HYBRIDISM
The alleged sterility of hybrids a stumbling-block to
evolutionists—Huxley’s views—Wallace on the sterility of hybrids—Darwin
on the same—Wallace’s theory that the infertility of hybrids has been
caused by Natural Selection so as to prevent the evils of
intercrossing—Crosses between distinct species not necessarily
infertile—Fertile crosses between species of plants—Sterile plant
hybrids—Fertile mammalian hybrids—Fertile bird hybrids—Fertile hybrids
among amphibia—Limits of hybridisation—Multiple hybrids—Characters of
hybrids—Hybridism does not appear to have exercised much effect on the
origin of new species.
The alleged sterility of the hybrids produced by crossing different
species has long proved a great stumbling-block to evolutionists. Huxley,
in particular, felt the force of this objection to the Darwinian theory.
If the hybrids between natural species are sterile, while those of all
the varieties which the breeder has produced are perfectly fertile, it is
obviously quite useless for evolutionists to point with pride to the
results obtained by the breeder, and to declare that his products differ
from one another to a greater extent than do many well-recognised
species.
“After much consideration, and with no bias against Mr Darwin’s views,”
wrote Huxley to the _Westminster Review_ in 1860, “it is our clear
conviction that, as the evidence now stands, it is not absolutely proven
that a group of animals having all the characters exhibited by species in
nature, has ever been originated by selection, whether natural or
artificial. Groups having the morphological nature of species, distinct
and permanent races, in fact, have been so produced over and over again;
but there is no positive evidence at present that any group of animals
has, by variation and selective breeding, given rise to another group
which was in the least degree infertile with the first. Mr Darwin is
perfectly aware of this weak point, and brings forward a multitude of
ingenious and important arguments to diminish the force of the objection.
We admit the value of these arguments to the fullest extent; nay, we will
go so far as to express our belief that experiments, conducted by a
skilful physiologist, would very probably obtain the desired production
of mutually more or less infertile breeds from a common stock in a
comparatively few years; but still, as the case stands at present, this
little ‘rift within the lute’ is not to be disguised or overlooked.”
Alleged Sterility of Hybrids
Similarly Wallace writes, at the beginning of chapter vii. of his
_Darwinism_: “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 one another
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 inevitably to reasoning in a circle, and what
might be only a rather common fact was elevated into a law which had no
exceptions.”
Thus the sterility of hybrids was a zoological bogey which had to be
demolished. The plan of campaign adopted by Darwin and Wallace was,
firstly, to try to disprove the assertion that the hybrids between
different species are always sterile, and secondly, to find a reason for
the alleged sterility of these hybrids.
Fertile Hybrids
Darwin succeeded in obtaining some examples of crosses between botanical
species which were said to be fertile. These he quotes in chapter viii.
of _The Origin of Species_. As regards animals, he met with less success.
“Although,” he writes, “I do not know of any thoroughly
well-authenticated cases of perfectly fertile hybrid animals, I have some
reason to believe that the hybrids from _Cervulus vaginalis_ and
_reevesii_, and from _Phasianus colchicus_ and _P. torquatus_ and with
_P. versicolor_ are perfectly fertile. There is no doubt that these three
pheasants, namely, the common, the true ring-necked, and the Japan,
intercross, and are becoming blended together in the woods of several
parts of England. The hybrids from the common and Chinese geese (_A.
cygnoides_), species which are so different that they are generally
ranked in distinct genera, have often been bred in this country with
either pure parent, and in one single instance they have bred _inter se_.
This was effected by Mr Eyton, who raised two hybrids from the same
parents but from different hatches; and from these two birds he raised no
less than eight hybrids (grandchildren of the pure geese) from one nest.
In India, however, these cross-bred geese must be far more fertile; for I
am assured by two eminently capable judges, namely, Mr Blyth and Captain
Hutton, that whole flocks of these crossed geese are kept in various
parts of the country; and as they are kept for profit, where neither pure
parent species exists, they must certainly be highly fertile.[5] . . . So
again there is reason to believe that our European and the humped Indian
cattle are quite fertile together; and from facts communicated to me by
Mr Blyth, I think they must be considered as distinct species.”
Darwin does not seem to have been very satisfied with the evidence he had
collected, for he said: “Finally, looking to all the ascertained facts on
the intercrossing of plants and animals, it may be concluded that some
degree of sterility, both in first crosses and in hybrids, is an
extremely general result; but that it cannot, under our present state of
knowledge, be considered as absolutely universal.”
Similarly Wallace writes: “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.”
Darwin resorted to much ingenious argument in his attempt to explain what
he believed to be the almost universal sterility of hybrids, as opposed
to mongrels or crosses between varieties. He pointed out that changed
conditions tend to produce sterility, as is evidenced by the fact that
many creatures refuse to breed in confinement, and believed that the
crossing of distinct wild species produced a similar effect on the sexual
organs. He expressed his belief that the early death of the embryos is a
very frequent cause of sterility in first crosses.
Wallace thus summarises Darwin’s conclusions as to the cause of the
sterility of hybrids: “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 species 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.”
A Biological Bogey
But Wallace has not been content to let the matter remain where Darwin
left it. He has boldly tried to make an ally of this bogey of the
infertility of hybrids. On page 179 of _Darwinism_ he argues, most
ingeniously, that the sterility of hybrids has been actually produced by
natural selection to prevent the evils of the intercrossing of allied
species. We will not reproduce his argument for the simple reason that it
is now well-known, or should be well-known, that hybrids between allied
species are by no means always sterile. The doctrine of the infertility
of hybrids seems to have been founded on the fact that the hybrids best
known to breeders, namely the cross between the ass and the horse, and
those between the canary and other finches, are sterile.
Fertile Crosses between Species of Plants
In the case of plants the number of fertile hybrids between species is so
large that we cannot attempt to enumerate them. De Vries cites several
instances in Lecture IX of his _Species and Varieties: Their Origin by
Mutation_.
One of these—the hybrid between the purple and the yellow species of
Lucerne which is known to botanists as _Medicago media_ is, writes De
Vries, “cultivated in some parts of Germany on a large scale, as it is
more productive than the ordinary lucerne.” Other examples of perfectly
fertile plant hybrids cited by De Vries are the crosses between _Anemone
magellanica_ and _A. sylvestris_, between _Salix alba_ and _Salix
pentandra_, between _Rhododendron hirsutum_ and _R. ferrugineum_.
He gives an instance of a hybrid—_Ægilops speltæformis_, which, though
fertile, is not so fertile as a normal species would be. It is worthy of
note that Burbank of California has obtained a hybrid between the
blackberry and the raspberry, which is not only fertile, but quite
popular as producing a novel fruit.
Sterile Plant Hybrids
De Vries does not cite nearly so many examples of sterile hybrids,
presumably because they are not so easy to find. He mentions the sterile
“Gordon’s currant,” which is considered to be a hybrid between the
Californian and the Missouri species. He also gives _Cytisus adami_ as an
absolutely sterile hybrid, this being a cross between two species of
Labernum—the common and the purple.
In the case of animals the known hybrids are so much less numerous that
we are able to furnish a list which may be taken as fairly exhaustive.
Fertile Mammalian Hybrids
Taking the mammals first, we find that, in addition to those cited by
Darwin, there are several recorded cases of crosses between well-defined
species which are fertile.
There is the hybrid between the brown bear and the polar bear, which is
perfectly fertile. In the London Zoological Gardens there is a specimen
of this hybrid, also one of this individual’s offspring by a pure polar
bear.
The stoat has been crossed with the domestic ferret, a descendant of the
polecat, a very distinct species; the resulting hybrids have nevertheless
proved fertile.
The bull American bison produces with the domestic cow hybrids known as
“cataloes,” which are fertile. The reverse cross of the domestic bull
with the bison cow does not, however, succeed at all, which reminds us of
what happens in the case of finch-hybrids.
Bird fanciers when crossing the canary with wild species of finch, almost
invariably use a hen canary as the female parent, because domesticated
female animals breed more readily than do captive wild ones.
The domestic yak breeds frequently in the Himalayas with the perfectly
distinct zebu or humped cow of India, and the hybrids are fertile. Yet
the zebu and the Indian buffalo, living constantly side by side in the
plains of India, never interbreed at all.
Among wild ruminants of this hollow-horned family, the Himalayan Argali
(_Ovis ammon_) ram, a giant sheep of the size of a donkey, has been known
to appropriate a herd of ewes of the Urial (_O. vignei_), a very distinct
species of the size of a domestic sheep. Many hybrids were born, and
these, in turn, bred with the pure urials of the herd.
In our parks the little Sika deer of Japan (_Cervus sika_), a species
about the size of the fallow-deer, with an even more marked seasonal
change of colouration and antlers having only three tines, breeds with
the red deer, and the hybrids are fertile.
In certain parts of Asia Minor the natives cross the female one-humped
camel with the male of the bactrian or two-humped species. The hybrids
(which are one-humped) will breed with the pure species; but, although
the hybrids are strong and useful, the three-quarter bred beasts are
apparently of little value.
Fertile Bird Hybrids
Coming to birds, we are confronted by a longer list of fertile hybrids.
This is the natural outcome of the fact that a greater number of bird
species have been kept in captivity.
The oldest known fertile hybrid is that between the common and Chinese
geese above cited, but many others have since been recorded. Even among
birds so seldom bred, comparatively, as the parrot family, a fertile
hybrid has been produced, that between the Australian Rosella Parrakeet
(_Platycercus eximius_) and Pennant’s Parrakeet (_P. elegans_). The
hybrid was first described as a distinct species, the Red-mantled
Parrakeet (_P. erythropeplus_). These two parrakeets, though nearly
allied, are very distinct; Pennant’s being coloured red, blue, and black,
with a distinct young plumage of uniform dull green; the rosella in
addition to the above colours displays much yellow and some white and
green. It is, moreover, considerably smaller and has no distinct youthful
dress.
The Amherst Pheasant (_Chrysolophus amherstiæ_) and the Gold Pheasant
(_C. pictus_) have long been known as producing hybrids which are fertile
either _inter se_ or with the parents. Here the species are still more
distinct; not only are the leading colours of the Amherst white and
green, instead of red and gold, but it is a bigger bird with a larger
tail and smaller crest, and a bare patch round the eyes.
The Pintail Duck (_Dafila acuta_) and the Mallard or Wild Duck and its
domestic descendants (_Anas boscas_), when bred together, produce hybrids
which have been proved fertile between themselves and with the pure
pintail. Any sportsman or frequenter of our parks can see for himself the
distinctness of the species concerned.
The Pied Wagtail (_Motacilla lugubris_) and the Grey Wagtail (_M.
melanope_) have produced hybrids in aviaries, which have proved fertile.
The two species are distinct in every way, as all British ornithologists
know.
The Cut-throat Finch (_Amadina fasciata_) and Red-headed Finch (_A.
erythrocephala_) of Africa have hybridised in aviaries, and the produce
has proved fertile. The red-headed finch, among other differences, is far
larger than the cut-throat, and the males have the head all red, not
merely a throat-band of that colour.
The Japanese Greenfinch (_Ligurinus sinicus_) which is not green, but
brown and grey, with bolder yellow wing- and tail-markings than our
larger European greenfinch, has produced fertile hybrids with this latter
bird.
[Illustration: MALE AMHERST PHEASANT]
The Red Dove of India (_Oenopopilia tranquebarica_) has produced hybrids
with the tame Collared Dove (_T. risorius_) and these have bred again
when paired with the red species. _O. tranquebarica_, although presenting
a general similarity to the collared dove, is truly distinct, being much
smaller, with a shorter tail, and displaying a marked sex-difference (the
male only being red, and the female drab). Its voice is also utterly
unlike the well-known penetrating and musical _coo_ of the Collared Dove.
There is a large class of fertile wild hybrids produced between forms
differing only in colour, such as those between the Hooded Crow (_Corvus
cornix_) and Carrion Crow (_Corvus corone_), the various species of
_Molpastes_ bulbuls, and the Indian Roller (_Coracias indica_) and
Burmese Roller (_C. affinis_). Indeed, it may be said that wherever two
such colour-species meet they hybridize and become more or less fused.
In this connection sportsmen, as mentioned by Darwin, performed
unconsciously a most interesting experiment when, more than a century
ago, they introduced largely into their coverts the Chinese Ring-necked
Pheasant (_Phasianus torquatus_) and the Japanese _P. versicolor_. So
freely has the former bred with the common species already present there
(_Phasianus colchicus_) that nowadays nearly all our English pheasants
show traces of the cross in the shape of white feathers on the neck, or
the green tinge of the plumage of the lower back. The influence of the
Japanese Green Pheasant (_P. versicolor_) has been very slight.
It is, of course, open to anyone to assert that such crosses are not true
hybrids, as the species are not fully distinct, but mere
colour-mutations.
The fact of the intermingling, however, is a fatal blow to the theory of
recognition marks, since it demonstrates that merely distinctive
colouring is not a preventative of cross-breeding. To this matter we
shall return later.
Fertile Hybrids among Amphibia
Our Crested Newt (_Molge cristata_) and the Continental Marbled Newt (_M.
marmorata_) interbreed in France, in the wild state, and the resulting
hybrid was at first described as a distinct species, under the name of
_Molge blasii_. These two newts differ greatly in appearance. In the
Marbled Newt the colouration is brilliant green and black above, and
shows no orange below, thus differing much from that of the Crested Newt,
which is black above and mottled with orange beneath, while the crest of
the breeding-male of this species lacks the notches which are so
conspicuous in that of the Crested Newt.
[Illustration: HARLEQUIN QUAIL (Coturnix delegorguei)]
[Illustration: RAIN QUAIL (Coturnix coromandelica)]
Insects
Among insects, M. de Quatrefages states that the hybrid progeny of the
silk-moths _Bombyx cynthia_ and _B. arrindia_ are fertile for eight
generations when bred _inter se_.
Limits to the Possibilities of Hybridisation
Hybrids can apparently only be produced between species of the same
natural family. The stories of cat-rabbits, deer-ponies, fowl-ducks, and
similar distant crosses invariably break down on close examination. A
belief in such remote crosses characterized the ancient “bestiaries,” and
still lingers, as witness the falsely-reputed crosses alluded to above.
This belief has no doubt arisen from the fact that the domestic breeds of
dogs, fowls, etc., are popularly confounded with truly distinct species.
Mongrels are well known to be readily produced, and hence the notion
arises that hybrids between the most widely-separated species are
possible.
In practice, the most remote cross of which authenticated specimens exist
is that between the red grouse and the domestic fowl (bantam cock). It is
true that the grouse are commonly ranked by ornithologists as a family
distinct (_Tetraonidae_) from that of the pheasants and partridges
(_Phasianidae_), to which the fowl belongs; but the relationship is
admittedly very close, and we doubt if general zoologists would
countenance the maintenance of the families as distinct. Ornithologists
are notoriously apt to over-rate small differences when drawing up a
classification. It would be therefore safe to say, in the present state
of our knowledge, that species belonging to different natural families
cannot hybridize.
In some cases multiple hybrids have been produced. Thus, at the London
Zoological Gardens, many years ago, a hybrid between the Gayal of India
(_Bos frontalis_) and the Indian humped cow mentioned above was put to an
American bison, and produced a double hybrid calf.
M. G. Rogeron of Angers bred many hybrids from a male pochard and a duck
bred from a Mallard and a Gadwall.
More recently, Mr J. L. Bonhote has succeeded in combining the blood of
five wild species of ducks in one individual.
Mr J. T. Newman has also bred turtle-doves containing the blood of three
distinct species.
A cross, which usually results in sterile offspring, may in very rare
cases produce a fertile individual; thus, Mr A. Suchetet once succeeded
in obtaining a three-quarter-bred bird from the not uncommon hybrid of
the tame pigeon and tame collared dove (_Turtur risorius_), which is
usually barren, by pairing it with a dove; but the bird thus produced,
when again paired with a dove, was itself sterile. Some of the cases here
given seem to encourage Darwin’s view that domestication tends to
eliminate sterility; but it is doubtful if this can be upheld. The hybrid
between the Muscovy duck (_Cairina moschata_) and common duck is usually,
at all events, sterile, like that between the pigeon and dove; yet all
these birds have been long domesticated. The hybrid between the fowl and
the guinea-fowl is likewise barren, nor has the long domestication of the
horse and ass lessened the sterility of the mule.
Characters of Hybrids
Some facts may be noted respecting the characters of hybrids. In the
first place, it is important to notice that the characters of the hybrid
vary according to the sexes of the species concerned; thus, the “hinny,”
which is bred from a horse and a she-ass, is a different animal from the
true “mule,” which is bred from the jackass and mare, and is inferior to
it.
Similarly, Mr G. E. Weston, a great authority on British cage-birds and
their hybrids, informs us that when hybrids are bred from a male canary
and a hen goldfinch or siskin—contrary to the almost universal practice
of using the hen canary for crossing—the progeny are inferior in size and
colour to the hybrids obtained in the ordinary way.
Hybrids, in animals at all events, differ from crosses between mutations
or colour-variations in not exhibiting the phenomenon of alternative
inheritance; they do not follow one parent or the other exclusively, but
always exhibit some blending of the characters of both, which is, after
all, what might have been expected, since well-defined species usually
differ in more than one character.
Thus, the cross between the Amherst and gold pheasants chiefly resembles
the latter, but has the ruff white as in the Amherst, while the crest,
though in form it resembles that of the gold species, is not yellow as in
that species, nor red as in the Amherst, but of an intermediate tint,
brilliant orange.
The mule between the horse and ass, as all know, combines the shapes of
the two parents, though in colour it follows the horse rather than the
ass.
When two remote species, one or each of which possesses some distinctive
structural peculiarity, are crossed, the hybrid does not inherit such
points. The guinea-fowl has a helmet, and a pair of wattles on the upper
jaw; the common fowl a comb, and a pair of wattles on the lower jaw; but
in the hybrid no comb, helmet, or wattles are present.
The Muscovy drake has a bare red eye-patch, and the male of the common
duck curled middle-tail feathers; in the hybrid neither of these
peculiarities is reproduced.
In a cross between nearly-related forms, the peculiarity of one species
may be reproduced in a modified form in the hybrid; for instance, in that
between the blackcock (_Tetrao tetrix_) and the capercailzie (_T.
urogallus_), the forked tail of the former reappears to a small extent in
the hybrid.
Very interesting are those cases in which the hybrid resembles neither
parent, but tends to be like an altogether distinct species, or to have a
character of its own. Thus the hybrids between the pied European and
chestnut African sheldrakes (_Tadorna cornuta_ and _Casarca cana_), now
in the British Museum, bear a distinct resemblance to the grey Australian
sheldrake (_C. tadornoides_). In pheasants, also, the crosses between the
common and gold, common and Amherst, gold and Japanese, and gold and
Reeves’ pheasants, widely different as all these birds are in
colouration, are remarkably alike, being all chestnut-coloured birds with
buff median tail-feathers. These may be seen in the British Museum. This
phenomenon, together with the above-noted disappearance of specialised
features in hybrids, is possibly comparable to the “reversion” observed
when widely-distinct domestic breeds are crossed, and so may give us an
idea of the appearance of the ancestors of the groups of species
concerned.
In the few cases wherein several generations of hybrids have been bred
_inter se_, there seems to have been no reversion to the original pure
types, such as happens when colour-forms are crossed.
M. Suchetet bred hybrid gold = Amherst pheasants for four generations,
and they retained the hybrid character. The young bred by Darwin from a
pair of common = Chinese geese hybrids “resembled,” he says, “in every
detail their hybrid parents.”
Wild Hybrids
When hybrids have been—as has far more usually been the case—bred back to
one of the pure stocks, the hybrid characters have shown, as might be
expected, a tendency quickly to disappear. The three-quarter-bred polar
bear now in the London Zoological Gardens is a pure polar save for a
brown tinge on the back. A three-quarter Amherst = gold pheasant in the
British Museum is a pure Amherst save for the larger crest, and a patch
of red on the abdomen. When three-quarter-bred pintail = common duck
hybrids were bred back to the pintail, the offspring “lost all
resemblance to the common duck.” In the case of the Argali-urial herd of
wild sheep above-mentioned, after the usurping Argali ram had been killed
by wolves, the hybrids bred with the urials, with the result that the
herd renewed the appearance of pure urial.
Thus, except in the very improbable case of a family of hybrids going off
and starting a colony by themselves, the effect of hybridism on the
evolution of species seems likely to have been _nil_. It is, however,
curious that three-quarter-bred animals have rarely, if ever, been
recorded in a state of nature, though a good many wild-bred hybrids are
on record.
This points to some unfitness for the struggle for existence even in a
fertile hybrid. It is necessary to emphasise the fact that wild hybrids
are always exceedingly rare as individuals, in spite of what has been
said as to the number of recorded crosses.
More hybrid unions have been noted among the duck family than anywhere
else in the animal kingdom. Nevertheless Finn never once saw a hybrid
duck for sale in the Calcutta market, although for seven years he was
constantly on the look-out for such forms; nor does Hume record any such
specimen in his _Game Birds and Wild Fowl of India_.
The hybrid which occurs most commonly as an individual is that between
the blackcock and capercailzie, which is recorded yearly on the
Continent; but it appears to be sterile, and so has no influence on the
species.
Wild hybrids between mammals are far rarer even than bird hybrids, the
only ones which seem to be on record being those between the Argali and
Urial above alluded to; those between the brown and blue hares and the
common and Arctic foxes.
A consideration of the phenomena of hybridism thus leads us to the
conclusion that, although many hybrids are fertile, the crossing of
distinct species has exercised little or no effect on the origin of
species. Even where allied species, like the pintail and the mallard
ducks, whose hybrid offspring is known to be fertile, inhabit the same
breeding area and occasionally interbreed in nature, such crossing does
not, for some reason or other, appear to affect the purity of the
species.
Very different, of course, is the effect of crossing a mutation within a
species with the parent form; the offspring are, as we shall see, likely
to resemble one or other of the parents; so that, if the mutation occur
frequently enough and be favourable to the species, the new form may in
course of time replace the old one.