ON CROSSING.
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. (15/1. 'Communications to the Board of Agriculture' volume 1 page
367.) The celebrated agriculturist Marshall (15/2. 'Review of Reports, North
of England' 1808 page 200.) 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 (15/3. 'Saugethiere von Paraguay'
1830 s. 212.) 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. (15/4. Rengger
'Saugethiere' etc. s. 154.) 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 (15/5. White 'Regular Gradation in Man'
page 146.) 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. (15/6. Dr. W.F.
Edwards in his 'Caracteres Physiolog. des Races Humaines' page 24 first called
attention to this subject, and ably discussed it.) 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 (15/7. Rev. D. Tyerman and Bennett 'Journal of Voyages' 1821-1829 volume
1 page 300.); 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." (15/8. Mr. S.J. Salter 'Journal Linn. Soc.' volume 6 1862 page
71.) 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 (15/9. Sturm 'Ueber Racen, etc.'
1825 s. 107. Bronn 'Geschichte der Natur' b. 2 s. 170 gives a table of the
proportions of blood after successive crosses. Dr. P. Lucas 'L'Heredite Nat.'
tome 2 page 308.); 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. Gartner found (15/10. 'Bastarderzeugung' s. 463, 470.), 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 (15/11. 'Nova Acta
Petrop.' 1794 page 393: see also previous volume.) 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 Gartner and Kolreuter
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 (15/12.
'The Dog' 1867 pages 179-184.) 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 (15/13. As quoted in the 'True Principles
of Breeding' by C.H. Macknight and Dr. H. Madden 1865 page 11.) 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 Gartner 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. (15/14. 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 Muller and Delpino.) 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 (15/15. 'Teoria della
Riproduzione Vegetal' 1816 page 12.) 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 (15/16.
Verlot 'Des Varietes' 1865 page 72.) 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
(15/17. Duval Jouve 'Bull. Soc. Bot. de France' tome 10 1863 page 194. With
respect to the perfect flowers setting seed see Dr. Ascherson in 'Bot.
Zeitung' 1864 page 350.), 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. (15/18. Extract of a letter from Sir R. Heron 1838 given me
by Mr. Yarrell. With respect to mice see 'Annal. des Sc. Nat.' tome 1 page
180; and I have heard of other similar cases. For turtle-doves Boitard and
Corbie 'Les Pigeons' etc. page 238. For the Game fowl 'The Poultry Book' 1866
page 128. For crosses of tailless fowls see Bechstein 'Naturges. Deutsch.' b.
3 s. 403. Bronn 'Geschichte der Natur' b. 2 s. 170 gives analogous facts with
horses. On the hairless condition of crossed South American dogs see Rengger
'Saugethiere 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' volume 2 page 355. About the crossed pigs, extract of
letter from Sir R. Heron to Mr. Yarrell. For other cases see P. Lucas
'L'Hered. Nat.' tome 1 page 212.) 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. (15/19. 'Internat. Hort. and Bot. Congress of London' 1866.) 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. Gartner 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. (15/20. 'Bastarderzeugung' s. 307. Kolreuter 'Dritte
Fortsetszung' s. 34, 39 however, obtained intermediate tints from similar
crosses in the genus Verbascum. With respect to the turnips see Herbert
'Amaryllidaceae' 1837 page 370.) 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 (15/21.
'Nouvelles Archives du Museum' tome 1 page 100.) 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. (15/22.
Richardson 'Pigs' 1847 pages 37, 42; S. Sidney's edition of 'Youatt on the
Pig' 1860 page 3.) 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." (15/23. See Mr. W.C. Spooner's excellent paper on
Cross-Breeding 'Journal Royal Agricult. Soc.' volume 20 part 2: see also an
equally good article by Mr. Ch. Howard in 'Gardener's Chronicle' 1860 page
320.) To give an example, the "Oxfordshire Downs" now rank as an established
breed. (15/24. 'Gardener's Chronicle' 1857 pages 649, 652.) 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. (15/25. 'Bulletin de La Soc. d'Acclimat.' 1862 tome 9 page 463.
See also for other cases MM. Moll and Gayot 'Du Boeuf' 1860 page 32.) The
Sebright bantam, which breeds as true as any other kind of fowl, was formed
about sixty years ago by a complicated cross. (15/26. 'Poultry Chronicle'
volume 2 1854 page 36.) Dark Brahmas, which are believed by some fanciers to
constitute a distinct species, were undoubtedly formed (15/27. 'The Poultry
Book' by W.B. Tegetmeier 1866 page 58.) 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. (15/28. 'Gardener's Chronicle' 1852 page 765.)
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."
(15/29. Spooner in 'Journal Royal Agricult. Soc.' volume 20 part 2) 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. (15/30. See Colin 'Traite de Phys. Comp. des Animaux
Domestiques' tome 2 page 536, where this subject is well treated.)
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
Corbie (15/31. 'Les Pigeons' page 37.) 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' (15/32. Volume 1 1854 page 101.) 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 (15/33.
'Cottage Gardener' 1856 page 110.) 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. Gartner
(15/34. 'Bastarderzeugung' s. 553.) 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.