is fertile. Breeds of

this mixed race are numerous in the north of Europe."[55] Nothing

appears to be known of such hybrids either in Scandinavia or in Italy;

but Professor Giglioli of Florence has kindly given me some useful

references to works in which they are described. The following extract

from his letter is very interesting: "I need not tell you that there

being such hybrids is now generally accepted as a fact. Buffon

(_Supplements_, tom. iii. p. 7, 1756) obtained one such hybrid in 1751

and eight in 1752. Sanson (_La Culture_, vol. vi. p. 372, 1865) mentions

a case observed in the Vosges, France. Geoff. St. Hilaire (_Hist. Nat.

Gén. des reg. org._, vol. iii. p. 163) was the first to mention, I

believe, that in different parts of South America the ram is more

usually crossed with the she-goat than the sheep with the he-goat. The

well-known 'pellones' of Chile are produced by the second and third

generation of such hybrids (Gay, 'Hist, de Chile,' vol. i. p. 466,

_Agriculture_, 1862). Hybrids bred from goat and sheep are called

'chabin' in French, and 'cabruno' in Spanish. In Chile such hybrids are

called 'carneros lanudos'; their breeding _inter se_ appears to be not

always successful, and often the original cross has to be recommenced to

obtain the proportion of three-eighths of he-goat and five-eighths of

sheep, or of three-eighths of ram and five-eighths of she-goat; such

being the reputed best hybrids."

With these numerous facts recorded by competent observers we can hardly

doubt that races of hybrids between these very distinct species have

been produced, and that such hybrids are fairly fertile _inter se_; and

the analogous facts already given lead us to believe that whatever

amount of infertility may at first exist could be eliminated by careful

selection, if the crossed races were bred in large numbers and over a

considerable area of country. This case is especially valuable, as

showing how careful we should be in assuming the infertility of hybrids

when experiments have been made with the progeny of a single pair, and

have been continued only for one or two generations.

Among insects one case only appears to have been recorded. The hybrids

of two moths (Bombyx cynthia and B. arrindia) were proved in Paris,

according to M. Quatrefages, to be fertile _inter se_ for eight

generations.

_Fertility of Hybrids among Plants._

Among plants the cases of fertile hybrids are more numerous, owing, in

part, to the large scale on which they are grown by gardeners and

nurserymen, and to the greater facility with which experiments can be

made. Darwin tells us that Kölreuter found ten cases in which two plants

considered by botanists to be distinct species were quite fertile

together, and he therefore ranked them all as varieties of each other.

In some cases these were grown for six to ten successive generations,

but after a time the fertility decreased, as we saw to be the case in

animals, and presumably from the same cause, too close interbreeding.

Dean Herbert, who carried on experiments with great care and skill for

many years, found numerous cases of hybrids which were perfectly fertile

_inter se_. Crinum capense, fertilised by three other species--C.

pedunculatum, C. canaliculatum, or C. defixum--all very distinct from

it, produced perfectly fertile hybrids; while other species less

different in appearance were quite sterile with the same C. capense.

All the species of the genus Hippeastrum produce hybrid offspring which

are invariably fertile. Lobelia syphylitica and L. fulgens, two very

distinct species, have produced a hybrid which has been named Lobelia

speciosa, and which reproduces itself abundantly. Many of the beautiful

pelargoniums of our greenhouses are hybrids, such as P. ignescens from a

cross between P. citrinodorum and P. fulgidum, which is quite fertile,

and has become the parent of innumerable varieties of beautiful plants.

All the varied species of Calceolaria, however different in appearance,

intermix with the greatest readiness, and the hybrids are all more or

less fertile. But the most remarkable case is that of two species of

Petunia, of which Dean Herbert says: "It is very remarkable that,

although there is a great difference in the form of the flower,

especially of the tube, of P. nyctanigenaeflora and P. phoenicea the

mules between them are not only fertile, but I have found them seed much

more freely with me than either parent.... From a pod of the

above-mentioned mule, to which no pollen but its own had access, I had a

large batch of seedlings in which there was no variability or difference

from itself; and it is evident that the mule planted by itself, in a

congenial climate, would reproduce itself as a species; at least as much

deserving to be so considered as the various Calceolarias of different

districts of South America."[56]

Darwin was informed by Mr. C. Noble that he raises stocks for grafting

from a hybrid between Rhododendron ponticum and R. catawbiense, and that

this hybrid seeds as freely as it is possible to imagine. He adds that

horticulturists raise large beds of the same hybrid, and such alone are

fairly treated; for, by insect agency, the several individuals are

freely crossed with each other, and the injurious influence of close

interbreeding is thus prevented. 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.[57]

_Cases of Sterility of Mongrels._

The reverse phenomenon to the fertility of hybrids, the sterility of

mongrels or of the crosses between _varieties_ of the same species, is a

comparatively rare one, yet some undoubted cases have occurred. Gartner,

who believed in the absolute distinctness of species and varieties, had

two varieties of maize--one dwarf with yellow seeds, the other taller

with red seeds; yet they never naturally crossed, and, when fertilised

artificially, only a single head produced any seeds, and this one only

five grains. Yet these few seeds were fertile; so that in this case the

first cross was almost sterile, though the hybrid when at length

produced was fertile. In like manner, dissimilarly coloured varieties of

Verbascum or mullein have been found by two distinct observers to be

comparatively infertile. The two pimpernels (Anagallis arvensis and A.

coerulea), classed by most botanists as varieties of one species, have

been found, after repeated trials, to be perfectly sterile when crossed.

No cases of this kind are recorded among animals; but this is not to be

wondered at, when we consider how very few experiments have been made

with natural varieties; while there is good reason for believing that

domestic varieties are exceptionally fertile, partly because one of the

conditions of domestication was fertility under changed conditions, and

also because long continued domestication is believed to have the effect

of increasing fertility and eliminating whatever sterility may exist.

This is shown by the fact that, in many cases, domestic animals are

descended from two or more distinct species. This is almost certainly

the case with the dog, and probably with the hog, the ox, and the sheep;

yet the various breeds are now all perfectly fertile, although we have

every reason to suppose that there would be some degree of infertility

if the several aboriginal species were crossed together for the first

time.

_Parallelism between Crossing and Change of Conditions._

In the whole series of these phenomena, from the beneficial effects of

the crossing of different stocks and the evil effects of close

interbreeding, up to the partial or complete sterility induced by

crosses between species belonging to different genera, we have, as Mr.

Darwin points out, a curious parallelism with the effects produced by

change of physical conditions. It is well known that slight changes in

the conditions of life are beneficial to all living things. Plants, if

constantly grown in one soil and locality from their own seeds, are

greatly benefited by the importation of seed from some other locality.

The same thing happens with animals; and the benefit we ourselves

experience from "change of air" is an illustration of the same

phenomenon. But the amount of the change which is beneficial has its

limits, and then a greater amount is injurious. A change to a climate a

few degrees warmer or colder may be good, while a change to the tropics

or to the arctic regions might be injurious.

Thus we see that, both slight changes of conditions and a slight amount

of crossing, are beneficial; while extreme changes, and crosses between

individuals too far removed in structure or constitution, are injurious.

And there is not only a parallelism but an actual connection between the

two classes of facts, for, as we have already shown, many species of

animals and plants are rendered infertile, or altogether sterile, by the

change from their natural conditions which occurs in confinement or in

cultivation; while, on the other hand, the increased vigour or fertility

which is invariably produced by a judicious cross may be also effected

by a judicious change of climate and surroundings. We shall see in a

subsequent chapter, that this interchangeability of the beneficial

effects of crossing and of new conditions, serves to explain some very

puzzling phenomena in the forms and economy of flowers.

_Remarks on the Facts of Hybridity._

The facts that have now been adduced, though not very numerous, are

sufficiently conclusive to prove that the old belief, of the universal

sterility of hybrids and fertility of mongrels, is incorrect. The

doctrine that such a universal law existed was never more than a

plausible generalisation, founded on a few inconclusive facts derived

from domesticated animals and cultivated plants. The facts were, and

still are, inconclusive for several reasons. They are founded,

primarily, on what occurs among animals in domestication; and it has

been shown that domestication both tends to increase fertility, and was

itself rendered possible by the fertility of those particular species

being little affected by changed conditions. The exceptional fertility

of all the varieties of domesticated animals does not prove that a

similar fertility exists among natural varieties. In the next place, the

generalisation is founded on too remote crosses, as in the case of the

horse and the ass, the two most distinct and widely separated species of

the genus Equus, so distinct indeed that they have been held by some

naturalists to form distinct genera. Crosses between the two species of

zebra, or even between the zebra and the quagga, or the quagga and the

ass, might have led to a very different result. Again, in pre-Darwinian

times it was so universally the practice to argue in a circle, and

declare that the fertility of the offspring of a cross proved the

identity of species of the parents, that experiments in hybridity were

usually made between very remote species and even between species of

different genera, to avoid the possibility of the reply: "They are both

really the same species;" and the sterility of the hybrid offspring of

such remote crosses of course served to strengthen the popular belief.

Now that we have arrived at a different standpoint, and look upon a

species, not as a distinct entity due to special creation, but as an

assemblage of individuals which have become somewhat modified in

structure, form, and constitution so as to adapt them to slightly

different conditions of life; which can be differentiated from other

allied assemblages; which reproduce their like, and which usually breed

together--we require a fresh set of experiments calculated to determine

the matter of fact,--whether such species crossed with their near allies

do always produce offspring which are more or less sterile _inter se_.

Ample materials for such experiments exist, in the numerous

"representative species" inhabiting distinct areas on a continent or

different islands of a group; or even in those found in the same area

but frequenting somewhat different stations.

To carry out these experiments with any satisfactory result, it will be

necessary to avoid the evil effects of confinement and of too close

interbreeding. If birds are experimented with, they should be allowed as

much liberty as possible, a plot of ground with trees and bushes being

enclosed with wire netting overhead so as to form a large open aviary.

The species experimented with should be obtained in considerable

numbers, and by two separate persons, each making the opposite

reciprocal cross, as explained at p. 155. In the second generation these

two stocks might be themselves crossed to prevent the evil effects of

too close interbreeding. By such experiments, carefully carried out with

different groups of animals and plants, we should obtain a body of facts

of a character now sadly wanting, and without which it is hopeless to

expect to arrive at a complete solution of this difficult problem. There

are, however, some other aspects of the question that need to be

considered, and some theoretical views which require to be carefully

examined, having done which we shall be in a condition to state the

general conclusions to which the facts and reasonings at our command

seem to point.

_Sterility due to changed Conditions and usually correlated with other

Characters, especially with Colour._

The evidence already adduced as to the extreme susceptibility of the

reproductive system, and the curious irregularity with which infertility

or sterility appears in the crosses between some varieties or species

while quite absent in those between others, seem to indicate that

sterility is a characteristic which has a constant tendency to appear,

either by itself or in correlation with other characters. It is known to

be especially liable to occur under changed conditions of life; and, as

such change is usually the starting-point and cause of the development

of new species, we have already found a reason why it should so often

appear when species become fully differentiated.

In almost all the cases of infertility or sterility between varieties or

species, we have some external differences with which it is correlated;

and though these differences are sometimes slight, and the amount of the

infertility is not always, or even usually, proportionate to the

external difference between the two forms crossed, we must believe that

there is some connection between the two classes of facts. This is

especially the case as regards colour; and Mr. Darwin has collected a

body of facts which go far to prove that colour, instead of being an

altogether trifling and unimportant character, as was supposed by the

older naturalists, is really one of great significance, since it is

undoubtedly often correlated with important constitutional differences.

Now colour is one of the characters that most usually distinguishes

closely allied species; and when we hear that the most closely allied

species of plants are infertile together, while those more remote are

fertile, the meaning usually is that the former differ chiefly in the

_colour_ of their flowers, while the latter differ in the form of the

flowers or foliage, in habit, or in other structural characters.

It is therefore a most curious and suggestive fact, that in all the

recorded cases, in which a decided infertility occurs between varieties

of the same species, those varieties are distinguished by a difference

of colour. The infertile varieties of Verbascum were white and yellow

flowered respectively; the infertile varieties of maize were red and

yellow seeded; while the infertile pimpernels were the red and the blue

flowered varieties. So, the differently coloured varieties of

hollyhocks, though grown close together, each reproduce their own colour

from seed, showing that they are not capable of freely intercrossing.

Yet Mr. Darwin assures us that the agency of bees is necessary to carry

the pollen from one plant to another, because in each flower the pollen

is shed before the stigma is ready to receive it. We have here,

therefore, either almost complete sterility between varieties of

different colours, or a prepotent effect of pollen from a flower of the

same colour, bringing about the same result.

Similar phenomena have not been recorded among animals; but this is not

to be wondered at when we consider that most of our pure and valued

domestic breeds are characterised by definite colours which constitute

one of their distinctive marks, and they are, therefore, seldom crossed

with these of another colour; and even when they are so crossed, no

notice would be taken of any slight diminution of fertility, since this

is liable to occur from many causes. We have also reason to believe that

fertility has been increased by long domestication, in addition to the

fact of the original stocks being exceptionally fertile; and no

experiments have been made on the differently coloured varieties of wild

animals. There are, however, a number of very curious facts showing that

colour in animals, as in plants, is often correlated with constitutional

differences of a remarkable kind, and as these have a close relation to

the subject we are discussing, a brief summary of them will be here

given.

_Correlation of Colour with Constitutional Peculiarities._

The correlation of a white colour and blue eyes in male cats with

deafness, and of the tortoise-shell marking with the female sex of the

same animal, are two well-known but most extraordinary cases. Equally

remarkable is the fact, communicated to Darwin by Mr. Tegetmeier, that

white, yellow, pale blue, or dun pigeons, of all breeds, have the young

birds born naked, while in all other colours they are well covered with

down. Here we have a case in which colour seems of more physiological

importance than all the varied structural differences between the

varieties and breeds of pigeons. In Virginia there is a plant called the

paint-root (Lachnanthes tinctoria), which, when eaten by pigs, colours

their bones pink, and causes the hoofs of all but the black varieties to

drop off; so that black pigs only can be kept in the district.[58]

Buckwheat in flower is also said to be injurious to white pigs but not

to black. In the Tarentino, black sheep are not injured by eating the

Hypericum crispum--a species of St. John's-wort--which kills white

sheep. White terriers suffer most from distemper; white chickens from

the gapes. White-haired horses or cattle are subject to cutaneous

diseases from which the dark coloured are free; while, both in Thuringia

and the West Indies, it has been noticed that white or pale coloured

cattle are much more troubled by flies than are those which are brown or

black. The same law even extends to insects, for it is found that

silkworms which produce white cocoons resist the fungus disease much

better than do those which produce yellow cocoons.[59] Among plants, we

have in North America green and yellow-fruited plums not affected by a

disease that attacked the purple-fruited varieties. Yellow-fleshed

peaches suffer more from disease than white-fleshed kinds. In Mauritius,

white sugar-canes were attacked by a disease from which the red canes

were free. White onions and verbenas are most liable to mildew; and

red-flowered hyacinths were more injured by the cold during a severe

winter in Holland than any other kinds.[60]

These curious and inexplicable correlations of colour with

constitutional peculiarities, both in animals and plants, render it

probable that the correlation of colour with infertility, which has been

detected in several cases in plants, may also extend to animals in a

state of nature; and if so, the fact is of the highest importance as

throwing light on the origin of the infertility of many allied species.

This will be better understood after considering the facts which will be

now described.

_The Isolation of Varieties by Selective Association._

In the last chapter I have shown that the importance of geographical

isolation for the formation of new species by natural selection has been

greatly exaggerated, because the very change of conditions, which is

the initial power in starting such new forms, leads also to a local or

stational segregation of the forms acted upon. But there is also a very

powerful cause of isolation in the mental nature--the likes and

dislikes--of animals; and to this is probably due the fact of the

comparative rarity of hybrids in a state of nature. The differently

coloured herds of cattle in the Falkland Islands, each of which keeps

separate, have been already mentioned; and it may be added, that the

mouse-coloured variety seem to have already developed a physiological

peculiarity in breeding a month earlier than the others. Similar facts

occur, however, among our domestic animals and are well known to

breeders. Professor Low, one of the greatest authorities on our

domesticated animals, says: "The female of the dog, when not under

restraint, makes selection of her mate, the mastiff selecting the

mastiff, the terrier the terrier, and so on." And again: "The Merino

sheep and Heath sheep of Scotland, if two flocks are mixed together,

each will breed with its own variety." Mr. Darwin has collected many

facts illustrating this point. One of the chief pigeon-fanciers in

England informed him that, if free to choose, each breed would prefer

pairing with its own kind. Among the wild horses in Paraguay those of

the same colour and size associate together; while in Circassia there

are three races of horses which have received special names, and which,

when living a free life, almost always refuse to mingle and cross, and

will even attack one another. On one of the Faroe Islands, not more than

half a mile in diameter, the half-wild native black sheep do not readily

mix with imported white sheep. In the Forest of Dean, and in the New

Forest, the dark and pale coloured herds of fallow deer have never been

known to mingle; and even the curious Ancon sheep of quite modern origin

have been observed to keep together, separating themselves from the rest

of the flock when put into enclosures with other sheep. The same rule

applies to birds, for Darwin was informed by the Rev. W.D. Fox that his

flocks of white and Chinese geese kept distinct.[61]

This constant preference of animals for their like, even in the case of

slightly different varieties of the same species, is evidently a fact

of great importance in considering the origin of species by natural

selection, since it shows us that, so soon as a slight differentiation

of form or colour has been effected, isolation will at once arise by the

selective association of the animals themselves; and thus the great

stumbling-block of "the swamping effects of intercrossing," which has

been so prominently brought forward by many naturalists, will be

completely obviated.

If now we combine with this fact the correlation of colour with

important constitutional peculiarities, and, in some cases, with

infertility; and consider, further, the curious parallelism that has

been shown to exist between the effects of changed conditions and the

intercrossing of varieties in producing either an increase or a decrease

of fertility, we shall have obtained, at all events, a starting-point

for the production of that infertility which is so characteristic a

feature of distinct species when intercrossed. All we need, now, is some

means of increasing or accumulating this initial tendency; and to a

discussion of this problem we will therefore address ourselves.

_The Influence of Natural Selection upon Sterility and Fertility._

It will occur to many persons that, as the infertility or sterility of

incipient species would be useful to them when occupying the same or

adjacent areas, by neutralising the effects of intercrossing, this

infertility might have been increased by the action of natural

selection; and this will be thought the more probable if we admit, as we

have seen reason to do, that variations in fertility occur, perhaps as

frequently as other variations. Mr. Darwin tells us that, at one time,

this appeared to him probable, but he found the problem to be one of

extreme complexity; and he was also influenced against the view by many

considerations which seemed to render such an origin of the sterility or

infertility of species when intercrossed very improbable. The fact that

species which occupy distinct areas, and which nowhere come in contact

with each other, are often sterile when crossed, is one of the

difficulties; but this may perhaps be overcome by the consideration

that, though now isolated, they may, and often must, have been in

contact at their origination. More important is the objection that

natural selection could not possibly have produced the difference that

often occurs between reciprocal crosses, one of these being sometimes

fertile, while the other is sterile. The extremely different amounts of

infertility or sterility between different species of the same genus,

the infertility often bearing no proportion to the difference between

the species crossed, is also an important objection. But none of these

objections would have much weight if it could be clearly shown that

natural selection _is_ able to increase the infertility variations of

incipient species, as it is certainly able to increase and develop all

useful variations of form, structure, instincts, or habits. Ample causes

of infertility have been shown to exist, in the nature of the organism

and the laws of correlation; the agency of natural selection is only

needed to accumulate the effects produced by these causes, and to render

their final results more uniform and more in accordance with the facts

that exist.

About twenty years ago I had much correspondence and discussion with Mr.

Darwin on this question. I then believed that I was able to demonstrate

the action of natural selection in accumulating infertility; but I could

not convince him, owing to the extreme complexity of the process under

the conditions which he thought most probable. I have recently returned

to the question; and, with the fuller knowledge of the facts of

variation we now possess, I think it may be shown that natural selection

_is_, in some probable cases at all events, able to accumulate

variations in infertility between incipient species.

The simplest case to consider, will be that in which two forms or

varieties of a species, occupying an extensive area, are in process of

adaptation to somewhat different modes of life within the same area. If

these two forms freely intercross with each other, and produce mongrel

offspring which are quite fertile _inter se_, then the further

differentiation of the forms into two distinct species will be retarded,

or perhaps entirely prevented; for the offspring of the crossed unions

will be, perhaps, more vigorous on account of the cross, although less

perfectly adapted to the conditions of existence than either of the pure

breeds; and this would certainly establish a powerful antagonistic

influence to the further differentiation of the two forms.

Now, let us suppose that a partial sterility of the hybrids between the

two forms arises, in correlation with the different modes of life and

the slight external or internal peculiarities that exist between them,

both of which we have seen to be real causes of infertility. The result

will be that, even if the hybrids between the two forms are still freely

produced, these hybrids will not themselves increase so rapidly as the

two pure forms; and as these latter are, by the terms of the problem,

better suited to their conditions of life than are the hybrids between

them, they will not only increase more rapidly, but will also tend to

supplant the hybrids altogether whenever the struggle for existence

becomes exceptionally severe. Thus, the more complete the sterility of

the hybrids the more rapidly will they die out and leave the two parent

forms pure. Hence it will follow that, if there is greater infertility

between the two forms in one part of the area than the other, these

forms will be kept more pure wherever this greater infertility prevails,

will therefore have an advantage at each recurring period of severe

struggle for existence, and will thus ultimately supplant the less

infertile or completely fertile forms that may exist in other portions

of the area. It thus appears that, in such a case as here supposed,

natural selection would preserve those portions of the two breeds which

were most infertile with each other, or whose hybrid offspring were most

infertile; and would, therefore, if variations in fertility continued to

arise, tend to increase that infertility. It must particularly be noted

that this effect would result, not by the preservation of the infertile

variations on account of their infertility, but by the inferiority of

the hybrid offspring, both as being fewer in numbers, less able to

continue their race, and less adapted to the conditions of existence

than either of the pure forms. It is this inferiority of the hybrid

offspring that is the essential point; and as the number of these

hybrids will be permanently less where the infertility is greatest,

therefore those portions of the two forms in which infertility is

greatest will have the advantage, and will ultimately survive in the

struggle for existence.

The differentiation of the two forms into distinct species, with the

increase of infertility between them, would be greatly assisted by two

other important factors in the problem. It has already been shown that,

with each modification of form and habits, and especially with

modifications of colour, there arises a disinclination of the two forms

to pair together; and this would produce an amount of isolation which

would greatly assist the specialisation of the forms in adaptation to

their different conditions of life. Again, evidence has been adduced

that change of conditions or of mode of life is a potent cause of

disturbance of the reproductive system, and, consequently, of

infertility. We may therefore assume that, as the two forms adopted more

and more different modes of life, and perhaps acquired also decided

peculiarities of form and coloration, the infertility between them would

increase or become more general; and as we have seen that every such

increase of infertility would give that portion of the species in which

it arose an advantage over the remaining portions in which the two

varieties were more fertile together, all this induced infertility would

maintain itself, and still further increase the general infertility

between the two forms of the species.

It follows, then, that specialisation to separate conditions of life,

differentiation of external characters, disinclination to cross-unions,

and the infertility of the hybrid produce of these unions, would all

proceed _pari passu_, and would ultimately lead to the production of two

distinct forms having all the characteristics, physiological as well as

structural, of true species.

In the case now discussed it has been supposed, that some amount of

general infertility might arise in correlation with the different modes

of life of two varieties or incipient species. A considerable body of

facts already adduced renders it probable that this _is_ the mode in

which any widespread infertility would arise; and, if so, it has been

shown that, by the influence of natural selection and the known laws

which affect varieties, the infertility would be gradually increased.

But, if we suppose the infertility to arise sporadically within the two

forms, and to affect only a small proportion of the individuals in any

area, it will be difficult, if not impossible, to show that such

infertility would have any tendency to increase, or would produce any

but a prejudicial effect. If, for example, five per cent of each form

thus varied so as to be infertile with the other form, the result would

be hardly perceptible, because the individuals which formed cross-unions

and produced hybrids would constitute a very small portion of the whole

species; and the hybrid offspring, being at a disadvantage in the

struggle for existence and being themselves infertile, would soon die

out, while the much more numerous fertile portion of the two forms would

increase rapidly, and furnish a sufficient number of pure-bred offspring

of each form to take the place of the somewhat inferior hybrids between

them whenever the struggle for existence became severe. We must suppose

that the normal fertile forms would transmit their fertility to their

progeny, and the few infertile forms their infertility; but the latter

would necessarily lose half their proper increase by the sterility of

their hybrid offspring whenever they crossed with the other form, and

when they bred with their own form the tendency to sterility would die

out except in the very minute proportion of the five per cent

(one-twentieth) that chance would lead to pair together. Under these

circumstances the incipient sterility between the two forms would

rapidly be eliminated, and could never rise much above the numbers which

were produced by sporadic variation each year.

It was, probably, by a consideration of some such case as this that Mr.

Darwin came to the conclusion that infertility arising between incipient

species could not be increased by natural selection; and this is the

more likely, as he was always disposed to minimise both the frequency

and the amount even of structural variations.

We have yet to notice another mode of action of natural selection in

favouring and perpetuating any infertility that may arise between two

incipient species. If several distinct species are undergoing

modification at the same time and in the same area, to adapt them to

some new conditions that have arisen there, then any species in which

the structural or colour differences that have arisen between it and its

varieties or close allies were correlated with infertility of the

crosses between them, would have an advantage over the corresponding

varieties of other species in which there was no such physiological

peculiarity. Thus, incipient species which were infertile together would

have an advantage over other incipient species which were fertile, and,

whenever the struggle for existence became severe, would prevail over

them and take their place. Such infertility, being correlated with

constitutional or structural differences, would probably, as already

suggested, go on increasing as these differences increased; and thus, by

the time the new species became fully differentiated from its parent

form (or brother variety) the infertility might have become as well

marked as we usually find it to be between distinct species.

This discussion has led us to some conclusions of the greatest

importance as bearing on the difficult problem of the cause of the

sterility of the hybrids between distinct species. Accepting, as highly

probable, the fact of variations in fertility occurring in correlation

with variations in habits, colour, or structure, we see, that so long as

such variations occurred only sporadically, and affected but a small

proportion of the individuals in any area, the infertility could not be

increased by natural selection, but would tend to die out almost as fast

as it was produced. If, however, it was so closely correlated with

physical variations or diverse modes of life as to affect, even in a

small degree, a considerable proportion of the individuals of the two

forms in definite areas, it would be preserved by natural selection, and

the portion of the varying species thus affected would increase at the

expense of those portions which were more fertile when crossed. Each

further variation towards infertility between the two forms would be

again preserved, and thus the incipient infertility of the hybrid

offspring might be increased till it became so great as almost to amount

to sterility. Yet further, we have seen that if several competing

species in the same area were being simultaneously modified, those

between whose varieties infertility arose would have an advantage over

those whose varieties remained fertile _inter se_, and would ultimately

supplant them.

The preceding argument, it will be seen, depends entirely upon the

assumption that some amount of infertility characterises the distinct

varieties which are in process of differentiation into species; and it

may be objected that of such infertility there is no proof. This is

admitted; but it is urged that facts have been adduced which render such

infertility probable, at least in some cases, and this is all that is

required. It is by no means necessary that _all_ varieties should

exhibit incipient infertility, but only, some varieties; for we know

that, of the innumerable varieties that occur but few become developed

into distinct species, and it may be that the absence of infertility, to

obviate the effects of intercrossing, is one of the usual causes of

their failure. All I have attempted to show is, that _when_ incipient

infertility does occur in correlation with other varietal differences,

that infertility can be, and in fact must be, increased by natural

selection; and this, it appears to me, is a decided step in advance in

the solution of the problem.[62]

_Physiological Selection._

Another form of infertility has been suggested by Professor G.J. Romanes

as having aided in bringing about the characteristic infertility or

sterility of hybrids. It is founded on the fact, already noticed, that

certain individuals of some species possess what may be termed selective

sterility--that is, while fertile with some individuals of the species

they are sterile with others, and this altogether independently of any

differences of form, colour, or structure. The phenomenon, in the only

form in which it has been observed, is that of "infertility or absolute

sterility between two individuals, each of which is perfectly fertile

with all other individuals;" but Mr. Romanes thinks that "it would not

be nearly so remarkable, or physiologically improbable, that such

incompatibility should run through a whole race or strain."[63]

Admitting that this may be so, though we have at present no evidence

whatever in support of it, it remains to be considered whether such

physiological varieties could maintain themselves, or whether, as in the

cases of sporadic infertility already discussed, they would necessarily

die out unless correlated with useful characters. Mr. Romanes thinks

that they would persist, and urges that "whenever this one kind of

variation occurs _it cannot escape the preserving agency_ of

physiological selection. Hence, even if it be granted that the variation

which affects the reproductive system in this particular way is a

variation of comparatively rare occurrence, still, as _it must always be

preserved_ whenever it does occur, its influence in the manufacture of

specific types _must be cumulative_." The very positive statements which

I have italicised would lead most readers to believe that the alleged

fact had been demonstrated by a careful working out of the process in

some definite supposed cases. This, however, has nowhere been done in

Mr. Romanes' paper; and as it is _the_ vital theoretical point on which

any possible value of the new theory rests, and as it appears so opposed

to the self-destructive effects of simple infertility, which we have

already demonstrated when it occurs between the intermingled portion of

two varieties, it must be carefully examined. In doing so, I will

suppose that the required variation is not of "rare occurrence," but of

considerable amount, and that it appears afresh each year to about the

same extent, thus giving the theory every possible advantage.

Let us then suppose that a given species consists of 100,000 individuals

of each sex, with only the usual amount of fluctuating external

variability. Let a physiological variation arise, so that 10 per cent of

the whole number--10,000 individuals of each sex--while remaining

fertile _inter se_ become quite sterile with the remaining 90,000. This

peculiarity is not correlated with any external differences of form or

colour, or with inherent peculiarities of likes or dislikes leading to

any choice as to the pairing of the two sets of individuals. We have now

to inquire, What would be the result?

Taking, first, the 10,000 pairs of the physiological or abnormal

variety, we find that each male of these might pair with any one of the

whole 100,000 of the opposite sex. If, therefore, there was nothing to

limit their choice to particular individuals of either variety, the

probabilities are that 9000 of them would pair with the opposite

variety, and only 1000 with their own variety--that is, that 9000 would

form sterile unions, and only _one_ thousand would form fertile unions.

Taking, next, the 90,000 normal individuals of either sex, we find, that

each male of these has also a choice of 100,000 to pair with. The

probabilities are, therefore, that nine-tenths of them--that is,

81,000--would pair with their normal fellows, while 9000 would pair with

the opposite abnormal variety forming the above-mentioned sterile

unions.

Now, as the number of individuals forming a species remains constant,

generally speaking, from year to year, we shall have next year also

100,000 pairs, of which the two physiological varieties will be in the

proportion of eighty-one to one, or 98,780 pairs of the normal variety

to 1220[64] of the abnormal, that being the proportion of the fertile

unions of each. In this year we shall find, by the same rule of

probabilities, that only 15 males of the abnormal variety will pair with

their like and be fertile, the remaining 1205 forming sterile unions

with some of the normal variety. The following year the total 100,000

pairs will consist of 99,984 of the normal, and only 16 of the abnormal

variety; and the probabilities, of course, are, that the whole of these

latter will pair with some of the enormous preponderance of normal

individuals, and, their unions being sterile, the physiological variety

will become extinct in the third year.

If now in the second and each succeeding year a similar proportion as at

first (10 per cent) of the physiological variety is produced afresh from

the ranks of the normal variety, the same rate of diminution will go on,

and it will be found that, on the most favourable estimate, the

physiological variety can never exceed 12,000 to the 88,000 of the

normal form of the species, as shown by the following table:--

1st Year. 10,000 of physiological variety to 90,000 of normal variety.

2d " 1,220 + 10,000 again produced.

3d " 16 + 1,220 + 10,000 do. = 11,236

4th " O + 16 + 1,220 + 10,000 do. = 11,236

5th " O + 16 + 1,220 + 10,000 = 11,236

and so on for any number of generations.

In the preceding discussion we have given the theory the advantage of

the large proportion of 10 per cent of this very exceptional variety

arising in its midst year by year, and we have seen that, even under

these favourable conditions, it is unable to increase its numbers much

above its starting-point, and that it remains wholly dependent on the

continued renewal of the variety for its existence beyond a few years.

It appears, then, that this form of inter-specific sterility cannot be

increased by natural or any other known form of selection, but that it

contains within itself its own principle of destruction. If it is

proposed to get over the difficulty by postulating a larger percentage

of the variety annually arising within the species, we shall not affect

the law of decrease until we approach equality in the numbers of the two

varieties. But with any such increase of the physiological variety the

species itself would inevitably suffer by the large proportion of

sterile unions in its midst, and would thus be at a great disadvantage

in competition with other species which were fertile throughout. Thus,

natural selection will always tend to weed out any species with too

great a tendency to sterility among its own members, and will therefore

prevent such sterility from becoming the general characteristic of

varying species, which this theory demands should be the case.

On the whole, then, it appears clear that no form of infertility or

sterility between the individuals of a species, can be increased by

natural selection unless correlated with some useful variation, while

all infertility not so correlated has a constant tendency to effect its

own elimination. But the opposite property, fertility, is of vital

importance to every species, and gives the offspring of the individuals

which possess it, in consequence of their superior numbers, a greater

chance of survival in the battle of life. It is, therefore, directly

under the control of natural selection, which acts both by the

self-preservation of fertile and the self-destruction of infertile

stocks--except always where correlated as above, when they become

useful, and therefore subject to be increased by natural selection.

_Summary and Concluding Remarks on Hybridity._

The facts which are of the greatest importance to a comprehension of

this very difficult subject are those which show the extreme

susceptibility of the reproductive system both in plants and animals. We

have seen how both these classes of organisms may be rendered infertile,

by a change of conditions which does not affect their general health, by

captivity, or by too close interbreeding. We have seen, also, that

infertility is frequently correlated with a difference of colour, or

with other characters; that it is not proportionate to divergence of

structure; that it varies in reciprocal crosses between pairs of the

same species; while in the cases of dimorphic and trimorphic plants the

different crosses between the same pair of individuals may be fertile or

sterile at the same time. It appears as if fertility depended on such a

delicate adjustment of the male and female elements to each other, that,

unless constantly kept up by the preservation of the most fertile

individuals, sterility is always liable to arise. This preservation

always occurs within the limits of each species, both because fertility

is of the highest importance to the continuance of the race, and also

because sterility (and to a less extent infertility) is self-destructive

as well as injurious to the species.

So long therefore as a species remains undivided, and in occupation of a

continuous area, its fertility is kept up by natural selection; but the

moment it becomes separated, either by geographical or selective

isolation, or by diversity of station or of habits, then, while each

portion must be kept fertile _inter se_, there is nothing to prevent

infertility arising between the two separated portions. As the two

portions will necessarily exist under somewhat different conditions of

life, and will usually have acquired some diversity of form and

colour--both which circumstances we know to be either the cause of

infertility or to be correlated with it,--the fact of some degree of

infertility usually appearing between closely allied but locally or

physiologically segregated species is exactly what we should expect.

The reason why varieties do not usually exhibit a similar amount of

infertility is not difficult to explain. The popular conclusions on this

matter have been drawn chiefly from what occurs among domestic animals,

and we have seen that the very first essential to their becoming

domesticated was that they should continue fertile under changed

conditions of life. During the slow process of the formation of new

varieties by conscious or unconscious selection, fertility has always

been an essential character, and has thus been invariably preserved or

increased; while there is some evidence to show that domestication

itself tends to increase fertility.

Among plants, wild species and varieties have been more frequently

experimented on than among animals, and we accordingly find numerous

cases in which distinct species of plants are perfectly fertile when

crossed, their hybrid offspring being also fertile _inter se_. We also

find some few examples of the converse fact--varieties of the same

species which when crossed are infertile or even sterile.

The idea that either infertility or geographical isolation is absolutely

essential to the formation of new species, in order to prevent the

swamping effects of intercrossing, has been shown to be unsound, because

the varieties or incipient species will, in most cases, be sufficiently

isolated by having adopted different habits or by frequenting different

stations; while selective association, which is known to be general

among distinct varieties or breeds of the same species, will produce an

effective isolation even when the two forms occupy the same area.

From the various considerations now adverted to, Mr. Darwin arrived at

the conclusion that the sterility or infertility of species with each

other, whether manifested in the difficulty of obtaining first crosses

between them or in the sterility of the hybrids thus obtained, is not a

constant or necessary result of specific difference, but is incidental

on unknown peculiarities of the reproductive system. These peculiarities

constantly tend to arise under changed conditions owing to the extreme

susceptibility of that system, and they are usually correlated with

variations of form or of colour. Hence, as fixed differences of form and

colour, slowly gained by natural selection in adaptation to changed

conditions, are what essentially characterise distinct species, some

amount of infertility between species is the usual result.

Here the problem was left by Mr. Darwin; but we have shown that its

solution may be carried a step further. If we accept the association of

some degree of infertility, however slight, as a not unfrequent

accompaniment of the external differences which always arise in a state

of nature between varieties and incipient species, it has been shown

that natural selection _has_ power to increase that infertility just as

it has power to increase other favourable variations. Such an increase

of infertility will be beneficial, whenever new species arise in the

same area with the parent form; and we thus see how, out of the

fluctuating and very unequal amounts of infertility correlated with

physical variations, there may have arisen that larger and more constant

amount which appears usually to characterise well-marked species.

The great body of facts of which a condensed account has been given in

the present chapter, although from an experimental point of view very

insufficient, all point to the general conclusion we have now reached,

and afford us a not unsatisfactory solution of the great problem of

hybridism in relation to the origin of species by means of natural

selection. Further experimental research is needed in order to complete

the elucidation of the subject; but until these additional facts are

forthcoming no new theory seems required for the explanation of the

phenomena.

FOOTNOTES:

[Footnote 51: Darwin's _Animals and Plants under Domestication_, vol.

ii. pp. 163-170.]

[Footnote 52: For a full account of these interesting facts and of the

various problems to which they give rise, the reader must consult

Darwin's volume on _The Different Forms of Flowers in Plants of the same

Species_, chaps, i.-iv.]

[Footnote 53: See _Nature_, vol. xxi. p. 207.]

[Footnote 54: Low's _Domesticated Animals of Great Britain_,

Introduction, p. lxiv.]

[Footnote 55: Low's _Domesticated Animals_, p. 28.]

[Footnote 56: _Amaryllidaceae_, by the Hon. and Rev. William Herbert, p.

379.]

[Footnote 57: _Origin of Species_, p. 239.]

[Footnote 58: _Origin of Species_, sixth edition, p. 9.]

[Footnote 59: In the _Medico-Chirurgical Transactions_, vol. liii.

(1870), Dr. Ogle has adduced some curious physiological facts bearing on

the presence or absence of white colours in the higher animals. He

states that a dark pigment in the olfactory region of the nostrils is

essential to perfect smell, and that this pigment is rarely deficient

except when the whole animal is pure white, and the creature is then

almost without smell or taste. He observes that there is no proof that,

in any of the cases given above, the black animals actually eat the

poisonous root or plant; and that the facts are readily understood if

the senses of smell and taste are dependent on a pigment which is absent

in the white animals, who therefore eat what those gifted with normal

senses avoid. This explanation however hardly seems to cover the facts.

We cannot suppose that almost all the sheep in the world (which are

mostly white) are without smell or taste. The cutaneous disease on the

white patches of hair on horses, the special liability of white terriers

to distemper, of white chickens to the gapes, and of silkworms which

produce yellow silk to the fungus, are not explained by it. The

analogous facts in plants also indicate a real constitutional relation

with colour, not an affection of the sense of smell and taste only.]

[Footnote 60: For all these facts, see _Animals and Plants under

Domestication_, vol. ii. pp. 335-338.]

[Footnote 61: _Animals and Plants under Domestication_, vol. ii. pp.

102, 103.]

[Footnote 62: As this argument is a rather difficult one to follow,

while its theoretical importance is very great, I add here the following

briefer exposition of it, in a series of propositions; being, with a few

verbal alterations, a copy of what I wrote on the subject about twenty

years back. Some readers may find this easier to follow than the fuller

discussion in the text:--

_Can Sterility of Hybrids have been Produced by Natural

Selection?_

1. Let there be a species which has varied into _two forms_ each

adapted to certain existing conditions better than the parent

form, which they soon supplant.

2. If these _two forms_, which are supposed to coexist in the

same district, do not intercross, natural selection will

accumulate all favourable variations till they become well

suited to their conditions of life, and form two slightly

differing species.

3. But if these _two forms_ freely intercross with each other,

and produce hybrids, which are also quite fertile _inter se_,

then the formation of the two distinct races or species will be

retarded, or perhaps entirely prevented; for the offspring of

the crossed unions will be _more vigorous_ owing to the cross,

although _less adapted_ to their conditions of life than either

of the pure breeds.

4. Now, let a partial sterility of the hybrids of some

considerable proportion of these two forms arise; and, as this

would probably be due to some special conditions of life, we may

fairly suppose it to arise in some definite portion of the area

occupied by the two forms.

5. The result will be that, in that area, the hybrids (although

continually produced by first crosses almost as freely as

before) will not themselves increase so rapidly as the two pure

forms; and as the two pure forms are, by the terms of the

problem, better suited to their several conditions of life than

the hybrids, they will inevitably increase more rapidly, and

will continually tend to supplant the hybrids altogether at

every recurrent severe struggle for existence.

6. We may fairly suppose, also, that as soon as any sterility

appears some disinclination to _cross unions_ will appear, and

this will further tend to the diminution of the production of

hybrids.

7. In the other part of the area, however, where hybridism

occurs with perfect freedom, hybrids of various degrees may

increase till they equal or even exceed in number the pure

species--that is, the incipient species will be liable to be

swamped by intercrossing.

8. The first result, then, of a partial sterility of crosses

appearing in one part of the area occupied by the two forms,

will be--that the great majority of the individuals will there

consist of the two pure forms only, while in the remaining part

these will be in a minority,--which is the same as saying that

the new _physiological variety_ of the two forms will be better

suited to the conditions of existence than the remaining portion

which has not varied physiologically.

9. But when the struggle for existence becomes severe, that

variety which is best adapted to the conditions of existence

always supplants that which is imperfectly adapted; therefore,

_by natural selection_ the _varieties_ which are _sterile_ when

crossed will become established as the only ones.

10. Now let variations in the _amount of sterility_ and in

the _disinclination to crossed unions_ continue to occur--also

in certain parts of the area: exactly the same result must

recur, and the progeny of this new physiological variety will in

time occupy the whole area.

11. There is yet another consideration that would facilitate the

process. It seems probable that the _sterility variations_

would, to some extent, concur with, and perhaps depend upon, the

_specific variations_; so that, just in proportion as the _two

forms_ diverged and became better adapted to the conditions of

existence, they would become more sterile when intercrossed. If

this were the case, then natural selection would act with double

strength; and those which were better adapted to survive both

structurally and physiologically would certainly do so.]

[Footnote 63: Cases of this kind are referred to at p. 155. It must,

however, be noted, that such sterility in first crosses appears to be

equally rare between different species of the same genus as between

individuals of the same species. Mules and other hybrids are freely

produced between very distinct species, but are themselves infertile or

quite sterile; and it is this infertility or sterility of the hybrids

that is the characteristic--and was once thought to be the criterion--of

species, not the sterility of their first crosses. Hence we should not

expect to find any constant infertility in the first crosses between the

distinct strains or varieties that formed the starting-point of new

species, but only a slight amount of infertility in their mongrel

offspring. It follows, that Mr. Romanes' theory of _Physiological

Selection_--which assumes sterility or infertility between first crosses

as the fundamental fact in the origin of species--does not accord with

the general phenomena of hybridism in nature.]

[Footnote 64: The exact number is 1219.51, but the fractions are omitted

for clearness.]