The Variation of Animals and Plants under Domestication
by
Charles Darwin
Part 2 out of 12
alone; but we do not know the history of the first appearance of such
characters. In the chapter on Sheep, we have seen that the males of certain
races differ greatly from the females in the shape of their horns, these being
absent in the ewes of some breeds; they differ also in the development of fat
in the tail and in the outline of the forehead. These differences, judging
from the character of the allied wild species, cannot be accounted for by
supposing that they have been derived from distinct parent forms. There is,
also, a great difference between the horns of the two sexes in one Indian
breed of goats. The bull zebu is said to have a larger hump than the cow. In
the Scotch deer-hound the two sexes differ in size more than in any other
variety of the dog (14/28. W. Scrope 'Art of Deer Stalking' page 354.) and,
judging from analogy, more than in the aboriginal parent-species. The peculiar
colour called tortoise-shell is very rarely seen in a male cat; the males of
this variety being of a rusty tint.
In various breeds of the fowl the males and females often differ greatly; and
these differences are far from being the same with those which distinguish the
two sexes of the parent-species, the Gallus bankiva; and consequently have
originated under domestication. In certain sub-varieties of the Game race we
have the unusual case of the hens differing from each other more than the
cocks. In an Indian breed of a white colour shaded with black, the hens
invariably have black skins, and their bones are covered by a black
periosteum, whilst the cocks are never or most rarely thus characterised.
Pigeons offer a more interesting case; for throughout the whole great family
the two sexes do not often differ much; and the males and females of the
parent-form, the C. livia, are undistinguishable: yet we have seen that with
pouters the male has the characteristic quality of pouting more strongly
developed than the female; and in certain sub-varieties the males alone are
spotted or striated with black, or otherwise differ in colour. When male and
female English carrier-pigeons are exhibited in separate pens, the difference
in the development of the wattle over the beak and round the eyes is
conspicuous. So that here we have instances of the appearance of secondary
sexual characters in the domesticated races of a species in which such
differences are naturally quite absent.]
On the other hand, secondary sexual characters which belong to the species in
a state of nature are sometimes quite lost, or greatly diminished, under
domestication. We see this in the small size of the tusks in our improved
breeds of the pig, in comparison with those of the wild boar. There are sub-
breeds of fowls, in which the males have lost the fine-flowing tail-feathers
and hackles; and others in which there is no difference in colour between the
two sexes. In some cases the barred plumage, which in gallinaceous birds is
commonly the attribute of the hen, has been transferred to the cock, as in the
cuckoo sub-breeds. In other cases masculine characters have been partly
transferred to the female, as with the splendid plumage of the golden-spangled
Hamburgh hen, the enlarged comb of the Spanish hen, the pugnacious disposition
of the Game hen, and as in the well-developed spurs which occasionally appear
in the hens of various breeds. In Polish fowls both sexes are ornamented with
a topknot, that of the male being formed of hackle-like feathers, and this is
a new male character in the genus Gallus. On the whole, as far as I can judge,
new characters are more apt to appear in the males of our domesticated animals
than in the females (14/29. I have given in my 'Descent of Man' 2nd edition
page 223 sufficient evidence that male animals are usually more variable than
the females.), and afterwards to be inherited exclusively or more strongly by
the males. Finally, in accordance with the principle of inheritance as limited
by sex, the preservation and augmentation of secondary sexual characters in
natural species offers no especial difficulty, as this would follow through
that form of selection which I have called sexual selection.
INHERITANCE AT CORRESPONDING PERIODS OF LIFE.
This is an important subject. Since the publication of my 'Origin of Species'
I have seen no reason to doubt the truth of the explanation there given of one
of the most remarkable facts in biology, namely, the difference between the
embryo and the adult animal. The explanation is, that variations do not
necessarily or generally occur at a very early period of embryonic growth, and
that such variations are inherited at a corresponding age. As a consequence of
this the embryo, even after the parent-form has undergone great modification,
is left only slightly modified; and the embryos of widely-different animals
which are descended from a common progenitor remain in many important respects
like one another and probably like their common progenitor. We can thus
understand why embryology throws a flood of light on the natural system of
classification, as this ought to be as far as possible genealogical. When the
embryo leads an independent life, that is, becomes a larva, it has to be
adapted to the surrounding conditions in its structure and instincts,
independently of those of its parents; and the principle of inheritance at
corresponding periods of life renders this possible.
This principle is, indeed, in one way so obvious that it escapes attention. We
possess a number of races of animals and plants, which, when compared with one
another and with their parent-forms, present conspicuous differences, both in
their immature and mature states. Look at the seeds of the several kinds of
peas, beans, maize, which can be propagated truly, and see how they differ in
size, colour, and shape, whilst the full-grown plants differ but little.
Cabbages, on the other hand, differ greatly in foliage and manner of growth,
but hardly at all in their seeds; and generally it will be found that the
differences between cultivated plants at different periods of growth are not
necessarily closely connected together, for plants may differ much in their
seeds and little when full-grown, and conversely may yield seeds hardly
distinguishable, yet differ much when full-grown. In the several breeds of
poultry, descended from a single species, differences in the eggs and chickens
whilst covered with down, in the plumage at the first and subsequent moults,
as well as in the comb and wattles, are all inherited. With man peculiarities
in the milk and second teeth (of which I have received the details) are
inheritable, and longevity is often transmitted. So again with our improved
breeds of cattle and sheep, early maturity, including the early development of
the teeth, and with certain breeds of fowl the early appearance of secondary
sexual characters, all come under the same head of inheritance at
corresponding periods.
Numerous analogous facts could be given. The silk-moth, perhaps, offers the
best instance; for in the breeds which transmit their characters truly, the
eggs differ in size, colour, and shape: the caterpillars differ, in moulting
three or four times, in colour, even in having a dark-coloured mark like an
eyebrow, and in the loss of certain instincts;--the cocoons differ in size,
shape, and in the colour and quality of the silk; these several differences
being followed by slight or barely distinguishable differences in the mature
moth.
But it may be said that, if in the above cases a new peculiarity is inherited,
it must be at the corresponding stage of development; for an egg or seed can
resemble only an egg or seed, and the horn in a full-grown ox can resemble
only a horn. The following cases show inheritance at corresponding periods
more plainly, because they refer to peculiarities which might have supervened,
as far as we can see, earlier or later in life, yet are inherited at the same
period at which they first appeared.
[In the Lambert family the porcupine-like excrescences appeared in the father
and sons at the same age, namely, about nine weeks after birth. (14/30.
Prichard 'Phys. Hist. of Mankind' 1851 volume 1 page 349.) In the
extraordinary hairy family described by Mr. Crawfurd (14/31. 'Embassy to the
Court of Ava' volume 1 page 320. The third generation is described by Capt.
Yule in his 'Narrative of the Mission to the Court of Ava' 1855 page 94.),
children were produced during three generations with hairy ears; in the father
the hair began to grow over his body at six years old; in his daughter
somewhat earlier, namely, at one year; and in both generations the milk teeth
appeared late in life, the permanent teeth being afterwards singularly
deficient. Greyness of hair at an unusually early age has been transmitted in
some families. These cases border on diseases inherited at corresponding
periods of life, to which I shall immediately refer.
It is a well-known peculiarity with almond-tumbler pigeons, that the full
beauty and peculiar character of the plumage does not appear until the bird
has moulted two or three times. Neumeister describes and figures a brace of
pigeons in which the whole body is white except the breast, neck, and head;
but in their first plumage all the white feathers have coloured edges. Another
breed is more remarkable: its first plumage is black, with rusty-red wing-bars
and a crescent-shaped mark on the breast; these marks then become white, and
remain so during three or four moults; but after this period the white spreads
over the body, and the bird loses its beauty. (14/32. 'Das Ganze der
Taubenzucht' 1837 s. 24 tab. 4 figure 2 s. 21 tab. 1 figure 4.) Prize canary-
birds have their wings and tail black: "this colour, however, is only retained
until the first moult, so that they must be exhibited ere the change takes
place. Once moulted, the peculiarity has ceased. Of course all the birds
emanating from this stock have black wings and tails the first year." (14/33.
Kidd 'Treatise on the Canary' page 18.) A curious and somewhat analogous
account has been given (14/34. Charlesworth 'Mag. of Nat. Hist.' volume 1 1837
page 167.) of a family of wild pied rooks which were first observed in 1798,
near Chalfont, and which every year from that date up to the period of the
published notice, viz., 1837 "have several of their brood particoloured, black
and white. This variegation of the plumage, however, disappears with the first
moult; but among the next young families there are always a few pied ones."
These changes of plumage, which are inherited at various corresponding periods
of life in the pigeon, canary-bird, and rook, are remarkable, because the
parent-species passes through no such change.
Inherited diseases afford evidence in some respects of less value than the
foregoing cases, because diseases are not necessarily connected with any
change in structure; but in other respects of more value, because the periods
have been more carefully observed. Certain diseases are communicated to the
child apparently by a process like inoculation, and the child is from the
first affected; such cases may be here passed over. Large classes of diseases
usually appear at certain ages, such as St. Vitus's dance in youth,
consumption in early mid-life, gout later, and apoplexy still later; and these
are naturally inherited at the same period. But even in diseases of this
class, instances have been recorded, as with St. Vitus's dance, showing that
an unusually early or late tendency to the disease is inheritable. (14/35. Dr.
Prosper Lucas 'Hered. Nat.' tome 2 page 713.) In most cases the appearance of
any inherited disease is largely determined by certain critical periods in
each person's life, as well as by unfavourable conditions. There are many
other diseases, which are not attached to any particular period, but which
certainly tend to appear in the child at about the same age at which the
parent was first attacked. An array of high authorities, ancient and modern,
could be given in support of this proposition. The illustrious Hunter believed
in it; and Piorry (14/36. 'L'Hered. dans les Maladies' 1840 page 135. For
Hunter see Harlan 'Med. Researches' page 530.) cautions the physician to look
closely to the child at the period when any grave inheritable disease attacked
the parent. Dr. Prosper Lucas (14/37. 'L'Hered. Nat.' tome 2 page 850.), after
collecting facts from every source, asserts that affections of all kinds,
though not related to any particular period of life, tend to reappear in the
offspring at whatever period of life they first appeared in the progenitor.
As the subject is important, it may be well to give a few instances, simply as
illustrations, not as proof; for proof, recourse must be had to the
authorities above quoted. Some of the following cases have been selected for
the sake of showing that, when a slight departure from the rule occurs, the
child is affected somewhat earlier in life than the parent. In the family of
Le Compte blindness was inherited through three generations, and no less than
twenty-seven children and grandchildren were all affected at about the same
age; their blindness in general began to advance about the fifteenth or
sixteenth year, and ended in total deprivation of sight at the age of about
twenty-two. (14/38. Sedgwick 'Brit. and For. Med.-Chirurg. Review' April 1861
page 485. In some accounts the number of children and grandchildren is given
as 37; but this seems to be an error judging from the paper first published in
the 'Baltimore Med. and Phys. Reg.' 1809 of which Mr. Sedgwick has been so
kind as to send me a copy.) In another case a father and his four children all
became blind at twenty-one years old; in another, a grandmother grew blind at
thirty-five, her daughter at nineteen, and three grandchildren at the ages of
thirteen and eleven. (14/39. Prosper Lucas 'Hered. Nat.' tome 1 page 400.) So
with deafness, two brothers, their father and paternal grandfather, all became
deaf at the age of forty. (14/40. Sedgwick ibid July 1861 page 202.)
Esquirol gives several striking instances of insanity coming on at the same
age, as that of a grandfather, father, and son, who all committed suicide near
their fiftieth year. Many other cases could be given, as of a whole family who
became insane at the age of forty. (14/41. Piorry page 109; Prosper Lucas tome
2 page 759.) Other cerebral affections sometimes follow the same rule,--for
instance, epilepsy and apoplexy. A woman died of the latter disease when
sixty-three years old; one of her daughters at forty-three, and the other at
sixty-seven: the latter had twelve children, who all died from tubercular
meningitis. (14/42. Prosper Lucas tome 2 page 748.) I mention this latter case
because it illustrates a frequent occurrence, namely, a change in the precise
nature of an inherited disease, though still affecting the same organ.
Asthma has attacked several members of the same family when forty years old,
and other families during infancy. The most different diseases, such as angina
pectoris, stone in the bladder, and various affections of the skin, have
appeared in successive generations at nearly the same age. The little finger
of a man began from some unknown cause to grow inwards, and the same finger in
his two sons began at the same age to bend inwards in a similar manner.
Strange and inexplicable neuralgic affections have caused parents and children
to suffer agonies at about the same period of life. (14/43. Prosper Lucas tome
3 pages 678, 700, 702; Sedgwick ibid April 1863 page 449 and July 1863 page
162. Dr. J. Steinan 'Essay on Hereditary Disease' 1843 pages 27, 34.)
I will give only two other cases, which are interesting as illustrating the
disappearance as well as the appearance of disease at the same age. Two
brothers, their father, their paternal uncles, seven cousins, and their
paternal grandfather, were all similarly affected by a skin-disease, called
pityriasis versicolor; "the disease, strictly limited to the males of the
family (though transmitted through the females), usually appeared at puberty,
and disappeared at about the age of forty or forty-five years." The second
case is that of four brothers, who when about twelve years old suffered almost
every week from severe headaches, which were relieved only by a recumbent
position in a dark room. Their father, paternal uncles, paternal grandfather,
and granduncles all suffered in the same way from headaches, which ceased at
the age of fifty-four or fifty-five in all those who lived so long. None of
the females of the family were affected. (14/44. These cases are given by Mr.
Sedgwick on the authority of Dr. H. Stewart in 'Med.-Chirurg. Review' April
1863 pages 449, 477.)]
It is impossible to read the foregoing accounts, and the many others which
have been recorded, of diseases coming on during three or even more
generations in several members of the same family at the same age, especially
in the case of rare affections in which the coincidence cannot be attributed
to chance, and to doubt that there is a strong tendency to inheritance in
disease at corresponding periods of life. When the rule fails, the disease is
apt to come on earlier in the child than in the parent; the exceptions in the
other direction being very much rarer. Dr. Lucas (14/45. 'Hered. Nat.' tome 2
page 852.) alludes to several cases of inherited diseases coming on at an
earlier period. I have already given one striking instance with blindness
during three generations; and Mr. Bowman remarks that this frequently occurs
with cataract. With cancer there seems to be a peculiar liability to earlier
inheritance: Sir J. Paget, who has particularly attended to this subject, and
tabulated a large number of cases, informs me that he believes that in nine
cases out of ten the later generation suffers from the disease at an earlier
period than the previous generation. He adds, "In the instances in which the
opposite relation holds, and the members of later generations have cancer at a
later age than their predecessors, I think it will be found that the non-
cancerous parents have lived to extreme old ages." So that the longevity of a
non-affected parent seems to have the power of influencing the fatal period in
the offspring; and we thus apparently get another element of complexity in
inheritance.
The facts, showing that with certain diseases the period of inheritance
occasionally or even frequently advances, are important with respect to the
general descent-theory, for they render it probable that the same thing would
occur with ordinary modifications of structure. The final result of a long
series of such advances would be the gradual obliteration of characters proper
to the embryo and larva, which would thus come to resemble more and more
closely the mature parent-form. But any structure which was of service to the
embryo or larva would be preserved by the destruction at this stage of growth
of each individual which manifested any tendency to lose its proper character
at too early an age.
Finally, from the numerous races of cultivated plants and domestic animals, in
which the seeds or eggs, the young or old, differ from one another and from
those of the parent-species;--from the cases in which new characters have
appeared at a particular period, and afterwards been inherited at the same
period;--and from what we know with respect to disease, we must believe in the
truth of the great principle of inheritance at corresponding periods of life.
SUMMARY OF THE THREE PRECEDING CHAPTERS.
Strong as is the force of inheritance, it allows the incessant appearance of
new characters. These, whether beneficial or injurious,--of the most trifling
importance, such as a shade of colour in a flower, a coloured lock of hair, or
a mere gesture,--or of the highest importance, as when affecting the brain, or
an organ so perfect and complex as the eye,--or of so grave a nature as to
deserve to be called a monstrosity,--or so peculiar as not to occur normally
in any member of the same natural class,--are often inherited by man, by the
lower animals, and plants. In numberless cases it suffices for the inheritance
of a peculiarity that one parent alone should be thus characterised.
Inequalities in the two sides of the body, though opposed to the law of
symmetry, may be transmitted. There is ample evidence that the effects of
mutilations and of accidents, especially or perhaps exclusively when followed
by disease, are occasionally inherited. There can be no doubt that the evil
effects of the long-continued exposure of the parent to injurious conditions
are sometimes transmitted to the offspring. So it is, as we shall see in a
future chapter, with the effects of the use and disuse of parts, and of mental
habits. Periodical habits are likewise transmitted, but generally, as it would
appear, with little force.
Hence we are led to look at inheritance as the rule, and non-inheritance as
the anomaly. But this power often appears to us in our ignorance to act
capriciously, transmitting a character with inexplicable strength or
feebleness. The very same peculiarity, as the weeping habit of trees, silky
feathers, etc., may be inherited either firmly or not at all by different
members of the same group, and even by different individuals of the same
species, though treated in the same manner. In this latter case we see that
the power of transmission is a quality which is merely individual in its
attachment. As with single characters, so it is with the several concurrent
slight differences which distinguish sub-varieties or races; for of these,
some can be propagated almost as truly as species, whilst others cannot be
relied on. The same rule holds good with plants, when propagated by bulbs,
offsets, etc., which in one sense still form parts of the same individual, for
some varieties retain or inherit through successive bud-generations their
character far more truly than others.
Some characters not proper to the parent-species have certainly been inherited
from an extremely remote epoch, and may therefore be considered as firmly
fixed. But it is doubtful whether length of inheritance in itself gives
fixedness of character; though the chances are obviously in favour of any
character which has long been transmitted true or unaltered still being
transmitted true as long as the conditions of life remain the same. We know
that many species, after having retained the same character for countless
ages, whilst living under their natural conditions, when domesticated have
varied in the most diversified manner, that is, have failed to transmit their
original form; so that no character appears to be absolutely fixed. We can
sometimes account for the failure of inheritance by the conditions of life
being opposed to the development of certain characters; and still oftener, as
with plants cultivated by grafts and buds, by the conditions causing new and
slight modifications incessantly to appear. In this latter case it is not that
inheritance wholly fails, but that new characters are continually superadded.
In some few cases, in which both parents are similarly characterised,
inheritance seems to gain so much force by the combined action of the two
parents, that it counteracts its own power, and a new modification is the
result.
In many cases the failure of the parents to transmit their likeness is due to
the breed having been at some former period crossed; and the child takes after
his grandparent or more remote ancestor of foreign blood. In other cases, in
which the breed has not been crossed, but some ancient character has been lost
through variation, it occasionally reappears through reversion, so that the
parents apparently fail to transmit their own likeness. In all cases, however,
we may safely conclude that the child inherits all its characters from its
parents, in whom certain characters are latent, like the secondary sexual
characters of one sex in the other. When, after a long succession of bud-
generations, a flower or fruit becomes separated into distinct segments,
having the colours or other attributes of both parent-forms, we cannot doubt
that these characters were latent in the earlier buds, though they could not
then be detected, or could be detected only in an intimately commingled state.
So it is with animals of crossed parentage, which with advancing years
occasionally exhibit characters derived from one of their two parents, of
which not a trace could at first be perceived. Certain monstrosities, which
resemble what naturalists call the typical form of the group in question,
apparently come under the same law of reversion. It is assuredly an
astonishing fact that the male and female sexual elements, that buds, and even
full-grown animals, should retain characters, during several generations in
the case of crossed breeds, and during thousands of generations in the case of
pure breeds, written as it were in invisible ink, yet ready at any time to be
evolved under certain conditions.
What these conditions precisely are, we do not know. But any cause which
disturbs the organisation or constitution seems to be sufficient. A cross
certainly gives a strong tendency to the reappearance of long-lost characters,
both corporeal and mental. In the case of plants, this tendency is much
stronger with those species which have been crossed after long cultivation and
which therefore have had their constitutions disturbed by this cause as well
as by crossing, than with species which have always lived under their natural
conditions and have then been crossed. A return, also, of domesticated animals
and cultivated plants to a wild state favours reversion; but the tendency
under these circumstances has been much exaggerated.
When individuals of the same family which differ somewhat, and when races or
species are crossed, the one is often prepotent over the other in transmitting
its character. A race may possess a strong power of inheritance, and yet when
crossed, as we have seen with trumpeter-pigeons, yield to the prepotency of
every other race. Prepotency of transmission may be equal in the two sexes of
the same species, but often runs more strongly in one sex. It plays an
important part in determining the rate at which one race can be modified or
wholly absorbed by repeated crosses with another. We can seldom tell what
makes one race or species prepotent over another; but it sometimes depends on
the same character being present and visible in one parent, and latent or
potentially present in the other.
Characters may first appear in either sex, but oftener in the male than in the
female, and afterwards be transmitted to the offspring of the same sex. In
this case we may feel confident that the peculiarity in question is really
present though latent in the opposite sex! hence the father may transmit
through his daughter any character to his grandson; and the mother conversely
to her granddaughter. We thus learn, and the fact is an important one, that
transmission and development are distinct powers. Occasionally these two
powers seem to be antagonistic, or incapable of combination in the same
individual; for several cases have been recorded in which the son has not
directly inherited a character from his father, or directly transmitted it to
his son, but has received it by transmission through his non-affected mother,
and transmitted it through his non-affected daughter. Owing to inheritance
being limited by sex, we see how secondary sexual characters may have arisen
under nature; their preservation and accumulation being dependent on their
service to either sex.
At whatever period of life a new character first appears, it generally remains
latent in the offspring until a corresponding age is attained, and then is
developed. When this rule fails, the child generally exhibits the character at
an earlier period than the parent. On this principle of inheritance at
corresponding periods, we can understand how it is that most animals display
from the germ to maturity such a marvellous succession of characters.
Finally, though much remains obscure with respect to Inheritance, we may look
at the following laws as fairly well established.
FIRSTLY, a tendency in every character, new and old, to be transmitted by
seminal and bud generation, though often counteracted by various known and
unknown causes.
SECONDLY, reversion or atavism, which depends on transmission and development
being distinct powers: it acts in various degrees and manners through both
seminal and bud generation.
THIRDLY, prepotency of transmission, which may be confined to one sex, or be
common to both sexes.
FOURTHLY, transmission, as limited by sex, generally to the same sex in which
the inherited character first appeared; and this in many, probably most cases,
depends on the new character having first appeared at a rather late period of
life.
FIFTHLY, inheritance at corresponding periods of life, with some tendency to
the earlier development of the inherited character.
In these laws of Inheritance, as displayed under domestication, we see an
ample provision for the production, through variability and natural selection,
of new specific forms.
CHAPTER 2.XV.
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.
CHAPTER 2.XVI.
CAUSES WHICH INTERFERE WITH THE FREE CROSSING OF VARIETIES--INFLUENCE OF
DOMESTICATION ON FERTILITY.
DIFFICULTIES IN JUDGING OF THE FERTILITY OF VARIETIES WHEN CROSSED.
VARIOUS CAUSES WHICH KEEP VARIETIES DISTINCT, AS THE PERIOD OF BREEDING AND
SEXUAL PREFERENCE.
VARIETIES OF WHEAT SAID TO BE STERILE WHEN CROSSED.
VARIETIES OF MAIZE, VERBASCUM, HOLLYHOCK, GOURDS, MELONS, AND TOBACCO,
RENDERED IN SOME DEGREE MUTUALLY STERILE.
DOMESTICATION ELIMINATES THE TENDENCY TO STERILITY NATURAL TO SPECIES WHEN
CROSSED.
ON THE INCREASED FERTILITY OF UNCROSSED ANIMALS AND PLANTS FROM DOMESTICATION
AND CULTIVATION.
The domesticated races of both animals and plants, when crossed, are, with
extremely few exceptions, quite prolific,--in some cases even more so than the
purely-bred parent-races. The offspring, also, raised from such crosses are
likewise, as we shall see in the following chapter, generally more vigorous
and fertile than their parents. On the other hand, species when crossed, and
their hybrid offspring, are almost invariably in some degree sterile; and here
there seems to exist a broad and insuperable distinction between races and
species. The importance of this subject as bearing on the origin of species is
obvious; and we shall hereafter recur to it.
It is unfortunate how few precise observations have been made on the fertility
of mongrel animals and plants during several successive generations. Dr. Broca
(16/1. 'Journal de Physiolog.' tome 2 1859 page 385.) has remarked that no one
has observed whether, for instance, mongrel dogs, bred inter se, are
indefinitely fertile; yet, if a shade of infertility be detected by careful
observation in the offspring of natural forms when crossed, it is thought that
their specific distinction is proved. But so many breeds of sheep, cattle,
pigs, dogs, and poultry, have been crossed and recrossed in various ways, that
any sterility, if it had existed, would from being injurious almost certainly
have been observed. In investigating the fertility of crossed varieties many
sources of doubt occur. Whenever the least trace of sterility between two
plants, however closely allied, was observed by Kolreuter, and more especially
by Gartner, who counted the exact number of seed in each capsule, the two
forms were at once ranked as distinct species; and if this rule be followed,
assuredly it will never be proved that varieties when crossed are in any
degree sterile. We have formerly seen that certain breeds of dogs do not
readily pair together; but no observations have been made whether, when
paired, they produce the full number of young, and whether the latter are
perfectly fertile inter se; but, supposing that some degree of sterility were
found to exist, naturalists would simply infer that these breeds were
descended from aboriginally distinct species; and it would be scarcely
possible to ascertain whether or not this explanation was the true one.
The Sebright Bantam is much less prolific than any other breed of fowls, and
is descended from a cross between two very distinct breeds, recrossed by a
third sub-variety. But it would be extremely rash to infer that the loss of
fertility was in any manner connected with its crossed origin, for it may with
more probability be attributed either to long-continued close interbreeding,
or to an innate tendency to sterility correlated with the absence of hackles
and sickle tail-feathers.
Before giving the few recorded cases of forms, which must be ranked as
varieties, being in some degree sterile when crossed, I may remark that other
causes sometimes interfere with varieties freely intercrossing. Thus they may
differ too greatly in size, as with some kinds of dogs and fowls: for
instance, the editor of the 'Journal of Horticulture, etc.' (16/2. December
1863 page 484.) says that he can keep Bantams with the larger breeds without
much danger of their crossing, but not with the smaller breeds, such as Games,
Hamburghs, etc. With plants a difference in the period of flowering serves to
keep varieties distinct, as with the various kinds of maize and wheat: thus
Colonel Le Couteur (16/3. On 'The Varieties of Wheat' page 66.) remarks, "the
Talavera wheat, from flowering much earlier than any other kind, is sure to
continue pure." In different parts of the Falkland Islands the cattle are
breaking up into herds of different colours; and those on the higher ground,
which are generally white, usually breed, as I am informed by Sir J. Sulivan,
three months earlier than those on the lowland; and this would manifestly tend
to keep the herds from blending.
Certain domestic races seem to prefer breeding with their own kind; and this
is a fact of some importance, for it is a step towards that instinctive
feeling which helps to keep closely allied species in a state of nature
distinct. We have now abundant evidence that, if it were not for this feeling,
many more hybrids would be naturally produced than in this case. We have seen
in the first chapter that the alco dog of Mexico dislikes dogs of other
breeds; and the hairless dog of Paraguay mixes less readily with the European
races, than the latter do with each other. In Germany the female Spitz-dog is
said to receive the fox more readily than will other dogs; a female Australian
Dingo in England attracted the wild male foxes. But these differences in the
sexual instinct and attractive power of the various breeds may be wholly due
to their descent from distinct species. In Paraguay the horses have much
freedom, and an excellent observer (16/4. Rengger 'Saugethiere von Paraguay'
s. 336.) believes that the native horses of the same colour and size prefer
associating with each other, and that the horses which have been imported from
Entre Rios and Banda Oriental into Paraguay likewise prefer associating
together. In Circassia six sub-races of the horse have received distinct
names; and a native proprietor of rank (16/5. See a memoir by MM. Lherbette
and De Quatrefages in 'Bull. Soc. d'Acclimat.' tome 8 July 1861 page 312.)
asserts that horses of three of these races, whilst living a free life, almost
always refuse to mingle and cross, and will even attack one another.
It has been observed, in a district stocked with heavy Lincolnshire and light
Norfolk sheep, that both kinds; though bred together, when turned out, "in a
short time separate to a sheep;" the Lincolnshires drawing off to the rich
soil, and the Norfolks to their own dry light soil; and as long as there is
plenty of grass, "the two breeds keep themselves as distinct as rooks and
pigeons." In this case different habits of life tend to keep the races
distinct. On one of the Faroe islands, not more than half a mile in diameter,
the half-wild native black sheep are said not to have readily mixed with the
imported white sheep. It is a more curious fact that the semi-monstrous ancon
sheep of modern origin "have been observed to keep together, separating
themselves from the rest of the flock, when put into enclosures with other
sheep." (16/6. For the Norfolk sheep see Marshall 'Rural Economy of Norfolk'
volume 2 page 136. See Rev. L. Landt 'Description of Faroe' page 66. For the
ancon sheep see 'Phil. Transact.' 1813 page 90.) With respect to fallow-deer,
which live in a semi-domesticated condition, Mr. Bennett (16/7. White 'Nat.
Hist. of Selbourne' edited by Bennett page 39. With respect to the origin of
the dark-coloured deer see 'Some Account of English Deer Parks' by E.P.
Shirley, Esq.) states that the dark and pale coloured herds, which have long
been kept together in the Forest of Dean, in High Meadow Woods, and in the New
Forest, have never been known to mingle: the dark-coloured deer, it may be
added, are believed to have been first brought by James I. from Norway, on
account of their greater hardiness. I imported from the island of Porto Santo
two of the feral rabbits, which differ, as described in the fourth chapter,
from common rabbits; both proved to be males, and, though they lived during
some years in the Zoological Gardens, the superintendent, Mr. Bartlett, in
vain endeavoured to make them breed with various tame kinds; but whether this
refusal to breed was due to any change in the instinct, or simply to their
extreme wildness, or whether confinement had rendered them sterile, as often
occurs, cannot be determined.
Whilst matching for the sake of experiment many of the most distinct breeds of
pigeons, it frequently appeared to me that the birds, though faithful to their
marriage vow, retained some desire after their own kind. Accordingly I asked
Mr. Wicking, who has kept a larger stock of various breeds together than any
man in England, whether he thought that they would prefer pairing with their
own kind, supposing that there were males and females enough of each; and he
without hesitation answered that he was convinced that this was the case. It
has often been noticed that the dovecote pigeon seems to have an actual
aversion towards the several fancy breeds (16/8. 'The Dovecote' by the Rev.
E.S. Dixon page 155; Bechstein 'Naturgesch. Deutschlands' b. 4 1795 page 17.)
yet all have certainly sprung from a common progenitor. The Rev. W.D. Fox
informs me that his flocks of white and common Chinese geese kept distinct.
These facts and statements, though some of them are incapable of proof,
resting only on the opinion of experienced observers, show that some domestic
races are led by different habits of life to keep to a certain extent
separate, and that others prefer coupling with their own kind, in the same
manner as species in a state of nature, though in a much less degree.
[With respect to sterility from the crossing of domestic races, I know of no
well-ascertained case with animals. This fact, seeing the great difference in
structure between some breeds of pigeons, fowls, pigs, dogs, etc., is
extraordinary, in contrast with the sterility of many closely allied natural
species when crossed; but we shall hereafter attempt to show that it is not so
extraordinary as it at first appears. And it may be well here to recall to
mind that the amount of external difference between two species is not a safe
guide for predicting whether or not they will breed together,--some closely
allied species when crossed being utterly sterile, and others which are
extremely unlike being moderately fertile. I have said that no case of
sterility in crossed races rests on satisfactory evidence; but here is one
which at first seems trustworthy. Mr. Youatt (16/9. 'Cattle' page 202.) and a
better authority cannot be quoted, states, that formerly in Lancashire crosses
were frequently made between longhorn and shorthorn cattle; the first cross
was excellent, but the produce was uncertain; in the third or fourth
generation the cows were bad milkers; "in addition to which, there was much
uncertainty whether the cows would conceive; and full one-third of the cows
among some of these half-breds failed to be in calf." This at first seems a
good case: but Mr. Wilkinson states (16/10. Mr. J. Wilkinson in 'Remarks
addressed to Sir J. Sebright' 1820 page 38.), that a breed derived from this
same cross was actually established in another part of England; and if it had
failed in fertility, the fact would surely have been noticed. Moreover,
supposing that Mr. Youatt had proved his case, it might be argued that the
sterility was wholly due to the two parent-breeds being descended from
primordially distinct species.
In the case of plants Gartner states that he fertilised thirteen heads (and
subsequently nine others) on a dwarf maize bearing yellow seed (16/11.
'Bastarderzeugung' s. 87, 169. See also the Table at the end of volume.) with
pollen of a tall maize having red seed; and one head alone produced good seed,
but only five in number. Though these plants are monoecious, and therefore do
not require castration, yet I should have suspected some accident in the
manipulation, had not Gartner expressly stated that he had during many years
grown these two varieties together, and they did not spontaneously cross; and
this, considering that the plants are monoecious and abound with pollen, and
are well known generally to cross freely, seems explicable only on the belief
that these two varieties are in some degree mutually infertile. The hybrid
plants raised from the above five seeds were intermediate in structure,
extremely variable, and perfectly fertile. (16/12. 'Bastarderzeugung' s. 87,
577.) In like manner Prof. Hildebrand (16/13. 'Bot. Zeitung' 1868 page 327.)
could not succeed in fertilising the female flowers of a plant bearing brown
grains with pollen from a certain kind bearing yellow grains; although other
flowers on the same plant, which were fertilised with their own pollen,
yielded good seed. No one, I believe, even suspects that these varieties of
maize are distinct species; but had the hybrids been in the least sterile, no
doubt Gartner would at once have so classed them. I may here remark, that with
undoubted species there is not necessarily any close relation between the
sterility of a first cross and that of the hybrid offspring. Some species can
be crossed with facility, but produce utterly sterile hybrids; others can be
crossed with extreme difficulty, but the hybrids when produced are moderately
fertile. I am not aware, however, of any instance quite like this of the
maize, namely, of a first cross made with difficulty, but yielding perfectly
fertile hybrids. (16/14. Mr. Shirreff formerly thought ('Gardener's Chronicle'
1858 page 771) that the offspring from a cross between certain varieties of
wheat became sterile in the fourth generation; but he now admits ('Improvement
of the Cereals' 1873) that this was an error.)
The following case is much more remarkable, and evidently perplexed Gartner,
whose strong wish it was to draw a broad line of distinction between species
and varieties. In the genus Verbascum, he made, during eighteen years, a vast
number of experiments, and crossed no less than 1085 flowers and counted their
seeds. Many of these experiments consisted in crossing white and yellow
varieties of both V. lychnitis and V. blattaria with nine other species and
their hybrids. That the white and yellow flowered plants of these two species
are really varieties, no one has doubted; and Gartner actually raised in the
case of both species one variety from the seed of the other. Now in two of his
works (16/15. 'Kenntniss der Befruchtung' s. 137; 'Bastarderzeugung' s. 92,
181. On raising the two varieties from seed see s. 307.) he distinctly asserts
that crosses between similarly-coloured flowers yield more seed than between
dissimilarly-coloured; so that the yellow-flowered variety of either species
(and conversely with the white-flowered variety), when crossed with pollen of
its own kind, yields more seed than when crossed with that of the white
variety; and so it is when differently coloured species are crossed. The
general results may be seen in the Table at the end of his volume. In one
instance he gives (16/16. 'Bastarderzeugung' s. 216.) the following details;
but I must premise that Gartner, to avoid exaggerating the degree of sterility
in his crosses, always compares the MAXIMUM number obtained from a cross with
the AVERAGE number naturally given by the pure mother-plant. The white variety
of V. lychnitis, naturally fertilised by its own pollen, gave from an AVERAGE
of twelve capsules ninety-six good seeds in each; whilst twenty flowers
fertilised with pollen from the yellow variety of this same species, gave as
the MAXIMUM only eighty-nine good seeds; so that we have the proportion of
1000 to 908, according to Gartner's usual scale. I should have thought it
possible that so small a difference in fertility might have been accounted for
by the evil effects of the necessary castration; but Gartner shows that the
white variety of V. lychnitis, when fertilised first by the white variety of
V. blattaria, and then by the yellow variety of this species, yielded seed in
the proportion of 622 to 438; and in both these cases castration was
performed. Now the sterility which results from the crossing of the
differently coloured varieties of the same species, is fully as great as that
which occurs in many cases when distinct species are crossed. Unfortunately
Gartner compared the results of the first unions alone, and not the sterility
of the two sets of hybrids produced from the white variety of V. lychnitis
when fertilised by the white and yellow varieties of V. blattaria, for it is
probable that they would have differed in this respect.
Mr. J. Scott has given me the results of a series of experiments on Verbascum,
made by him in the Botanic Gardens of Edinburgh. (16/17. The results have
since been published in 'Journ. Asiatic Soc. of Bengal' 1867 page 145.) He
repeated some of Gartner's experiments on distinct species, but obtained only
fluctuating results, some confirmatory, the greater number contradictory;
nevertheless these seem hardly sufficient to overthrow the conclusion arrived
at by Gartner from experiments tried on a larger scale. Mr. Scott also
experimented on the relative fertility of unions between similarly and
dissimilarly-coloured varieties of the same species. Thus he fertilised six
flowers of the yellow variety of V. lychnitis by its own pollen, and obtained
six capsules; and calling, for the sake of comparison, the average number of
good seed in each of their capsules one hundred, he found that this same
yellow variety, when fertilised by the white variety, yielded from seven
capsules an average of ninety-four seed. On the same principle, the white
variety of V. lychnitis by its own pollen (from six capsules), and by the
pollen of the yellow variety (eight capsules), yielded seed in the proportion
of 100 to 82. The yellow variety of V. thapsus by its own pollen (eight
capsules), and by that of the white variety (only two capsules), yielded seed
in the proportion of 100 to 94. Lastly, the white variety of V. blattaria by
its own pollen (eight capsules), and by that of the yellow variety (five
capsules), yielded seed in the proportion of 100 to 79. So that in every case
the unions of similarly-coloured varieties of the same species were more
fertile than the unions of dissimilarly-coloured varieties; when all the cases
are grouped together, the difference of fertility is as 100 to 86. Some
additional trials were made, and altogether thirty-six similarly-coloured
unions yielded thirty-five good capsules; whilst thirty-five dissimilarly-
coloured unions yielded only twenty-six good capsules. Besides the foregoing
experiments, the purple V. phoeniceum was crossed by a rose-coloured and a
white variety of the same species; these two varieties were also crossed
together, and these several unions yielded less seed than V. phoeniceum by its
own pollen. Hence it follows from Mr. Scott's experiments, that in the genus
Verbascum the similarly and dissimilarly-coloured varieties of the same
species behave, when crossed, like closely allied but distinct species.
(16/18. The following facts, given by Kolreuter in his 'Dritte Fortsetzung'
ss. 34, 39, appear at first sight strongly to confirm Mr. Scott's and
Gartner's statements; and to a certain limited extent they do so. Kolreuter
asserts, from innumerable observations, that insects incessantly carry pollen
from one species and variety of Verbascum to another; and I can confirm this
assertion; yet he found that the white and yellow varieties of Verbascum
lychnitis often grew wild mingled together: moreover, he cultivated these two
varieties in considerable numbers during four years in his garden, and they
kept true by seed; but when he crossed them, they produced flowers of an
intermediate tint. Hence it might have been thought that both varieties must
have a stronger elective affinity for the pollen of their own variety than for
that of the other; this elective affinity, I may add of each species for its
own pollen (Kolreuter 'Dritte Forts.' s. 39 and Gartner 'Bastarderz.' passim)
being a perfectly well-ascertained power. But the force of the foregoing facts
is much lessened by Gartner's numerous experiments, for, differently from
Kolreuter, he never once got ('Bastarderz.' s. 307) an intermediate tint when
he crossed the yellow and white flowered varieties of Verbascum. So that the
fact of the white and yellow varieties keeping true to their colour by seed
does not prove that they were not mutually fertilised by the pollen carried by
insects from one to the other.)
This remarkable fact of the sexual affinity of similarly-coloured varieties,
as observed by Gartner and Mr. Scott, may not be of very rare occurrence; for
the subject has not been attended to by others. The following case is worth
giving, partly to show how difficult it is to avoid error. Dr. Herbert (16/19.
'Amaryllidaceae' 1837 page 366. Gartner has made a similar observation.) has
remarked that variously-coloured double varieties of the Hollyhock (Althea
rosea) may be raised with certainty by seed from plants growing close
together. I have been informed that nurserymen who raise seed for sale do not
separate their plants; accordingly I procured seed of eighteen named
varieties; of these, eleven varieties produced sixty-two plants all perfectly
true to their kind; and seven produced forty-nine plants, half of which were
true and half false. Mr. Masters of Canterbury has given me a more striking
case; he saved seed from a great bed of twenty-four named varieties planted in
closely adjoining rows, and each variety reproduced itself truly with only
sometimes a shade of difference in tint. Now in the hollyhock the pollen,
which is abundant, is matured and nearly all shed before the stigma of the
same flower is ready to receive it (16/20. Kolreuter first observed this fact,
'Mem. de l'Acad. de St. Petersburg' volume 3 page 127. See also C.K. Sprengel
'Das Entdeckte Geheimniss' s. 345.); and as bees covered with pollen
incessantly fly from plant to plant, it would appear that adjoining varieties
could not escape being crossed. As, however, this does not occur, it appeared
to me probable that the pollen of each variety was prepotent on its own stigma
over that of all other varieties, but I have no evidence on this point. Mr. C.
Turner of Slough, well known for his success in the cultivation of this plant,
informs me that it is the doubleness of the flowers which prevents the bees
gaining access to the pollen and stigma; and he finds that it is difficult
even to cross them artificially. Whether this explanation will fully account
for varieties in close proximity propagating themselves so truly by seed, I do
not know.
The following cases are worth giving, as they relate to monoecious forms,
which do not require, and consequently cannot have been injured by,
castration. Girou de Buzareingues crossed what he designates three varieties
of gourd (16/21. Namely Barbarines, Pastissons, Giraumous: 'Annal. des Sc.
Nat.' tome 30 1833 pages 398 and 405.), and asserts that their mutual
fertilisation is less easy in proportion to the difference which they present.
I am aware how imperfectly the forms in this group were until recently known;
but Sageret (16/22. 'Memoire sur les Cucurbitaceae' 1826 pages 46, 55.), who
ranked them according to their mutual fertility, considers the three forms
above alluded to as varieties, as does a far higher authority, namely, M.
Naudin. (16/23. 'Annales des Sc. Nat.' 4th series tome 6. M. Naudin considers
these forms as undoubtedly varieties of Cucurbita pepo.) Sageret (16/24. 'Mem.
Cucurb.' page 8.) has observed that certain melons have a greater tendency,
whatever the cause may be, to keep true than others; and M. Naudin, who has
had such immense experience in this group, informs me that he believes that
certain varieties intercross more readily than others of the same species; but
he has not proved the truth of this conclusion; the frequent abortion of the
pollen near Paris being one great difficulty. Nevertheless, he has grown close
together, during seven years, certain forms of Citrullus, which, as they could
be artificially crossed with perfect facility and produced fertile offspring,
are ranked as varieties; but these forms when not artificially crossed kept
true. Many other varieties, on the other hand, in the same group cross with
such facility, as M. Naudin repeatedly insists, that without being grown far
apart they cannot be kept in the least true.
Another case, though somewhat different, may be here given, as it is highly
remarkable, and is established on excellent evidence. Kolreuter minutely
describes five varieties of the common tobacco (16/25. 'Zweite Forts.' s. 53
namely Nicotiana major vulgaris; (2) perennis; (3) transylvanica; (4) a sub-
var. of the last; (5) major latifol. fl. alb.) which were reciprocally
crossed, and the offspring were intermediate in character and as fertile as
their parents: from this fact Kolreuter inferred that they are really
varieties; and no one, as far as I can discover, seems to have doubted that
such is the case. He also crossed reciprocally these five varieties with N.
glutinosa, and they yielded very sterile hybrids; but those raised from the
var. perennis, whether used as the father or mother plant, were not so sterile
as the hybrids from the four other varieties. (16/26. Kolreuter was so much
struck with this fact that he suspected that a little pollen of N. glutinosa
in one of his experiments might have accidentally got mingled with that of
var. perennis, and thus aided its fertilising power. But we now know
conclusively from Gartner ('Bastarderz.' s. 34, 43) that the pollen of two
species never acts CONJOINTLY on a third species; still less will the pollen
of a distinct species, mingled with a plant's own pollen, if the latter be
present in sufficient quantity, have any effect. The sole effect of mingling
two kinds of pollen is to produce in the same capsule seeds which yield
plants, some taking after the one and some after the other parent.) So that
the sexual capacity of this one variety has certainly been in some degree
modified, so as to approach in nature that of N. glutinosa. (16/27. Mr. Scott
has made some observations on the absolute sterility of a purple and white
primrose (Primula vulgaris) when fertilised by pollen from the common primrose
('Journal of Proc. of Linn. Soc.' volume 8 1864 page 98); but these
observations require confirmation. I raised a number of purple-flowered long-
styled seedlings from seed kindly sent me by Mr. Scott, and, though they were
all in some degree sterile, they were much more fertile with pollen taken from
the common primrose than with their own pollen. Mr. Scott has likewise
described a red equal-styled cowslip (P. veris ibid page 106), which was found
by him to be highly sterile when crossed with the common cowslip; but this was
not the case with several equal-styled red seedlings raised by me from his
plant. This variety of the cowslip presents the remarkable peculiarity of
combining male organs in every respect like those of the short-styled form,
with female organs resembling in function and partly in structure those of the
long-styled form; so that we have the singular anomaly of the two forms
combined in the same flower. Hence it is not surprising that these flowers
should be spontaneously self-fertile in a high degree.)
These facts with respect to plants show that in some few cases certain
varieties have had their sexual powers so far modified, that they cross
together less readily and yield less seed than other varieties of the same
species. We shall presently see that the sexual functions of most animals and
plants are eminently liable to be affected by the conditions of life to which
they are exposed; and hereafter we shall briefly discuss the conjoint bearing
of this fact, and others, on the difference in fertility between crossed
varieties and crossed species.
DOMESTICATION ELIMINATES THE TENDENCY TO STERILITY WHICH IS GENERAL WITH
SPECIES WHEN CROSSED.
This hypothesis was first propounded by Pallas (16/28. 'Act. Acad. St.
Petersburg' 1780 part 2 pages 84, 100.), and has been adopted by several
authors. I can find hardly any direct facts in its support; but unfortunately
no one has compared, in the case of either animals or plants, the fertility of
anciently domesticated varieties, when crossed with a distinct species, with
that of the wild parent-species when similarly crossed. No one has compared,
for instance, the fertility of Gallus bankiva and of the domesticated fowl,
when crossed with a distinct species of Gallus or Phasianus; and the
experiment would in all cases be surrounded by many difficulties. Dureau de la
Malle, who has so closely studied classical literature, states (16/29.
'Annales des Sc. Nat.' tome 21 1st series page 61.) that in the time of the
Romans the common mule was produced with more difficulty than at the present
day; but whether this statement may be trusted I know not. A much more
important, though somewhat different, case is given by M. Groenland (16/30.
'Bull. Bot. Soc. de France' December 27, 1861 tome 8 page 612.), namely, that
plants, known from their intermediate character and sterility to be hybrids
between Aegilops and wheat, have perpetuated themselves under culture since
1857, WITH A RAPID BUT VARYING INCREASE OF FERTILITY IN EACH GENERATION. In
the fourth generation the plants, still retaining their intermediate
character, had become as fertile as common cultivated wheat.
The indirect evidence in favour of the Pallasian doctrine appears to me to be
extremely strong. In the earlier chapters I have shown that our various breeds
of the dog are descended from several wild species; and this probably is the
case with sheep. There can be no doubt that the Zebu or humped Indian ox
belongs to a distinct species from European cattle: the latter, moreover, are
descended from two forms, which may be called either species or races. We have
good evidence that our domesticated pigs belong to at least two specific
types, S. scrofa and indicus. Now a widely extended analogy leads to the
belief that if these several allied species, when first reclaimed, had been
crossed, they would have exhibited, both in their first unions and in their
hybrid offspring, some degree of sterility. Nevertheless, the several
domesticated races descended from them are now all, as far as can be
ascertained, perfectly fertile together. If this reasoning be trustworthy, and
it is apparently sound, we must admit the Pallasian doctrine that long-
continued domestication tends to eliminate that sterility which is natural to
species when crossed in their aboriginal state.
ON INCREASED FERTILITY FROM DOMESTICATION AND CULTIVATION.
Increased fertility from domestication, without any reference to crossing, may
be here briefly considered. This subject bears indirectly on two or three
points connected with the modification of organic beings. As Buffon long ago
remarked (16/31. Quoted by Isid. Geoffroy St. Hilaire 'Hist. Naturelle
Generale' tome 3 page 476. Since this MS. has been sent to press a full
discussion on the present subject has appeared in Mr. Herbert Spencer's
'Principles of Biology' volume 2 1867 page 457 et seq.), domestic animals
breed oftener in the year and produce more young at a birth than wild animals
of the same species; they, also, sometimes breed at an earlier age. The case
would hardly have deserved further notice, had not some authors lately
attempted to show that fertility increases and decreases in an inverse ratio
with the amount of food. This strange doctrine has apparently arisen from
individual animals when supplied with an inordinate quantity of food, and from
plants of many kinds when grown on excessively rich soil, as on a dunghill,
becoming sterile: but to this latter point I shall have occasion presently to
return. With hardly an exception, our domesticated animals, which have been
long habituated to a regular and copious supply of food, without the labour of
searching for it, are more fertile than the corresponding wild animals. It is
notorious how frequently cats and dogs breed, and how many young they produce
at a birth. The wild rabbit is said generally to breed four times yearly, and
to produce each time at most six young; the tame rabbit breeds six or seven
times yearly, producing each time from four to eleven young; and Mr. Harrison
Weir tells me of a case of eighteen young having been produced at a birth, all
of which survived. The ferret, though generally so closely confined, is more
prolific than its supposed wild prototype. The wild sow is remarkably
prolific; she often breeds twice in the year, and bears from four to eight and
sometimes even twelve young; but the domestic sow regularly breeds twice a
year, and would breed oftener if permitted; and a sow that produces less than
eight at a birth "is worth little, and the sooner she is fattened for the
butcher the better." The amount of food affects the fertility of the same
individual: thus sheep, which on mountains never produce more than one lamb at
a birth, when brought down to lowland pastures frequently bear twins. This
difference apparently is not due to the cold of the higher land, for sheep and
other domestic animals are said to be extremely prolific in Lapland. Hard
living, also, retards the period at which animals conceive; for it has been
found disadvantageous in the northern islands of Scotland to allow cows to
bear calves before they are four years old. (16/32. For cats and dogs etc. see
Bellingeri in 'Annal. des Sc. Nat.' 2nd series, Zoolog. tome 12 page 155. For
ferrets Bechstein 'Naturgeschichte Deutschlands' b. 1 1801 s. 786, 795. For
rabbits ditto s. 1123, 1131; and Bronn 'Geschichte der Natur.' b. 2 s. 99. For
mountain sheep ditto s. 102. For the fertility of the wild sow, see Bechstein
'Naturgesch. Deutschlands' b. 1 1801 s. 534; for the domestic pig Sidney's
edition of 'Youatt on the Pig' 1860 page 62. With respect to Lapland see
Acerbi 'Travels to the North Cape' English translation volume 2 page 222.
About the Highland cows see 'Hogg on Sheep' page 263.)
[Birds offer still better evidence of increased fertility from domestication:
the hen of the wild Gallus bankiva lays from six to ten eggs, a number which
would be thought nothing of with the domestic hen. The wild duck lays from
five to ten eggs; the tame one in the course of the year from eighty to one
hundred. The wild grey-lag goose lays from five to eight eggs; the tame from
thirteen to eighteen, and she lays a second time; as Mr. Dixon has remarked,
"high-feeding, care, and moderate warmth induce a habit of prolificacy which
becomes in some measure hereditary." Whether the semi-domesticated dovecote
pigeon is more fertile than the wild rock-pigeon, C. livia, I know not; but
the more thoroughly domesticated breeds are nearly twice as fertile as
dovecotes: the latter, however, when caged and highly fed, become equally
fertile with house pigeons. I hear from Judge Caton that the wild turkey in
the United States does not breed when a year old, as the domesticated turkeys
there invariably do. The peahen alone of domesticated birds is rather more
fertile, according to some accounts, when wild in its native Indian home, than
in Europe when exposed to our much colder climate. (16/33. For the eggs of
Gallus bankiva see Blyth in 'Annals and Mag. of Nat. Hist.' 2nd series volume
1 1848 page 456. For wild and tame ducks Macgillivray 'British Birds' volume 5
page 37; and 'Die Enten' s. 87. For wild geese L. Lloyd 'Scandinavian
Adventures' volume 2 1854 page 413; and for tame geese 'Ornamental Poultry' by
Rev. E.S. Dixon page 139. On the breeding of Pigeons Pistor 'Das Ganze der
Taubenzucht' 1831 s. 48; and Boitard and Corbie 'Les Pigeons' page 158. With
respect to peacocks, according to Temminck 'Hist. Nat. Gen. des Pigeons' etc.
1813 tome 2 page 41, the hen lays in India even as many as twenty eggs; but
according to Jerdon and another writer quoted in Tegetmeier 'Poultry Book'
1866 pages 280, 282, she there lays only from four to nine or ten eggs: in
England she is said, in the 'Poultry Book' to lay five or six, but another
writer says from eight to twelve eggs.)
With respect to plants, no one would expect wheat to tiller more, and each ear
to produce more grain, in poor than in rich soil; or to get in poor soil a
heavy crop of peas or beans. Seeds vary so much in number that it is difficult
to estimate them; but on comparing beds of carrots in a nursery garden with
wild plants, the former seemed to produce about twice as much seed. Cultivated
cabbages yielded thrice as many pods by measure as wild cabbages from the
rocks of South Wales. The excess of berries produced by the cultivated
asparagus in comparison with the wild plant is enormous. No doubt many highly
cultivated plants, such as pears, pineapples, bananas, sugar-cane, etc., are
nearly or quite sterile; and I am inclined to attribute this sterility to
excess of food and to other unnatural conditions; but to this subject I shall
recur.]
In some cases, as with the pig, rabbit, etc., and with those plants which are
valued for their seed, the direct selection of the more fertile individuals
has probably much increased their fertility; and in all cases this may have
occurred indirectly, from the better chance of some of the numerous offspring
from the more fertile individuals having been preserved. But with cats,
ferrets, and dogs, and with plants like carrots, cabbages, and asparagus,
which are not valued for their prolificacy, selection can have played only a
subordinate part; and their increased fertility must be attributed to the more
favourable conditions of life under which they have long existed.
CHAPTER 2.XVII.
ON THE GOOD EFFECTS OF CROSSING, AND ON THE EVIL EFFECTS OF CLOSE
INTERBREEDING.
DEFINITION OF CLOSE INTERBREEDING.
AUGMENTATION OF MORBID TENDENCIES.
GENERAL EVIDENCE OF THE GOOD EFFECTS DERIVED FROM CROSSING, AND ON THE EVIL
EFFECTS FROM CLOSE INTERBREEDING.
CATTLE, CLOSELY INTERBRED; HALF-WILD CATTLE LONG KEPT IN THE SAME PARKS.
SHEEP.
FALLOW-DEER.
DOGS, RABBITS, PIGS.
MAN, ORIGIN OF HIS ABHORRENCE OF INCESTUOUS MARRIAGES.
FOWLS.
PIGEONS.
HIVE-BEES.
PLANTS, GENERAL CONSIDERATIONS ON THE BENEFITS DERIVED FROM CROSSING.
MELONS, FRUIT-TREES, PEAS, CABBAGES, WHEAT, AND FOREST-TREES.
ON THE INCREASED SIZE OF HYBRID PLANTS, NOT EXCLUSIVELY DUE TO THEIR
STERILITY.
ON CERTAIN PLANTS WHICH EITHER NORMALLY OR ABNORMALLY ARE SELF-IMPOTENT, BUT
ARE FERTILE, BOTH ON THE MALE AND FEMALE SIDE, WHEN CROSSED WITH DISTINCT
INDIVIDUALS EITHER OF THE SAME OR ANOTHER SPECIES.
CONCLUSION.
The gain in constitutional vigour, derived from an occasional cross between
individuals of the same variety, but belonging to distinct families, or
between distinct varieties, has not been so largely or so frequently
discussed, as have the evil effects of too close interbreeding. But the former
point is the more important of the two, inasmuch as the evidence is more
decisive. The evil results from close interbreeding are difficult to detect,
for they accumulate slowly, and differ much in degree with different species;
whilst the good effects which almost invariably follow a cross are from the
first manifest. It should, however, be clearly understood that the advantage
of close interbreeding, as far as the retention of character is concerned, is
indisputable, and often outweighs the evil of a slight loss of constitutional
vigour. In relation to the subject of domestication, the whole question is of
some importance, as too close interbreeding interferes with the improvement of
old races. It is important as indirectly bearing on Hybridism; and possibly on
the extinction of species, when any form has become so rare that only a few
individuals remain within a confined area. It bears in an important manner on
the influence of free intercrossing, in obliterating individual differences,
and thus giving uniformity of character to the individuals of the same race or
species; for if additional vigour and fertility be thus gained, the crossed
offspring will multiply and prevail, and the ultimate result will be far
greater than otherwise would have occurred. Lastly, the question is of high
interest, as bearing on mankind. I shall therefore discuss this subject at
full length. As the facts which prove the evil effects of close interbreeding
are more copious, though less decisive, than those on the good effects of
crossing, I shall, under each group of beings, begin with the former.
There is no difficulty in defining what is meant by a cross; but this is by no
means easy in regard to "breeding in and in" or "too close interbreeding,"
because, as we shall see, different species of animals are differently
affected by the same degree of interbreeding. The pairing of a father and
daughter, or mother and son, or brothers and sisters, if carried on during
several generations, is the closest possible form of interbreeding. But some
good judges, for instance Sir J. Sebright, believe that the pairing of a
brother and sister is much closer than that of parents and children; for when
the father is matched with his daughter he crosses, as is said, with only half
his own blood. The consequences of close interbreeding carried on for too long
a time, are, as is generally believed, loss of size, constitutional vigour,
and fertility, sometimes accompanied by a tendency to malformation. Manifest
evil does not usually follow from pairing the nearest relations for two,
three, or even four generations; but several causes interfere with our
detecting the evil--such as the deterioration being very gradual, and the
difficulty of distinguishing between such direct evil and the inevitable
augmentation of any morbid tendencies which may be latent or apparent in the
related parents. On the other hand, the benefit from a cross, even when there
has not been any very close interbreeding, is almost invariably at once
conspicuous. There is good reason to believe, and this was the opinion of that
most experienced observer Sir J. Sebright (17/1. 'The Art of Improving the
Breed, etc.' 1809 page 16.), that the evil effects of close interbreeding may
be checked or quite prevented by the related individuals being separated for a
few generations and exposed to different conditions of life. This conclusion
is now held by many breeders; for instance Mr. Carr (17/2. 'The History of the
Rise and Progress of the Killerby, etc. Herds' page 41.) remarks, it is a
well-known "fact that a change of soil and climate effects perhaps almost as
great a change in the constitution as would result from an infusion of fresh
blood." I hope to show in a future work that consanguinity by itself counts
for nothing, but acts solely from related organisms generally having a similar
constitution, and having been exposed in most cases to similar conditions.
That any evil directly follows from the closest interbreeding has been denied
by many persons; but rarely by any practical breeder; and never, as far as I
know, by one who has largely bred animals which propagate their kind quickly.
Many physiologists attribute the evil exclusively to the combination and
consequent increase of morbid tendencies common to both parents; and that this
is an active source of mischief there can be no doubt. It is unfortunately too
notorious that men and various domestic animals endowed with a wretched
constitution, and with a strong hereditary disposition to disease, if not
actually ill, are fully capable of procreating their kind. Close
interbreeding, on the other hand, often induces sterility; and this indicates
something quite distinct from the augmentation of morbid tendencies common to
both parents. The evidence immediately to be given convinces me that it is a
great law of nature, that all organic beings profit from an occasional cross
with individuals not closely related to them in blood; and that, on the other
hand, long-continued close interbreeding is injurious.
Various general considerations have had much influence in leading me to this
conclusion; but the reader will probably rely more on special facts and
opinions. The authority of experienced observers, even when they do not
advance the grounds of their belief, is of some little value. Now almost all
men who have bred many kinds of animals and have written on the subject, such
as Sir J. Sebright, Andrew Knight, etc. (17/3. For Andrew Knight see A. Walker
on 'Intermarriage' 1838 page 227. Sir J. Sebright 'Treatise' has just been
quoted.), have expressed the strongest conviction on the impossibility of
long-continued close interbreeding. Those who have compiled works on
agriculture, and have associated much with breeders, such as the sagacious
Youatt, Low, etc., have strongly declared their opinion to the same effect.
Prosper Lucas, trusting largely to French authorities, has come to a similar
conclusion. The distinguished German agriculturist Hermann von Nathusius, who
has written the most able treatise on this subject which I have met with,
concurs; and as I shall have to quote from this treatise, I may state that
Nathusius is not only intimately acquainted with works on agriculture in all
languages, and knows the pedigrees of our British breeds better than most
Englishmen, but has imported many of our improved animals, and is himself an
experienced breeder.
Evidence of the evil effects of close interbreeding can most readily be
acquired in the case of animals, such as fowls, pigeons, etc., which propagate
quickly, and, from being kept in the same place, are exposed to the same
conditions. Now I have inquired of very many breeders of these birds, and I
have hitherto not met with a single man who was not thoroughly convinced that
an occasional cross with another strain of the same sub-variety was absolutely
necessary. Most breeders of highly improved or fancy birds value their own
strain, and are most unwilling, at the risk, in their opinion, of
deterioration, to make a cross. The purchase of a first-rate bird of another
strain is expensive, and exchanges are troublesome; yet all breeders, as far
as I can hear, excepting those who keep large stocks at different places for
the sake of crossing, are driven after a time to take this step.
Another general consideration which has had great influence on my mind is,
that with all hermaphrodite animals and plants, which it might have been
thought would have perpetually fertilised themselves and been thus subjected
for long ages to the closest interbreeding, there is not a single species, as
far as I can discover, in which the structure ensures self-fertilisation. On
the contrary, there are in a multitude of cases, as briefly stated in the
fifteenth chapter, manifest adaptations which favour or inevitably lead to an
occasional cross between one hermaphrodite and another of the same species;
and these adaptive structures are utterly purposeless, as far as we can see,
for any other end.
[With CATTLE there can be no doubt that extremely close interbreeding may be
long carried on advantageously with respect to external characters, and with
no manifest evil as far as constitution is concerned. The case of Bakewell's
Longhorns, which were closely interbred for a long period, has often been
quoted; yet Youatt says (17/4. 'Cattle' page 199.) the breed "had acquired a
delicacy of constitution inconsistent with common management," and "the
propagation of the species was not always certain." But the Shorthorns offer
the most striking case of close interbreeding; for instance, the famous bull
Favourite (who was himself the offspring of a half-brother and sister from
Foljambe) was matched with his own daughter, granddaughter, and great-
granddaughter; so that the produce of this last union, or the great-great-
granddaughter, had 15-16ths, or 93.75 per cent of the blood of Favourite in
her veins. This cow was matched with the bull Wellington, having 62.5 per cent
of Favourite blood in his veins, and produced Clarissa; Clarissa was matched
with the bull Lancaster, having 68.75 of the same blood, and she yielded
valuable offspring. (17/5. I give this on the authority of Nathusius 'Ueber
Shorthorn Rindvieh' 1857 s. 71, see also 'Gardeners Chronicle' 1860 page 270.
But Mr. J. Storer, a large breeder of cattle, informs me that the parentage of
Clarissa is not well authenticated. In the first volume of the 'Herd Book' she
was entered as having six descents from Favourite, "which was a palpable
mistake," and in all subsequent editions she was spoken of as having only four
descents. Mr. Storer doubts even about the four, as no names of the dams are
given. Moreover, Clarissa bore "only two bulls and one heifer, and in the next
generation her progeny became extinct." Analogous cases of close interbreeding
are given in a pamphlet published by Mr. C. Macknight and Dr. H. Madden 'On
the True Principles of Breeding' Melbourne Australia 1865.) Nevertheless
Collings, who reared these animals, and was a strong advocate for close
breeding, once crossed his stock with a Galloway, and the cows from this cross
realised the highest prices. Bates's herd was esteemed the most celebrated in
the world. For thirteen years he bred most closely in and in; but during the
next seventeen years, though he had the most exalted notion of the value of
his own stock, he thrice infused fresh blood into his herd: it is said that he
did this, not to improve the form of his animals, but on account of their
lessened fertility. Mr. Bates's own view, as given by a celebrated breeder
(17/6. Mr. Willoughby Wood in 'Gardener's Chronicle' 1855 page 411; and 1860
page 270. See the very clear tables and pedigrees given in Nathusius
'Rindvieh' s. 72-77.), was, that "to breed in-and-in from a bad stock was ruin
and devastation; yet that the practice may be safely followed within certain
limits when the parents so related are descended from first-rate animals." We
thus see that there has been much close interbreeding with Shorthorns; but
Nathusius, after the most careful study of their pedigrees, says that he can
find no instance of a breeder who has strictly followed this practice during
his whole life. From this study and his own experience, he concludes that
close interbreeding is necessary to ennoble the stock; but that in effecting
this the greatest care is necessary, on account of the tendency to infertility
and weakness. It may be added, that another high authority (17/7. Mr. Wright
'Journal of Royal Agricult. Soc.' volume 7 1846 page 204. Mr. J. Downing (a
successful breeder of Shorthorns in Ireland) informs me that the raisers of
the great families of Shorthorns carefully conceal their sterility and want of
constitution. He adds that Mr. Bates, after he had bred his herd in-and-in for
some years, "lost in one season twenty-eight calves solely from want of
constitution.") asserts that many more calves are born cripples from
Shorthorns than from other and less closely interbred races of cattle.
Although by carefully selecting the best animals (as Nature effectually does
by the law of battle) close interbreeding may be long carried on with cattle,
yet the good effects of a cross between almost any two breeds is at once shown
by the greater size and vigour of the offspring; as Mr. Spooner writes to me,
"crossing distinct breeds certainly improves cattle for the butcher." Such
crossed animals are of course of no value to the breeder; but they have been
raised during many years in several parts of England to be slaughtered (17/8.
'Youatt on Cattle' page 202.); and their merit is now so fully recognised,
that at fat-cattle shows a separate class has been formed for their reception.
The best fat ox at the great show at Islington in 1862 was a crossed animal.
The half-wild cattle, which have been kept in British parks probably for 400
or 500 years, or even for a longer period, have been advanced by Culley and
others as a case of long-continued interbreeding within the limits of the same
herd without any consequent injury. With respect to the cattle at Chillingham,
the late Lord Tankerville owned that they were bad breeders. (17/9. 'Report
British Assoc. Zoolog. Sect.' 1838.) The agent, Mr. Hardy, estimates (in a
letter to me, dated May, 1861) that in the herd of about fifty the average
number annually slaughtered, killed by fighting, and dying, is about ten, or
one in five. As the herd is kept up to nearly the same average number, the
annual rate of increase must be likewise about one in five. The bulls, I may
add, engage in furious battles, of which battles the present Lord Tankerville
has given me a graphic description, so that there will always be rigorous
selection of the most vigorous males. I procured in 1855 from Mr. D. Gardner,
agent to the Duke of Hamilton, the following account of the wild cattle kept
in the Duke's park in Lanarkshire, which is about 200 acres in extent. The
number of cattle varies from sixty-five to eighty; and the number annually
killed (I presume by all causes) is from eight to ten; so that the annual rate
of increase can hardly be more than one in six. Now in South America, where
the herds are half-wild, and therefore offer a nearly fair standard of
comparison, according to Azara the natural increase of the cattle on an
estancia is from one-third to one-fourth of the total number, or one in
between three and four and this, no doubt, applies exclusively to adult
animals fit for consumption. Hence the half-wild British cattle which have
long interbred within the limits of the same herd are relatively far less
fertile. Although in an unenclosed country like Paraguay there must be some
crossing between the different herds, yet even there the inhabitants believe
that the occasional introduction of animals from distant localities is
necessary to prevent "degeneration in size and diminution of fertility."
(17/10. Azara 'Quadrupedes du Paraguay' tome 2 pages 354, 368.) The decrease
in size from ancient times in the Chillingham and Hamilton cattle must have
been prodigious, for Professor Rutimeyer has shown that they are almost
certainly the descendants of the gigantic Bos primigenius. No doubt this
decrease in size may be largely attributed to less favourable conditions of
life; yet animals roaming over large parks, and fed during severe winters, can
hardly be considered as placed under very unfavourable conditions.
With SHEEP there has often been long-continued interbreeding within the limits
of the same flock; but whether the nearest relations have been matched so
frequently as in the case of Shorthorn cattle, I do not know. The Messrs.
Brown during fifty years have never infused fresh blood into their excellent
flock of Leicesters. Since 1810 Mr. Barford has acted on the same principle
with the Foscote flock. He asserts that half a century of experience has
convinced him that when two nearly related animals are quite sound in
constitution, in-and-in breeding does not induce degeneracy; but he adds that
he "does not pride himself on breeding from the nearest affinities." In France
the Naz flock has been bred for sixty years without the introduction of a
single strange ram. (17/11. For the case of the Messrs. Brown see 'Gardener's
Chronicle' 1855 page 26. For the Foscote flock 'Gardener's Chronicle' 1860
page 416. For the Naz flock 'Bull. de la Soc. d'Acclimat.' 1860 page 477.)
Nevertheless, most great breeders of sheep have protested against close
interbreeding prolonged for too great a length of time. (17/12. Nathusius
'Rindvieh' s. 65; 'Youatt on Sheep' page 495.) The most celebrated of recent
breeders, Jonas Webb, kept five separate families to work on, thus "retaining
the requisite distance of relationship between the sexes" (17/13. 'Gardener's
Chronicle' 1861 page 631.); and what is probably of greater importance, the
separate flocks will have been exposed to somewhat different conditions.
Although by the aid of careful selection the near interbreeding of sheep may
be long continued without any manifest evil, yet it has often been the
practice with farmers to cross distinct breeds to obtain animals for the
butcher, which plainly shows that good of some kind is derived from this
practice. We have excellent evidence on this head from Mr. S. Druce (17/14.
'Journal R. Agricult. Soc.' volume 14 1853 page 212.), who gives in detail the
comparative numbers of four pure breeds and of a cross-breed which can be
supported on the same ground, and he gives their produce in fleece and
carcase. A high authority, Mr. Pusey, sums up the result in money value during
an equal length of time, namely (neglecting shillings), for Cotswolds 248
pounds, for Leicesters 223 pounds, for Southdowns 204 pounds, for Hampshire
Downs 264 pounds, and for the crossbred 293 pounds. A former celebrated
breeder, Lord Somerville, states that his half-breeds from Ryelands and
Spanish sheep were larger animals than either the pure Ryelands or pure
Spanish sheep. Mr. Spooner concludes his excellent Essay on Crossing by
asserting that there is a pecuniary advantage in judicious cross-breeding,
especially when the male is larger than the female. (17/15. Lord Somerville
'Facts on Sheep and Husbandry' page 6. Mr. Spooner in 'Journal of Royal
Agricult. Soc. of England' volume 20 part 2. See also an excellent paper on
the same subject in 'Gardener's Chronicle' 1860 page 321 by Mr. Charles
Howard.)
As some of our British parks are ancient, it occurred to me that there must
have been long-continued close interbreeding with the fallow-deer (Cervus
dama) kept in them; but on inquiry I find that it is a common practice to
infuse new blood by procuring bucks from other parks. Mr. Shirley (17/16.
'Some Account of English Deer Parks' by Evelyn P. Shirley 1867.), who has
carefully studied the management of deer, admits that in some parks there has
been no admixture of foreign blood from a time beyond the memory of man. But
he concludes "that in the end the constant breeding in-and-in is sure to tell
to the disadvantage of the whole herd, though it may take a very long time to
prove it; moreover, when we find, as is very constantly the case, that the
introduction of fresh blood has been of the very greatest use to deer, both by
improving their size and appearance, and particularly by being of service in
removing the taint of 'rickback,' if not of other diseases, to which deer are
sometimes subject when the blood has not been changed, there can, I think, be
no doubt but that a judicious cross with a good stock is of the greatest
consequence, and is indeed essential, sooner or later, to the prosperity of
every well-ordered park."
Mr. Meynell's famous foxhounds have been adduced, as showing that no ill
effects follow from close interbreeding; and Sir J. Sebright ascertained from
him that he frequently bred from father and daughter, mother and son, and
sometimes even from brothers and sisters. With greyhounds also there has been
much close interbreeding, but the best breeders agree that it may be carried
too far. (17/17. Stonehenge 'The Dog' 1867 pages 175-188.) But Sir J. Sebright
declares (17/18. 'The Art of Improving the Breed' etc. page 13. With respect
to Scotch deerhounds see Scrope 'Art of Deer Stalking' pages 350-353.), that
by breeding in-and-in, by which he means matching brothers and sisters, he has
actually seen the offspring of strong spaniels degenerate into weak and
diminutive lapdogs. The Rev. W.D. Fox has communicated to me the case of a
small lot of bloodhounds, long kept in the same family, which had become very
bad breeders, and nearly all had a bony enlargement in the tail. A single
cross with a distinct strain of bloodhounds restored their fertility, and
drove away the tendency to malformation in the tail. I have heard the
particulars of another case with bloodhounds, in which the female had to be
held to the male. Considering how rapid is the natural increase of the dog, it
is difficult to understand the large price of all highly improved breeds,
which almost implies long-continued close interbreeding, except on the belief
that this process lessens fertility and increases liability to distemper and
other diseases. A high authority, Mr. Scrope, attributes the rarity and
deterioration in size of the Scotch deerhound (the few individuals formerly
existing throughout the country being all related) in large part to close
interbreeding.
With all highly-bred animals there is more or less difficulty in getting them
to procreate quickly, and all suffer much from delicacy of constitution. A
great judge of rabbits (17/19. 'Cottage Gardener' 1861 page 327.) says, "the
long-eared does are often too highly bred or forced in their youth to be of
much value as breeders, often turning out barren or bad mothers." They often
desert their young, so that it is necessary to have nurse-rabbits, but I do
not pretend to attribute all these evil results to close interbreeding.
(17/20. Mr. Huth gives ('The Marriage of Near Kin' 1875 page 302) from the
'Bulletin de l'Acad. R. de Med. de Belgique' (volume 9 1866 pages 287, 305),
several statements made by a M. Legrain with respect to crossing brother and
sister rabbits for five or six successive generations with no consequent evil
results. I was so much surprised at this account, and at M. Legrain's
invariable success in his experiments, that I wrote to a distinguished
naturalist in Belgium to inquire whether M. Legrain was a trustworthy
observer. In answer, I have heard that, as doubts were expressed about the
authenticity of these experiments, a commission of inquiry was appointed, and
that at a succeeding meeting of the Society ('Bull. de l'Acad. R. de Med. de
Belgique' 1867 3rd series tome 1 no. 1 to 5), Dr. Crocq reported "qu'il etait
materiellement impossible que M. Legrain ait fait les experiences qu'il
annonce." To this public accusation no satisfactory answer was made.)
With respect to PIGS there is more unanimity amongst breeders on the evil
effects of close interbreeding than, perhaps, with any other large animal. Mr.
Druce, a great and successful breeder of the Improved Oxfordshires (a crossed
race), writes, "without a change of boars of a different tribe, but of the
same breed, constitution cannot be preserved." Mr. Fisher Hobbs, the raiser of
the celebrated Improved Essex breed, divided his stock into three separate
families, by which means he maintained the breed for more than twenty years,
"by judicious selection from the THREE DISTINCT FAMILIES." (17/21. Sidney's
edition of 'Youatt on the Pig' 1860 page 30; page 33 quotation from Mr. Druce;
page 29 on Lord Western's case.) Lord Western was the first importer of a
Neapolitan boar and sow. "From this pair he bred in-and-in, until the breed
was in danger of becoming extinct, a sure result (as Mr. Sidney remarks) of
in-and-in breeding." Lord Western then crossed his Neapolitan pigs with the
old Essex, and made the first great step towards the Improved Essex breed.
Here is a more interesting case. Mr. J. Wright, well known as a breeder,
crossed (17/22. 'Journal of Royal Agricult. Soc. of England' 1846 volume 7
page 205.) the same boar with the daughter, granddaughter, and great-
granddaughter, and so on for seven generations. The result was, that in many
instances the offspring failed to breed; in others they produced few that
lived; and of the latter many were idiotic, without sense, even to suck, and
when attempting to move could not walk straight. Now it deserves especial
notice, that the two last sows produced by this long course of interbreeding
were sent to other boars, and they bore several litters of healthy pigs. The
best sow in external appearance produced during the whole seven generations
was one in the last stage of descent; but the litter consisted of this one
sow. She would not breed to her sire, yet bred at the first trial to a
stranger in blood. So that, in Mr. Wright's case, long-continued and extremely
close interbreeding did not affect the external form or merit of the young;
but with many of them the general constitution and mental powers, and
especially the reproductive functions, were seriously affected.
Nathusius gives (17/23. 'Ueber Rindvieh' etc. s. 78. Col. Le Couteur, who has
done so much for the agriculture of Jersey, writes to me that from possessing
a fine breed of pigs he bred them very closely, twice pairing brothers and
sisters, but nearly all the young had fits and died suddenly.) an analogous
and even more striking case: he imported from England a pregnant sow of the
large Yorkshire breed, and bred the product closely in-and-in for three
generations: the result was unfavourable, as the young were weak in
constitution, with impaired fertility. One of the latest sows, which he
esteemed a good animal, produced, when paired with her own uncle (who was
known to be productive with sows of other breeds), a litter of six, and a
second time a litter of only five weak young pigs. He then paired this sow
with a boar of a small black breed, which he had likewise imported from
England; this boar, when matched with sows of his own breed, produced from
seven to nine young. Now, the sow of the large breed, which was so
unproductive when paired with her own uncle, yielded to the small black boar,
in the first litter twenty-one, and in the second litter eighteen young pigs;
so that in one year she produced thirty-nine fine young animals!
As in the case of several other animals already mentioned, even when no injury
is perceptible from moderately close interbreeding, yet, to quote the words of
Mr. Coate (who five times won the annual gold medal of the Smithfield Club
Show for the best pen of pigs), "Crosses answer well for profit to the farmer,
as you get more constitution and quicker growth; but for me, who sell a great
number of pigs for breeding purposes, I find it will not do, as it requires
many years to get anything like purity of blood again." (17/24. Sidney on the
'Pig' page 36. See also note page 34. Also Richardson on the 'Pig' 1847 page
26.)]
Almost all the animals as yet mentioned are gregarious, and the males must
frequently pair with their own daughters, for they expel the young males as
well as all intruders, until forced by old age and loss of strength to yield
to some stronger male. It is therefore not improbable that gregarious animals
may have been rendered less susceptible than non-social species to the evil
consequences of close interbreeding, so that they may be enabled to live in
herds without injury to their offspring. Unfortunately we do not know whether
an animal like the cat, which is not gregarious, would suffer from close
interbreeding in a greater degree than our other domesticated animals. But the
pig is not, as far as I can discover, strictly gregarious, and we have seen
that it appears eminently liable to the evil effects of close interbreeding.
Mr. Huth, in the case of the pig, attributes (page 285) these effects to their
having been "cultivated most for their fat," or to the selected individuals
having had a weak constitution; but we must remember that it is great breeders
who have brought forward the above cases, and who are far more familiar than
ordinary men can be, with the causes which are likely to interfere with the
fertility of their animals.
The effects of close interbreeding in the case of man is a difficult subject,
on which I will say but little. It has been discussed by various authors under
many points of view. (17/25. Dr. Dally has published an excellent article
(translated in the 'Anthropolog. Review' May 1864 page 65), criticising all
writers who have maintained that evil follows from consanguineous marriages.
No doubt on this side of the question many advocates have injured their cause
by inaccuracies: thus it has been stated (Devay 'Du Danger des Mariages' etc.
1862 page 141) that the marriages of cousins have been prohibited by the
legislature of Ohio; but I have been assured, in answer to inquiries made in
the United States, that this statement is a mere fable.) Mr. Tylor (17/26. See
his interesting work on the 'Early History of Man' 1865 chapter 10.) has shown
that with widely different races in the most distant quarters of the world,
marriages between relations--even between distant relations--have been
strictly prohibited. There are, however, many exceptions to the rule, which
are fully given by Mr. Huth (17/27. 'The Marriage of Near Kin' 1875. The
evidence given by Mr. Huth would, I think, have been even more valuable than
it is on this and some other points, if he had referred solely to the works of
men who had long resided in each country referred to, and who showed that they
possessed judgment and caution. See also Mr. W. Adam 'On Consanguinity in
Marriage' in the 'Fortnightly Review' 1865 page 710. Also Hofacker 'Ueber die
Eigenschaften' etc. 1828.) It is a curious problem how these prohibitions
arose during early and barbarous times. Mr. Tylor is inclined to attribute
them to the evil effects of consanguineous marriages having been observed; and
he ingeniously attempts to explain some apparent anomalies in the prohibition
not extending equally to the relations on the male and female side. He admits,
however, that other causes, such as the extension of friendly alliances, may
have come into play. Mr. W. Adam, on the other hand, concludes that related
marriages are prohibited and viewed with repugnance, from the confusion which
would thus arise in the descent of property, and from other still more
recondite reasons. But I cannot accept these views, seeing that incest is held
in abhorrence by savages such as those of Australia and South America (17/28.
Sir G. Grey 'Journal of Expeditions into Australia' volume 2 page 243; and
Dobrizhoffer 'On the Abipones of South America.'), who have no property to
bequeath, or fine moral feelings to confuse, and who are not likely to reflect
on distant evils to their progeny. According to Mr. Huth the feeling is the
indirect result of exogamy, inasmuch as when this practice ceased in any tribe
and it became endogamous, so that marriages were strictly confined to the same
tribe, it is not unlikely that a vestige of the former practice would still be
retained, so that closely-related marriages would be prohibited. With respect
to exogamy itself Mr. MacLennan believes that it arose from a scarcity of
women, owing to female infanticide, aided perhaps by other causes.
It has been clearly shown by Mr. Huth that there is no instinctive feeling in
man against incest any more than in gregarious animals. We know also how
readily any prejudice or feeling may rise to abhorrence, as shown by Hindus in
regard to objects causing defilement. Although there seems to be no strong
inherited feeling in mankind against incest, it seems possible that men during
primeval times may have been more excited by strange females than by those
with whom they habitually lived; in the same manner as according to Mr.
Cupples (17/29. 'Descent of Man' 2nd. edit page 524.), male deerhounds are
inclined towards strange females, while the females prefer dogs with whom they
have associated. If any such feeling formerly existed in man, this would have
led to a preference for marriages beyond the nearest kin, and might have been
strengthened by the offspring of such marriages surviving in greater numbers,
as analogy would lead us to believe would have occurred.
Whether consanguineous marriages, such as are permitted in civilised nations,
and which would not be considered as close interbreeding in the case of our
domesticated animals, cause any injury will never be known with certainty
until a census is taken with this object in view. My son, George Darwin, has
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