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Darwinism (1889) by Alfred Russel Wallace

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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, Koelreuter, 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. Koelreuter 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 versa_. 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. Allie, M. Aube,
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.
Gen. 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 Koelreuter 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.

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