The Project Gutenberg ebook of Darwinism (1889), by Alfred Russel Wallace

as of no importance in structure, are continually found to be

functionally important; and I have been especially struck with this fact

in the case of plants, to which my observations have, of late years,

been confined. Therefore it seems to me rather rash to consider slight

differences between representative species, for instance, those

inhabiting the different islands of the same archipelago, as of no

functional importance, and as not in any way due to natural selection"

_(Life of Darwin_, vol. iii. p. 161).]
[Footnote 48: See _Variation of Animals and Plants_, vol. i. p. 86.]
[Footnote 49: _Journal of the Linnean Society, Zoology,_ vol. xx. p.

215.]
[Footnote 50: In Mr. Gulick's last paper (_Journal of Linn. Soc. Zool._,

vol. xx. pp. 189-274) he discusses the various forms of isolation above

referred to, under no less than thirty-eight different divisions and

subdivisions, with an elaborate terminology, and he argues that these

will frequently bring about divergent evolution without any change in

the environment or any action of natural selection. The discussion of

the problem here given will, I believe, sufficiently expose the fallacy

of his contention; but his illustration of the varied and often

recondite modes by which practical isolation may be brought about, may

help to remove one of the popular difficulties in the way of the action

of natural selection in the origination of species.]

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.

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.

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._

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._

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).

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.

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_.

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]

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]

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