FREE INTERCROSSING OBLITERATES THE DIFFERENCES BETWEEN ALLIED
BREEDS—WHEN THE NUMBERS OF TWO COMMINGLING BREEDS ARE UNEQUAL, ONE
ABSORBS THE OTHER—THE RATE OF ABSORPTION DETERMINED BY PREPOTENCY OF
TRANSMISSION, BY THE CONDITIONS OF LIFE, AND BY NATURAL SELECTION—ALL
ORGANIC BEINGS OCCASIONALLY INTERCROSS; APPARENT EXCEPTIONS—ON CERTAIN
CHARACTERS INCAPABLE OF FUSION; CHIEFLY OR EXCLUSIVELY THOSE WHICH HAVE
SUDDENLY APPEARED IN THE INDIVIDUAL—ON THE MODIFICATION OF OLD RACES,
AND THE FORMATION OF NEW RACES BY CROSSING—SOME CROSSED RACES HAVE BRED
TRUE FROM THEIR FIRST PRODUCTION—ON THE CROSSING OF DISTINCT SPECIES IN
RELATION TO THE FORMATION OF DOMESTIC RACES.
In the two previous chapters, when discussing reversion and prepotency,
I was necessarily led to give many facts on crossing. In the present
chapter I shall consider the part which crossing plays in two opposed
directions,—firstly, in obliterating characters, and consequently in
preventing the formation of new races; and secondly, in the
modification of old races, or in the formation of new and intermediate
races, by a combination of characters. I shall also show that certain
characters are incapable of fusion.
The effects of free or uncontrolled breeding between the members of the
same variety or of closely allied varieties are important; but are so
obvious that they need not be discussed at much length. It is free
intercrossing which chiefly gives uniformity, both under nature and
under domestication, to the individuals of the same species or variety,
when they live mingled together and are not exposed to any cause
inducing excessive variability. The prevention of free crossing, and
the intentional matching of individual animals, are the corner-stones
of the breeder’s art. No man in his senses would expect to improve or
modify a breed in any particular manner, or keep an old breed true and
distinct, unless he separated his animals. The killing of inferior
animals in each generation comes to the same thing as their separation.
In savage and semi-civilised countries, where the inhabitants have not
the means of separating their animals, more than a single breed of the
same species rarely or never exists. In former times, even in the
United States, there were no distinct races of sheep, for all had been
mingled together.[1] The celebrated agriculturist Marshall[2] remarks
that “sheep that are kept within fences, as well as shepherded flocks
in open countries, have generally a similarity, if not a uniformity, of
character in the individuals of each flock;” for they breed freely
together, and are prevented from crossing with other kinds; whereas in
the unenclosed parts of England the unshepherded sheep, even of the
same flock, are far from true or uniform, owing to various breeds
having mingled and crossed. We have seen that the half-wild cattle in
each of the several British parks are nearly uniform in character; but
in the different parks, from not having mingled and crossed during many
generations, they differ to a certain small extent.
We cannot doubt that the extraordinary number of varieties and
sub-varieties of the pigeon, amounting to at least one hundred and
fifty, is partly due to their remaining, differently from other
domesticated birds, paired for life once matched. On the other hand,
breeds of cats imported into this country soon disappear, for their
nocturnal and rambling habits render it hardly possible to prevent free
crossing. Rengger[3] gives an interesting case with respect to the cat
in Paraguay: in all the distant parts of the kingdom it has assumed,
apparently from the effects of the climate, a peculiar character, but
near the capital this change has been prevented, owing, as he asserts,
to the native animal frequently crossing with cats imported from
Europe. In all cases like the foregoing, the effects of an occasional
cross will be augmented by the increased vigour and fertility of the
crossed offspring, of which fact evidence will hereafter be given; for
this will lead to the mongrels increasing more rapidly than the pure
parent-breeds.
When distinct breeds are allowed to cross freely, the result will be a
heterogeneous body; for instance, the dogs in Paraguay are far from
uniform, and can no longer be affiliated to their parent-races.[4] The
character which a crossed body of animals will ultimately assume must
depend on several contingencies,—namely, on the relative members of the
individuals belonging to the two or more races which are allowed to
mingle; on the prepotency of one race over the other in the
transmission of character; and on the conditions of life to which they
are exposed. When two commingled breeds exist at first in nearly equal
numbers, the whole will sooner or later become intimately blended, but
not so soon, both breeds being equally favoured in all respects, as
might have been expected. The following calculation[5] shows that this
is the case: if a colony with an equal number of black and white men
were founded, and we assume that they marry indiscriminately, are
equally prolific, and that one in thirty annually dies and is born;
then “in 65 years the number of blacks, whites, and mulattoes would be
equal. In 91 years the whites would be 1-10th, the blacks 1-10th, and
the mulattoes, or people of intermediate degrees of colour, 8-10ths of
the whole number. In three centuries not 1-100th part of the whites
would exist.”
When one of two mingled races exceed the other greatly in number, the
latter will soon be wholly, or almost wholly, absorbed and lost.[6]
Thus European pigs and dogs have been largely introduced in the islands
of the Pacific Ocean, and the native races have been absorbed and lost
in the course of about fifty or sixty years;[7] but the imported races
no doubt were favoured. Rats may be considered as semi-domesticated
animals. Some snake-rats (_Mus alexandrinus_) escaped in the Zoological
Gardens of London “and for a long time afterwards the keepers
frequently caught cross-bred rats, at first half-breds, afterwards with
less of the character of the snake-rat, till at length all traces of it
disappeared.”[8] On the other hand, in some parts of London, especially
near the docks, where fresh rats are frequently imported, an endless
variety of intermediate forms may be found between the brown, black,
and snake rat, which are all three usually ranked as distinct species.
How many generations are necessary for one species or race to absorb
another by repeated crosses has often been discussed;[9] and the
requisite number has probably been much exaggerated. Some writers have
maintained that a dozen or score, or even more generations, are
necessary; but this in itself is improbable, for in the tenth
generation there would be only 1-1024th part of foreign blood in the
offspring. Gärtner found,[10] that with plants, one species could be
made to absorb another in from three to five generations, and he
believes that this could always be effected in from six to seven
generations. In one instance, however, Kolreuter[11] speaks of the
offspring of _Mirabilis vulgaris,_ crossed during eight successive
generations by _M. longiflora,_ as resembling this latter species so
closely, that the most scrupulous observer could detect “vix aliquam
notabilem differentiam” or, as he says, he succeeded, “ad plenariam
fere transmutationem.” But this expression shows that the act of
absorption was not even then absolutely complete, though these crossed
plants contained only the 1-256th part of _M. vulgaris._ The
conclusions of such accurate observers as Gärtner and Kölreuter are of
far higher worth than those made without scientific aim by breeders.
The most precise account which I have met with is given by
Stonehenge[12] and is illustrated by photographs. Mr. Hanley crossed a
greyhound bitch with a bulldog; the offspring in each succeeding
generation being recrossed with first-rate greyhounds. As Stonehenge
remarks, it might naturally be supposed that it would take several
crosses to get rid of the heavy form of the bulldog; but Hysterics, the
gr-gr-granddaughter of a bulldog, showed no trace whatever of this
breed in external form. She and all of the same litter, however, were
“remarkably deficient in stoutness, though fast as well as clever.” I
believe clever refers to skill in turning. Hysterics was put to a son
of Bedlamite, “but the result of the fifth cross is not as yet, I
believe, more satisfactory than that of the fourth.” On the other hand,
with sheep, Fleischmann[13] shows how persistent the effects of a
single cross may be: he says “that the original coarse sheep (of
Germany) have 5500 fibres of wool on a square inch; grades of the third
or fourth Merino cross produced about 8000, the twentieth cross 27,000,
the perfect pure Merino blood 40,000 to 48,000.” So that common German
sheep crossed twenty times successively with Merino did not by any
means acquire wool as fine as that of the pure breed. But in all cases,
the rate of absorption will depend largely on the conditions of life
being favourable to any particular character; and we may suspect that
there would be a constant tendency to degeneration in the wool of
Merinos under the climate of Germany, unless prevented by careful
selection; and thus perhaps the foregoing remarkable case may be
explained. The rate of absorption must also depend on the amount of
distinguishable difference between the two forms which are crossed, and
especially, as Gärtner insists, on prepotency of transmission in the
one form over the other. We have seen in the last chapter that one of
two French breeds of sheep yielded up its character, when crossed with
Merinos, very much more slowly than the other; and the common German
sheep referred to by Fleischmann may be in this respect analogous. In
all cases there will be more or less liability to reversion during many
subsequent generations, and it is this fact which has probably led
authors to maintain that a score or more of generations are requisite
for one race to absorb another. In considering the final result of the
commingling of two or more breeds, we must not forget that the act of
crossing in itself tends to bring back long-lost characters not proper
to the immediate parent-forms.
With respect to the influence of the conditions of life on any two
breeds which are allowed to cross freely, unless both are indigenous
and have long been accustomed to the country where they live, they
will, in all probability, be unequally affected by the conditions, and
this will modify the result. Even with indigenous breeds, it will
rarely or never occur that both are equally well adapted to the
surrounding circumstances; more especially when permitted to roam
freely, and not carefully tended, as is generally the case with breeds
allowed to cross. As a consequence of this, natural selection will to a
certain extent come into action, and the best fitted will survive, and
this will aid in determining the ultimate character of the commingled
body.
How long a time it would require before such a crossed body of animals
would assume a uniform character within a limited area, no one can say;
that they would ultimately become uniform from free intercrossing, and
from the survival of the fittest, we may feel assured; but the
characters thus acquired would rarely or never, as may be inferred from
the previous considerations, be exactly intermediate between those of
the two parent-breeds. With respect to the very slight differences by
which the individuals of the same sub-variety, or even of allied
varieties, are characterised, it is obvious that free crossing would
soon obliterate such small distinctions. The formation of new
varieties, independently of selection, would also thus be prevented;
except when the same variation continually recurred from the action of
some strongly predisposing cause. We may therefore conclude that free
crossing has in all cases played an important part in giving uniformity
of character to all the members of the same domestic race and of the
same natural species, though largely governed by natural selection and
by the direct action of the surrounding conditions.
_On the possibility of all organic beings occasionally
intercrossing._—But it may be asked, can free crossing occur with
hermaphrodite animals and plants? All the higher animals, and the few
insects which have been domesticated, have separate sexes, and must
inevitably unite for each birth. With respect to the crossing of
hermaphrodites, the subject is too large for the present volume, but in
the ‘Origin of Species’ I have given a short abstract of the reasons
which induce me to believe that all organic beings occasionally cross,
though perhaps in some cases only at long intervals of time.[14] I will
merely recall the fact that many plants, though hermaphrodite in
structure, are unisexual in function;—such as those called by C.K.
Sprengel _dichogamous,_ in which the pollen and stigma of the same
flower are matured at different periods; or those called by me
_reciprocally dimorphic,_ in which the flower’s own pollen is not
fitted to fertilise its own stigma; or again, the many kinds in which
curious mechanical contrivances exist, effectually preventing
self-fertilisation. There are, however, many hermaphrodite plants which
are not in any way specially constructed to favour intercrossing, but
which nevertheless commingle almost as freely as animals with separated
sexes. This is the case with cabbages, radishes, and onions, as I know
from having experimented on them: even the peasants of Liguria say that
cabbages must be prevented “from falling in love” with each other. In
the orange tribe, Gallesio[15] remarks that the amelioration of the
various kinds is checked by their continual and almost regular
crossing. So it is with numerous other plants.
On the other hand, some cultivated plants rarely or never intercross,
for instance, the common pea and sweet-pea (_Lathyrus odoratus_); yet
their flowers are certainly adapted for cross fertilisation. The
varieties of the tomato and aubergine (_Solanum_) and the pimenta
(_Pimenta vulgaris?_) are said[16] never to cross, even when growing
alongside one another. But it should be observed that these are all
exotic plants, and we do not know how they would behave in their native
country when visited by the proper insects. With respect to the common
pea, I have ascertained that it is rarely crossed in this country owing
to premature fertilisation. There exist, however, some plants which
under their natural conditions appear to be always self-fertilised,
such as the Bee Ophrys (_Ophrys apifera_) and a few other Orchids; yet
these plants exhibit the plainest adaptations for cross-fertilisation.
Again, some few plants are believed to produce only closed flowers,
called cleistogene, which cannot possibly be crossed. This was long
thought to be the case with the _Leersia oryzoides,_[17] but this grass
is now known occasionally to produce perfect flowers, which set seed.
Although some plants, both indigenous and naturalised, rarely or never
produce flowers, or if they flower never produce seeds, yet no one
doubts that phanerogamic plants are adapted to produce flowers, and the
flowers to produce seed. When they fail, we believe that such plants
under different conditions would perform their proper function, or that
they formerly did so, and will do so again. On analogous grounds, I
believe that the flowers in the above specified anomalous cases which
do not now intercross, either would do so occasionally under different
conditions, or that they formerly did so—the means for affecting this
being generally still retained—and will again intercross at some future
period, unless indeed they become extinct. On this view alone, many
points in the structure and action of the reproductive organs in
hermaphrodite plants and animals are intelligible,—for instance, the
fact of the male and female organs never being so completely enclosed
as to render access from without impossible. Hence we may conclude that
the most important of all the means for giving uniformity to the
individuals of the same species, namely, the capacity of occasionally
intercrossing, is present, or has been formerly present, with all
organic beings, except, perhaps, some of the lowest.
_On certain Characters not blending._—When two breeds are crossed their
characters usually become intimately fused together; but some
characters refuse to blend, and are transmitted in an unmodified state
either from both parents or from one. When grey and white mice are
paired, the young are piebald, or pure white or grey, but not of an
intermediate tint; so it is when white and common collared turtle-doves
are paired. In breeding Game fowls, a great authority, Mr. J. Douglas,
remarks, “I may here state a strange fact: if you cross a black with a
white game, you get birds of both breeds of the clearest colour.” Sir
R. Heron crossed during many years white, black, brown, and
fawn-coloured Angora rabbits, and never once got these colours mingled
in the same animal, but often all four colours in the same litter.[18]
From cases like these, in which the colours of the two parents are
transmitted quite separately to the offspring, we have all sorts of
gradations, leading to complete fusion. I will give an instance: a
gentleman with a fair complexion, light hair but dark eyes, married a
lady with dark hair and complexion: their three children have very
light hair, but on careful search about a dozen black hairs were found
scattered in the midst of the light hair on the heads of all three.
When turnspit dogs and ancon sheep, both of which have dwarfed limbs,
are crossed with common breeds, the offspring are not intermediate in
structure, but take after either parent. When tailless or hornless
animals are crossed with perfect animals, it frequently, but by no
means invariably, happens that the offspring are either furnished with
these organs in a perfect state, or are quite destitute of them.
According to Rengger, the hairless condition of the Paraguay dog is
either perfectly or not at all transmitted to its mongrel offspring;
but I have seen one partial exception in a dog of this parentage which
had part of its skin hairy, and part naked, the parts being distinctly
separated as in a piebald animal. When Dorking fowls with five toes are
crossed with other breeds, the chickens often have five toes on one
foot and four on the other. Some crossed pigs raised by Sir R. Heron
between the solid-hoofed and common pig had not all four feet in an
intermediate condition, but two feet were furnished with properly
divided, and two with united hoofs.
Analogous facts have been observed with plants: Major Trevor Clarke
crossed the little, glabrous-leaved, annual stock (Matthiola), with
pollen of a large, red-flowered, rough-leaved, biennial stock, called
_cocardeau_ by the French, and the result was that half the seedlings
had glabrous and the other half rough leaves, but none had leaves in an
intermediate state. That the glabrous seedlings were the product of the
rough-leaved variety, and not accidentally of the mother-plant’s own
pollen, was shown by their tall and strong habit of growth.[19] in the
succeeding generations raised from the rough-leaved crossed seedlings,
some glabrous plants appeared, showing that the glabrous character,
though incapable of blending with and modifying the rough leaves, was
all the time latent in this family of plants. The numerous plants
formerly referred to, which I raised from reciprocal crosses between
the peloric and common Antirrhinum, offer a nearly parallel case; for
in the first generation all the plants resembled the common form, and
in the next generation, out of one hundred and thirty-seven plants, two
alone were in an intermediate condition, the others perfectly
resembling either the peloric or common form. Major Trevor Clarke also
fertilised the above-mentioned red-flowered stock with pollen from the
purple Queen stock, and about half the seedlings scarcely differed in
habit, and not at all in the red colour of the flower, from the
mother-plant, the other half bearing blossoms of a rich purple, closely
like those of the paternal plant. Gärtner crossed many white and
yellow-flowered species and varieties of Verbascum; and these colours
were never blended, but the offspring bore either pure white or pure
yellow blossoms; the former in the larger proportion.[20] Dr. Herbert
raised many seedlings, as he informed me, from Swedish turnips crossed
by two other varieties, and these never produced flowers of an
intermediate tint, but always like one of their parents. I fertilised
the purple sweet-pea (_Lathyrus odoratus_), which has a dark
reddish-purple standard-petal and violet-coloured wings and keel, with
pollen of the painted lady sweet-pea, which has a pale cherry-coloured
standard, and almost white wings and keel; and from the same pod I
twice raised plants perfectly resembling both sorts; the greater number
resembling the father. So perfect was the resemblance, that I should
have thought there had been some mistake, if the plants which were at
first identical with the paternal variety, namely, the painted-lady,
had not later in the season produced, as mentioned in a former chapter,
flowers blotched and streaked with dark purple. I raised grandchildren
and great-grandchildren from these crossed plants, and they continued
to resemble the painted-lady, but during later generations became
rather more blotched with purple, yet none reverted completely to the
original mother-plant, the purple sweet-pea. The following case is
slightly different, but still shows the same principle: Naudin[21]
raised numerous hybrids between the yellow _Linaria vulgaris_ and the
purple _L. purpurea,_ and during three successive generations the
colours kept distinct in different parts of the same flower.
From cases such as the foregoing, in which the offspring of the first
generation perfectly resemble either parent, we come by a small step to
those cases in which differently coloured flowers borne on the same
root resemble both parents, and by another step to those in which the
same flower or fruit is striped or blotched with the two parental
colours, or bears a single stripe of the colour or other characteristic
quality of one of the parent-forms. With hybrids and mongrels it
frequently or even generally happens that one part of the body
resembles more or less closely one parent and another part the other
parent; and here again some resistence to fusion, or, what comes to the
same thing, some mutual affinity between the organic atoms of the same
nature, apparently comes into play, for otherwise all parts of the body
would be equally intermediate in character. So again, when the
offspring of hybrids or mongrels, which are themselves nearly
intermediate in character, revert either wholly or by segments to their
ancestors, the principle of the affinity of similar, or the repulsion
of dissimilar atoms, must come into action. To this principle, which
seems to be extremely general, we shall recur in the chapter on
pangenesis.
It is remarkable, as has been strongly insisted upon by Isidore
Geoffroy St. Hilaire in regard to animals, that the transmission of
characters without fusion occurs very rarely when species are crossed;
I know of one exception alone, namely, with the hybrids naturally
produced between the common and hooded crow (_Corvus corone_ and
_cornix_), which, however, are closely allied species, differing in
nothing except colour. Nor have I met with any well-ascertained cases
of transmission of this kind, even when one form is strongly prepotent
over another, when two races are crossed which have been slowly formed
by man’s selection, and therefore resemble to a certain extent natural
species. Such cases as puppies in the same litter closely resembling
two distinct breeds, are probably due to superfoetation,—that is, to
the influence of two fathers. All the characters above enumerated,
which are transmitted in a perfect state to some of the offspring and
not to others,— such as distinct colours, nakedness of skin, smoothness
of leaves, absence of horns or tail, additional toes, pelorism, dwarfed
structure, etc.,—have all been known to appear suddenly in individual
animals and plants. From this fact, and from the several slight,
aggregated differences which distinguish domestic races and species
from one another, not being liable to this peculiar form of
transmission, we may conclude that it is in some way connected with the
sudden appearance of the characters in question.
_On the Modification of old Races and the Formation of new Races by
Crossing._—We have hitherto chiefly considered the effects of crossing
in giving uniformity of character; we must now look to an opposite
result. There can be no doubt that crossing, with the aid of rigorous
selection during several generations, has been a potent means in
modifying old races, and in forming new ones. Lord Orford crossed his
famous stud of greyhounds once with the bulldog, in order to give them
courage and perseverance. Certain pointers have been crossed, as I hear
from the Rev. W. D. Fox, with the foxhound, to give them dash and
speed. Certain strains of Dorking fowls have had a slight infusion of
Game blood; and I have known a great fancier who on a single occasion
crossed his turbit-pigeons with barbs, for the sake of gaining greater
breadth of beak.
In the foregoing cases breeds have been crossed once, for the sake of
modifying some particular character; but with most of the improved
races of the pig, which now breed true, there have been repeated
crosses,—for instance, the improved Essex owes its excellence to
repeated crosses with the Neapolitan, together probably with some
infusion of Chinese blood.[22] So with our British sheep: almost all
the races, except the Southdown, have been largely crossed; “this, in
fact, has been the history of our principal breeds.”[23] To give an
example, the “Oxfordshire Downs” now rank as an established breed.[24]
They were produced about the year 1830 by crossing “Hampshire and in
some instances Southdown ewes with Cotswold rams:” now the Hampshire
ram was itself produced by repeated crosses between the native
Hampshire sheep and Southdowns; and the long-woolled Cotswold were
improved by crosses with the Leicester, which latter again is believed
to have been a cross between several long-woolled sheep. Mr. Spooner,
after considering the various cases which have been carefully recorded,
concludes, “that from a judicious pairing of cross-bred animals it is
practicable to establish a new breed.” On the continent the history of
several crossed races of cattle and of other animals has been well
ascertained. To give one instance: the King of Wurtemburg, after
twenty-five years’ careful breeding, that is, after six or seven
generations, made a new breed of cattle from a cross between a Dutch
and a Swiss breed, combined with other breeds.[25] The Sebright bantam,
which breeds as true as any other kind of fowl, was formed about sixty
years ago by a complicated cross.[26] Dark Brahmas, which are believed
by some fanciers to constitute a distinct species, were undoubtedly
formed[27] in the United States, within a recent period, by a cross
between Chittagongs and Cochins. With plants there is little doubt that
the Swede-turnip originated from a cross; and the history of a variety
of wheat, raised from two very distinct varieties, and which after six
years’ culture presented an even sample, has been recorded on good
authority.[28]
Until lately, cautious and experienced breeders, though not averse to a
single infusion of foreign blood, were almost universally convinced
that the attempt to establish a new race, intermediate between two
widely distinct races, was hopeless “they clung with superstitious
tenacity to the doctrine of purity of blood, believing it to be the ark
in which alone true safety could be found.”[29] Nor was this conviction
unreasonable: when two distinct races are crossed, the offspring of the
first generation are generally nearly uniform in character; but even
this sometimes fails to be the case, especially with crossed dogs and
fowls, the young of which from the first are sometimes much
diversified. As cross-bred animals are generally of large size and
vigorous, they have been raised in great numbers for immediate
consumption. But for breeding they are found utterly useless; for
though they may themselves be uniform in character, they yield during
many generations astonishingly diversified offspring. The breeder is
driven to despair, and concludes that he will never form an
intermediate race. But from the cases already given, and from others
which have been recorded, it appears that patience alone is necessary;
as Mr. Spooner remarks, “nature opposes no barrier to successful
admixture; in the course of time, by the aid of selection and careful
weeding, it is practicable to establish a new breed.” After six or
seven generations the hoped-for result will in most cases be obtained;
but even then an occasional reversion, or failure to keep true, may be
expected. The attempt, however, will assuredly fail if the conditions
of life be decidedly unfavourable to the characters of either
parent-breed.[30]
Although the grandchildren and succeeding generations of cross-bred
animals are generally variable in an extreme degree, some curious
exceptions to the rule have been observed both with crossed races and
species. Thus Boitard and Corbié[31] assert that from a Pouter and a
Runt “a Cavalier will appear, which we have classed amongst pigeons of
pure race, because it transmits all its qualities to its posterity.”
The editor of the ‘Poultry Chronicle’[32] bred some bluish fowls from a
black Spanish cock and a Malay hen; and these remained true to colour
“generation after generation.” The Himalayan breed of rabbits was
certainly formed by crossing two sub-varieties of the silver-grey
rabbit; although it suddenly assumed its present character, which
differs much from that of either parent-breed, yet it has ever since
been easily and truly propagated. I crossed some Labrador and Penguin
ducks, and recrossed the mongrels with Penguins; afterwards most of the
ducks reared during three generations were nearly uniform in character,
being brown with a white crescentic mark on the lower part of the
breast, and with some white spots at the base of the beak; so that by
the aid of a little selection a new breed might easily have been
formed. With regard to crossed varieties of plants, Mr. Beaton[33]
remarks that “Melville’s extraordinary cross between the Scotch kale
and an early cabbage is as true and genuine as any on record;” but in
this case no doubt selection was practised. Gärtner[34] has given five
cases of hybrids, in which the progeny kept constant; and hybrids
between _Dianthus armeria_ and _deltoides_ remained true and uniform to
the tenth generation. Dr. Herbert likewise showed me a hybrid from two
species of Loasa which from its first production had kept constant
during several generations.
We have seen in the first chapter, that the several kinds of dogs are
almost certainly descended from more than one species, and so it is
with cattle, pigs and some other domesticated animals. Hence the
crossing of aboriginally distinct species probably came into play at an
early period in the formation of our present races. From Rutimeyer’s
observations there can be little doubt that this occurred with cattle;
but in most cases one form will probably have absorbed and obliterated
the other, for it is not likely that semi-civilised men would have
taken the necessary pains to modify by selection their commingled,
crossed, and fluctuating stock. Nevertheless, those animals which were
best adapted to their conditions of life would have survived through
natural selection; and by this means crossing will often have
indirectly aided in the formation of primeval domesticated breeds.
Within recent times, as far as animals are concerned, the crossing of
distinct species has done little or nothing towards the formation or
modification of our races. It is not yet known whether the several
species of silk-moth which have been recently crossed in France will
yield permanent races. With plants which can be multiplied by buds and
cuttings, hybridisation has done wonders, as with many kinds of Roses,
Rhododendrons, Pelargoniums, Calceolarias, and Petunias. Nearly all
these plants can be propagated by seed, most of them freely; but
extremely few or none come true by seed.
Some authors believe that crossing is the chief cause of
variability,—that is, of the appearance of absolutely new characters.
Some have gone so far as to look at it as the sole cause; but this
conclusion is disproved by the facts given in the chapter on
Bud-variation. The belief that characters not present in either parent
or in their ancestors frequently originate from crossing is doubtful;
that they occasionally do so is probable; but this subject will be more
conveniently discussed in a future chapter on the causes of
Variability.
A condensed summary of this and of the three following chapters,
together with some remarks on Hybridism, will be given in the
nineteenth chapter.
REFERENCES
[1] ‘Communications to the Board of Agriculture,’ vol. i. p. 367.
[2] ‘Review of Reports, North of England,’ 1808, p. 200.
[3] ‘Säugethiere von Paraguay,’ 1830, s. 212.
[4] Rengger, ‘Säugethiere,’ etc., s. 154.
[5] White, ‘Regular Gradation in Man,’ p. 146.
[6] Dr. W. F. Edwards, in his ‘Caractères Physiolog. des Races
Humaines,’ p. 24, first called attention to this subject, and ably
discussed it.
[7] Rev. D. Tyerman and Bennett, ‘Journal of Voyages,’ 1821-1829, vol.
i. p. 300.
[8] Mr. S. J. Salter, ‘Journal Linn. Soc.,’ vol. vi., 1862, p. 71.
[9] Sturm, ‘Ueber Racen, etc.,’ 1825, s. 107. Bronn, ‘Geschichte der
Natur,’ b. ii. s. 170, gives a table of the proportions of blood after
successive crosses. Dr. P. Lucas, ‘L’Hérédité Nat.,’ tom. ii. p. 308.
[10] ‘Bastarderzeugung,’ s. 463, 470.
[11] ‘Nova Acta Petrop.,’ 1794, p. 393: _see also_ previous volume.
[12] ‘The Dog,’ 1867, pp. 179-184.
[13] As quoted in the ‘True Principles of Breeding,’ by C. H.
Macknight and Dr. H. Madden, 1865, p. 11.
[14] With respect to plants, an admirable essay on this subject (Die
Geschlechter-Vertheilung bei den Pflanzen: 1867) has been published by
Dr. Hildebrand, who arrives at the same general conclusions as I have
done. Various other treatises have since appeared on the same subject,
more especially by Hermann Müller and Delpino.
[15] ‘Teoria della Riproduzione Vegetal,’ 1816, p. 12.
[16] Verlot ‘Des Variétés,’ 1865, p. 72.
[17] Duval Jouve, ‘Bull. Soc. Bot. de France,’ tom. x., 1863, p. 194.
With respect to the perfect flowers setting seed, _see_ Dr. Ascherson
in ‘Bot. Zeitung,’ 1864, p. 350.
[18] Extract of a letter from Sir R. Heron, 1838, given me by Mr.
Yarrell. With respect to mice, _see_ ‘Annal. des Sc. Nat.,’ tom. i. p.
180; and I have heard of other similar cases. For turtle-doves Boitard
and Corbié, ‘Les Pigeons,’ etc., p. 238. For the Game fowl, ‘The
Poultry Book,’ 1866, p. 128. For crosses of tailless fowls _see_
Bechstein, ‘Naturges. Deutsch.’ b. iii. s. 403. Bronn, ‘Geschichte der
Natur,’ b. ii. s. 170, gives analogous facts with horses. On the
hairless condition of crossed South American dogs, _see_ Rengger,
‘Säugethiere von Paraguay,’ s. 152; but I saw in the Zoological
Gardens mongrels, from a similar cross, which were hairless, quite
hairy, or hairy in patches, that is, piebald with hair. For crosses of
Dorking and other fowls, _see_ ‘Poultry Chronicle,’ vol. ii. p. 355.
About the crossed pigs, extract of letter from Sir R. Heron to Mr.
Yarrell. For other cases, _ see_ P. Lucas ‘L’Héréd. Nat.’ tom. i. p.
212.
[19] ‘Internat. Hort. and Bot. Congress of London,’ 1866.
[20] ‘Bastarderzeugung,’ s. 307. Kölreuter (‘Dritte Fortsetszung,’ s.
34, 39), however, obtained intermediate tints from similar crosses in
the genus Verbascum. With respect to the turnips, _see_ Herbert’s
‘Amaryllidaceæ,’ 1837, p. 370.
[21] ‘Nouvelles Archives du Muséum,’ tom. i. p. 100.
[22] Richardson, ‘Pigs,’ 1847, pp. 37, 42; S. Sidney’s edition of
‘Youatt on the Pig,’ 1860, p. 3.
23[] _See_ Mr. W. C. Spooner’s excellent paper on Cross-Breeding,
‘Journal Royal Agricult. Soc.,’ vol. xx., part ii.: _see also_ an
equally good article by Mr. Ch. Howard, in ‘Gardener’s Chronicle,’
1860, p. 320.
[24] ‘Gardener’s Chronicle,’ 1857, pp. 649, 652.
[25] ‘Bulletin de la Soc. d’Acclimat.,’ 1862, tom. ix. p. 463. _ See
also_ for other cases MM. Moll and Gayot, ‘Du Bœuf,’ 1860, p. 32.
[26] ‘Poultry Chronicle,’ vol. ii., 1854, p. 36.
[27] ‘The Poultry Book,’ by W. B. Tegetmeier, 1866, p. 58.
[28] ‘Gardener’s Chronicle,’ 1852, p. 765.
[29] Spooner, in ‘Journal Royal Agricult. Soc.,’ vol. xx., part ii.
[30] _See_ Colin’s ‘Traité de Phys. Comp. des Animaux Domestiques,’
tom. ii. p. 536, where this subject is well treated.
[31] ‘Les Pigeons,’ p. 37.
[32] Vol. i., 1854, p. 101.
[33] ‘Cottage Gardener,’ 1856, p. 110.
[34] ‘Bastarderzeugung,’ s. 553.