CONCLUDING REMARKS.
DOMESTICATION--NATURE AND CAUSES OF VARIABILITY--SELECTION--DIVERGENCE AND
DISTINCTNESS OF CHARACTER--EXTINCTION OF RACES--CIRCUMSTANCES FAVOURABLE TO
SELECTION BY MAN--ANTIQUITY OF CERTAIN RACES--THE QUESTION WHETHER EACH
PARTICULAR VARIATION HAS BEEN SPECIALLY PREORDAINED ... Page 405
INDEX ... Page 433
* * * * *
ERRATA.
Vol. II., pp. 18, 232, 258, for Crataegus oxycantha, read oxyacantha.
,, p. 98, 8 lines from top, for Dianthus armoria read armeria.
,, ,, 156, 15 lines from bottom, for Casuarinus read Casuarius.
,, ,, ,, 4 lines from bottom, for Grus cineria read cinerea.
,, ,, 168, 11 lines from top, for Oesculus read Aesculus.
,, ,, 300, 3 lines from top, for anastomising read anastomosing.
,, ,, ,, foot-note, for Birckell read Brickell.
* * * * *
{1} THE
VARIATION OF ANIMALS AND PLANTS
UNDER DOMESTICATION.
* * * * *
CHAPTER XII.
INHERITANCE.
WONDERFUL NATURE OF INHERITANCE--PEDIGREES OF OUR DOMESTICATED
ANIMALS--INHERITANCE NOT DUE TO CHANCE--TRIFLING CHARACTERS
INHERITED--DISEASES INHERITED--PECULIARITIES IN THE EYE
INHERITED--DISEASES IN THE HORSE--LONGEVITY AND VIGOUR--ASYMMETRICAL
DEVIATIONS OF STRUCTURE--POLYDACTYLISM AND REGROWTH OF SUPERNUMERARY
DIGITS AFTER AMPUTATION--CASES OF SEVERAL CHILDREN SIMILARLY AFFECTED
FROM NON-AFFECTED PARENTS--WEAK AND FLUCTUATING INHERITANCE: IN WEEPING
TREES, IN DWARFNESS, COLOUR OF FRUIT AND FLOWERS, COLOUR OF
HORSES--NON-INHERITANCE IN CERTAIN CASES--INHERITANCE OF STRUCTURE AND
HABITS OVERBORNE BY HOSTILE CONDITIONS OF LIFE, BY INCESSANTLY
RECURRING VARIABILITY, AND BY REVERSION--CONCLUSION.
The subject of inheritance is an immense one, and has been treated by many
authors. One work alone, 'De l'Heredite Naturelle,' by Dr. Prosper Lucas,
runs to the length of 1562 pages. We must confine ourselves to certain
points which have an important bearing on the general subject of variation,
both with domestic and natural productions. It is obvious that a variation
which is not inherited throws no light on the derivation of species, nor is
of any service to man, except in the case of perennial plants, which can be
propagated by buds.
If animals and plants had never been domesticated, and wild ones alone had
been observed, we should probably never have heard the saying, that "like
begets like." The proposition would have been as self-evident, as that all
the buds on the same tree are alike, though neither proposition is strictly
true. For, as has often been remarked, probably no two individuals are {2}
identically the same. All wild animals recognise each other, which shows
that there is some difference between them; and when the eye is well
practised, the shepherd knows each sheep, and man can distinguish a
fellow-man out of millions on millions of other men. Some authors have gone
so far as to maintain that the production of slight differences is as much
a necessary function of the powers of generation, as the production of
offspring like their parents. This view, as we shall see in a future
chapter, is not theoretically probable, though practically it holds good.
The saying that "like begets like" has in fact arisen from the perfect
confidence felt by breeders, that a superior or inferior animal will
generally reproduce its kind; but this very superiority or inferiority
shows that the individual in question has departed slightly from its type.
The whole subject of inheritance is wonderful. When a new character arises,
whatever its nature may be, it generally tends to be inherited, at least in
a temporary and sometimes in a most persistent manner. What can be more
wonderful than that some trifling peculiarity, not primordially attached to
the species, should be transmitted through the male or female sexual cells,
which are so minute as not to be visible to the naked eye, and afterwards
through the incessant changes of a long course of development, undergone
either in the womb or in the egg, and ultimately appear in the offspring
when mature, or even when quite old, as in the case of certain diseases? Or
again, what can be more wonderful than the well-ascertained fact that the
minute ovule of a good milking cow will produce a male, from whom a cell,
in union with an ovule, will produce a female, and she, when mature, will
have large mammary glands, yielding an abundant supply of milk, and even
milk of a particular quality? Nevertheless, the real subject of surprise
is, as Sir H. Holland has well remarked,[1] not that a character should be
inherited, but that any should ever fail to be inherited. In a future
chapter, devoted to an hypothesis which I have termed pangenesis, an
attempt will be made to show the means by which characters of all kinds are
transmitted from generation to generation.
{3}
Some writers,[2] who have not attended to natural history, have attempted
to show that the force of inheritance has been much exaggerated. The
breeders of animals would smile at such simplicity; and if they
condescended to make any answer, might ask what would be the chance of
winning a prize if two inferior animals were paired together? They might
ask whether the half-wild Arabs were led by theoretical notions to keep
pedigrees of their horses? Why have pedigrees been scrupulously kept and
published of the Shorthorn cattle, and more recently of the Hereford breed?
Is it an illusion that these recently improved animals safely transmit
their excellent qualities even when crossed with other breeds? have the
Shorthorns, without good reason, been purchased at immense prices and
exported to almost every quarter of the globe, a thousand guineas having
been given for a bull? With greyhounds pedigrees have likewise been kept,
and the names of such dogs, as Snowball, Major, &c., are as well known to
coursers as those of Eclipse and Herod on the turf. Even with the Gamecock
pedigrees of famous strains were formerly kept, and extended back for a
century. With pigs, the Yorkshire and Cumberland breeders "preserve and
print pedigrees;" and to show how such highly-bred animals are valued, I
may mention that Mr. Brown, who won all the first prizes for small breeds
at Birmingham in 1850, sold a young sow and boar of his breed to Lord Ducie
for 43 guineas; the sow alone was afterwards sold to the Rev. F. Thursby
for 65 guineas; who writes, "she paid me very well, having sold her produce
for 300_l_., and having now four breeding sows from her."[3] Hard cash paid
down, over and over again, is an excellent test of inherited superiority.
In fact, the whole art of breeding, from which such great results have been
attained during the present century, depends on the inheritance of each
small {4} detail of structure. But inheritance is not certain; for if it
were, the breeder's art[4] would be reduced to a certainty, and there would
be little scope left for all that skill and perseverance shown by the men
who have left an enduring monument of their success in the present state of
our domesticated animals.
It is hardly possible, within a moderate compass, to impress on the mind of
those who have not attended to the subject, the full conviction of the
force of inheritance which is slowly acquired by rearing animals, by
studying the many treatises which have been published on the various
domestic animals, and by conversing with breeders. I will select a few
facts of the kind, which, as far as I can judge, have most influenced my
own mind. With man and the domestic animals, certain peculiarities have
appeared in an individual, at rare intervals, or only once or twice in the
history of the world, but have reappeared in several of the children and
grandchildren. Thus Lambert, "the porcupine-man," whose skin was thickly
covered with warty projections, which were periodically moulted, had all
his six children and two grandsons similarly affected.[5] The face and body
being covered with long hair, accompanied by deficient teeth (to which I
shall hereafter refer), occurred in three successive generations in a
Siamese family; but this case is not unique, as a woman[6] with a
completely hairy face was exhibited in London in 1663, and another instance
has recently occurred. Colonel Hallam[7] has described a race of two-legged
pigs, "the hinder extremities being entirely wanting;" and this deficiency
was transmitted through three generations. In fact, all races presenting
any remarkable peculiarity, such as solid-hoofed swine, Mauchamp sheep,
niata cattle, &c., are instances of the long-continued inheritance of rare
deviations of structure.
When we reflect that certain extraordinary peculiarities have {5} thus
appeared in a single individual out of many millions, all exposed in the
same country to the same general conditions of life, and, again, that the
same extraordinary peculiarity has sometimes appeared in individuals living
under widely different conditions of life, we are driven to conclude that
such peculiarities are not directly due to the action of the surrounding
conditions, but to unknown laws acting on the organisation or constitution
of the individual;--that their production stands in hardly closer relation
to the conditions than does life itself. If this be so, and the occurrence
of the same unusual character in the child and parent cannot be attributed
to both having been exposed to the same unusual conditions, then the
following problem is worth consideration, as showing that the result cannot
be due, as some authors have supposed, to mere coincidence, but must be
consequent on the members of the same family inheriting something in common
in their constitution. Let it be assumed that, in a large population, a
particular affection occurs on an average in one out of a million, so that
the _a priori_ chance that an individual taken at random will be so
affected is only one in a million. Let the population consist of sixty
millions, composed, we will assume, of ten million families, each
containing six members. On these data, Professor Stokes has calculated for
me that the odds will be no less than 8333 millions to 1 that in the ten
million families there will not be even a single family in which one parent
and two children will be affected by the peculiarity in question. But
numerous cases could be given, in which several children have been affected
by the same rare peculiarity with one of their parents; and in this case,
more especially if the grandchildren be included in the calculation, the
odds against mere coincidence become something prodigious, almost beyond
enumeration.
In some respects the evidence of inheritance is more striking when we
consider the reappearance of trifling peculiarities. Dr. Hodgkin formerly
told me of an English family in which, for many generations, some members
had a single lock differently coloured from the rest of the hair. I knew an
Irish gentleman, who, on the right side of his head, had a small white lock
in the midst of his dark hair: he assured me that his grandmother had {6} a
similar lock on the same side, and his mother on the opposite side. But it
is superfluous to give instances; every shade of expression, which may
often be seen alike in parents and children, tells the same story. On what
a curious combination of corporeal structure, mental character, and
training, must handwriting depend! yet every one must have noted the
occasional close similarity of the handwriting in father and son, although
the father had not taught his son. A great collector of franks assured me
that in his collection there were several franks of father and son hardly
distinguishable except by their dates. Hofacker, in Germany, remarks on the
inheritance of handwriting; and it has even been asserted that English boys
when taught to write in France naturally cling to their English manner of
writing.[8] Gait, gestures, voice, and general bearing are all inherited,
as the illustrious Hunter and Sir A. Carlisle have insisted.[9] My father
communicated to me two or three striking instances, in one of which a man
died during the early infancy of his son, and my father, who did not see
this son until grown up and out of health, declared that it seemed to him
as if his old friend had risen from the grave, with all his highly peculiar
habits and manners. Peculiar manners pass into tricks, and several
instances could be given of their inheritance; as in the case, often
quoted, of the father who generally slept on his back, with his right leg
crossed over the left, and whose daughter, whilst an infant in the cradle,
followed exactly the same habit, though an attempt was made to cure
her.[10] I will give one instance which has fallen under my own
observation, and which is curious from being a trick associated with a
peculiar state of mind, namely, pleasurable emotion. A boy had the singular
habit, when pleased, of rapidly moving his fingers parallel to each other,
and, when much excited, of raising both hands, with the fingers still
moving, to the sides of his face on a level with the eyes; this boy, when
almost an old man, could still hardly resist this trick when much pleased,
but from its absurdity concealed it. He had eight children. Of these, a
girl, when {7} pleased, at the age of four and a half years, moved her
fingers in exactly the same way, and what is still odder, when much
excited, the raised both her hands, with her fingers still moving, to the
sides of her face, in exactly the same manner as her father had done, and
sometimes even still continued to do when alone. I never heard of any one
excepting this one man and his little daughter who had this strange habit;
and certainly imitation was in this instance out of the question.
Some writers have doubted whether those complex mental attributes, on which
genius and talent depend, are inherited, even when both parents are thus
endowed. But he who will read Mr. Galton's able paper[11] on hereditary
talent will have his doubts allayed.
Unfortunately it matters not, as far as inheritance is concerned, how
injurious a quality or structure may be if compatible with life. No one can
read the many treatises[12] on hereditary disease and doubt this. The
ancients were strongly of this opinion, or, as Ranchin expresses it, _Omnes
Graeci, Arabes, et Latini in eo consentiunt_. A long catalogue could be
given of all sorts of inherited malformations and of predisposition to
various diseases. With gout, fifty per cent. of the cases observed in
hospital practice are, according to Dr. Garrod, inherited, and a greater
percentage in private practice. Every one knows how often insanity runs in
families, and some of the cases given by Mr. Sedgwick are awful,--as of a
surgeon, whose brother, father, and four paternal uncles were all insane,
the latter dying by suicide; of a Jew, whose father, mother, and six
brothers and sisters were all mad; and in some other cases several members
of the same family, during three or four successive generations, have
committed suicide. Striking instances {8} have been recorded of epilepsy,
consumption, asthma, stone in the bladder, cancer, profuse bleeding from
the slightest injuries, of the mother not giving milk, and of bad
parturition being inherited. In this latter respect I may mention an odd
case given by a good observer,[13] in which the fault lay in the offspring,
and not in the mother: in a part of Yorkshire the farmers continued to
select cattle with large hind-quarters, until they made a strain called
"Dutch-buttocked," and "the monstrous size of the buttocks of the calf was
frequently fatal to the cow, and numbers of cows were annually lost in
calving."
Instead of giving numerous details on various inherited malformations
and diseases, I will confine myself to one organ, that which is the
most complex, delicate, and probably best-known in the human frame,
namely, the eye, with its accessory parts. To begin with the latter: I
have heard of a family in which parents and children were affected by
drooping eyelids, in so peculiar a manner, that they could not see
without throwing their heads backwards; and Sir A. Carlisle[14]
specifies a pendulous fold to the eyelids as inherited. "In a family,"
says Sir H. Holland,[15] "where the father had a singular elongation of
the upper eyelid, seven or eight children were born with the same
deformity; two or three other children having it not." Many persons, as
I year from Mr. Paget, have two or three of the hairs in their eyebrows
(apparently corresponding with the vibrissae of the lower animals) much
longer than the others; and even so trifling a peculiarity as this
certainly runs in families.
With respect to the eye itself, the highest authority in England, Mr.
Bowman, has been so kind as to give me the following remarks on certain
inherited imperfections. First, hypermetropia, or morbidly long sight:
in this affection, the organ, instead of being spherical, is too flat
from front to back, and is often altogether too small, so that the
retina is brought too forward for the focus of the humours;
consequently a convex glass is required for clear vision of near
objects, and frequently even of distant ones. This state occurs
congenitally, or at a very early age, often in several children of the
same family, where one of the parents has presented it.[16] Secondly,
myopia, or short-sight, in which the eye is egg-shaped, and too long
from front to back; the retina in this case lies behind the focus, and
is therefore fitted to see distinctly only very near objects. This
condition is not commonly congenital, but comes on in youth, the
liability to it being well known to be transmissible from parent to
child. The change from the spherical to the ovoidal shape seems the
immediate {9} consequence of something like inflammation of the coats,
under which they yield, and there is ground for believing that it may
often originate in causes acting directly on the individual affected,
and may thenceforward become transmissible. When both parents are
myopic Mr. Bowman has observed the hereditary tendency in this
direction to be heightened, and some of the children to be myopic at an
earlier age or in a higher degree than their parents. Thirdly,
squinting is a familiar example of hereditary transmission: it is
frequently a result of such optical defects as have been above
mentioned; but the more primary and uncomplicated forms of it are also
sometimes in a marked degree transmitted in a family. Fourthly,
_Cataract_, or opacity of the crystalline lens, is commonly observed in
persons whose parents have been similarly affected, and often at an
earlier age in the children than in the parents. Occasionally more than
one child in a family is thus afflicted, one of whose parents or other
relation presents the senile form of the complaint. When cataract
affects several members of a family in the same generation, it is often
seen to commence at about the same age in each; _e.g._, in one family
several infants or young persons may suffer from it; in another,
several persons of middle age. Mr. Bowman also informs me that he has
occasionally seen, in several members of the same family, various
defects in either the right or left eye; and Mr. White Cooper has often
seen peculiarities of vision confined to one eye reappearing in the
same eye in the offspring.[17]
The following cases are taken from an able paper by Mr. W. Sedgwick,
and from Dr. Prosper Lucas.[18] Amaurosis, either congenital or coming
on late in life, and causing total blindness, is often inherited; it
has been observed in three successive generations. Congenital absence
of the iris has likewise been transmitted for three generations, a
cleft-iris for four generations, being limited in this latter case to
the males of the family. Opacity of the cornea and congenital smallness
of the eyes have been inherited. Portal records a curious case, in
which a father and two sons were rendered blind, whenever the head was
bent downwards, apparently owing to the crystalline lens, with its
capsule, slipping through an unusually large pupil into the anterior
chamber of the eye. Day-blindness, or imperfect vision under a bright
light, is inherited, as is night-blindness, or an incapacity to see
except under a strong light: a case has been recorded, by M. Cunier, of
this latter defect having affected eighty-five members of the same
family during six generations. The singular incapacity of
distinguishing colours, which has been called _Daltonism_, is
notoriously hereditary, and has been traced through five generations,
in which it was confined to the female sex.
With respect to the colour of the iris: deficiency of colouring matter
is well known to be hereditary in albinoes. The iris of one eye being
of a different colour from that of the other, and the iris being
spotted, are cases which have been inherited. Mr. Sedgwick gives, in
addition, on the {10} authority of Dr. Osborne,[19] the following
curious instance of strong inheritance: a family of sixteen sons and
five daughters all had eyes "resembling in miniature the markings on
the back of a tortoiseshell cat." The mother of this large family had
three sisters and a brother all similarly marked, and they derived this
peculiarity from their mother, who belonged to a family notorious for
transmitting it to their posterity.
Finally, Dr. Lucas emphatically remarks that there is not one single
faculty of the eye which is not subject to anomalies; and not one which
is not subjected to the principle of inheritance. Mr. Bowman agrees
with the general truth of this proposition; which of course does not
imply that all malformations are necessarily inherited; this would not
even follow if both parents were affected by an anomaly which in most
cases was transmissible.
Even if no single fact had been known with respect to the inheritance of
disease and malformations by man, the evidence would have been ample in the
case of the horse. And this might have been expected, as horses breed much
quicker than man, are matched with care, and are highly valued. I have
consulted many works, and the unanimity of the belief by veterinaries of
all nations in the transmission of various morbid tendencies is surprising.
Authors, who have had wide experience, give in detail many singular cases,
and assert that contracted feet, with the numerous contingent evils, of
ring-bones, curbs, splints, spavin, founder and weakness of the front legs,
roaring or broken and thick wind, melanosis, specific ophthalmia, and
blindness (the great French veterinary Hazard going so far as to say that a
blind race could soon be formed), crib-biting, jibbing, and ill-temper, are
all plainly hereditary. Youatt sums up by saying "there is scarcely a
malady to which the horse is subject which is not hereditary;" and M.
Bernard adds that the doctrine "that there is scarcely a disease which does
not run in the stock, is gaining new advocates every day."[20] So it {11}
is in regard to cattle, with consumption, good and bad teeth, fine skin,
&c. &c. But enough, and more than enough, has been said on disease. Andrew
Knight, from his own experience, asserts that disease is hereditary with
plants; and this assertion is endorsed by Lindley.[21]
Seeing how hereditary evil qualities are, it is fortunate that good health,
vigour, and longevity are equally inherited. It was formerly a well-known
practice, when annuities were purchased to be received during the lifetime
of a nominee, to search out a person belonging to a family of which many
members had lived to extreme old age. As to the inheritance of vigour and
endurance, the English race-horse offers an excellent instance. Eclipse
begot 334, and King Herod 497 winners. A "cock-tail" is a horse not purely
bred, but with only one-eighth or one-sixteenth impure blood in his veins,
yet very few instances have ever occurred of such horses having won a great
race. They are sometimes as fleet for short distances as thoroughbreds, but
as Mr. Robson, the great trainer, asserts, they are deficient in wind, and
cannot keep up the pace. Mr. Lawrence also remarks, "perhaps no instance
has ever occurred of a three-part-bred horse saving his '_distance_' in
running two miles with thoroughbred racers." It has been stated by Cecil,
that when unknown horses, whose parents were not celebrated, have
unexpectedly won great races, as in the case of Priam, they can always be
proved to be descended on both sides, through many generations, from
first-rate ancestors. On the Continent, Baron Cameronn challenges, in a
German veterinary periodical, the opponents of the English race-horse, to
name one good horse on the Continent which has not some English race-blood
in his veins.[22]
With respect to the transmission of the many slight, but {12} infinitely
diversified characters, by which the domestic races of animals and plants
are distinguished, nothing need be said; for the very existence of
persistent races proclaims the power of inheritance.
A few special cases, however, deserve some consideration. It might have
been anticipated, that deviations from the law of symmetry would not have
been inherited. But Anderson[23] states that a rabbit produced in a litter
a young animal having only one ear; and from this animal a breed was formed
which steadily produced one-eared rabbits. He also mentions a bitch, with a
single leg deficient, and she produced several puppies with the same
deficiency. From Hofacker's account[24] it appears that a one-horned stag
was seen in 1781 in a forest in Germany, in 1788 two, and afterwards, from
year to year, many were observed with only one horn on the right side of
the head. A cow lost a horn by suppuration,[25] and she produced three
calves which had on the same side of the head, instead of a horn, a small
bony lump attached merely to the skin; but we here approach the doubtful
subject of inherited mutilations. A man who is left-handed, and a shell in
which the spire turns in the wrong direction, are departures from the
normal though a symmetrical condition, and they are well known to be
inherited.
_Polydactylism._--Supernumerary fingers and toes are eminently liable,
as various authors have insisted, to transmission, but they are noticed
here chiefly on account of their occasional regrowth after amputation.
Polydactylism graduates[26] by multifarious steps from a mere cutaneous
appendage, not including any bone, to a double hand. But an additional
digit, supported on a metacarpal bone, and furnished with all the
proper muscles, nerves, and vessels, is sometimes so perfect, that it
escapes detection, unless the fingers are actually counted.
Occasionally there are several supernumerary digits; but usually only
one, making the total number six. This one may represent either a thumb
or finger, being attached to the inner or outer margin of the hand.
Generally, through the law of correlation, both hands and feet are
similarly affected. I have tabulated the cases recorded in various
works or privately communicated {13} to me, of forty-six persons with
extra digits on one or both hands and feet; if in each case all four
extremities had been similarly affected, the table would have shown a
total of ninety-two hands and ninety-two feet each with six digits. As
it is, seventy-three hands and seventy-five feet were thus affected.
This proves, in contradiction to the result arrived at by Dr.
Struthers,[27] that the hands are not more frequently affected than the
feet.
The presence of more than five digits is a great anomaly, for this
number is not normally exceeded by any mammal, bird, or existing
reptile.[28] Nevertheless, supernumerary digits are strongly inherited;
they have been transmitted through five generations; and in some cases,
after disappearing for one, two, or even three generations, have
reappeared through reversion. These facts are rendered, as Professor
Huxley has observed, more remarkable from its being known in most cases
that the affected person had not married one similarly affected. In
such cases the child of the fifth generation would have only 1-32nd
part of the blood of his first sedigitated ancestor. Other cases are
rendered remarkable by the affection gathering force, as Dr. Struthers
has shown, in each generation, though in each the affected person had
married one not affected; moreover such additional digits are often
amputated soon after birth, and can seldom have been strengthened by
use. Dr. Struthers gives the following instance: in the first
generation an additional digit appeared on one hand; in the second, on
both hands; in the third, three brothers had both hands, and one of the
brothers a foot affected; and in the fourth generation all four limbs
were affected. Yet we must not over-estimate the force of inheritance.
Dr. Struthers asserts that cases of non-inheritance and of the first
appearance of additional digits in unaffected families are much more
frequent than cases of inheritance. Many other deviations of structure,
of a nature almost as anomalous as supernumerary digits, such as
deficient phalanges, thickened joints, crooked fingers, &c., are in
like manner strongly inherited, and are equally subject to intermission
with reversion, though in such cases there is no reason to suppose that
both parents had been similarly affected.[29]
{14}
Additional digits have been observed in negroes as well as in other
races of man, and in several of the lower animals. Six toes have been
described on the hind feet of the newt (_Salamandra cristata_), and, as
it is said, of the frog. It deserves notice from what follows, that the
six-toed newt, though adult, had preserved some of its larval
characters; for part of the hyoidal apparatus, which is properly
absorbed during the act of metamorphosis, was retained. In the dog, six
toes on the hinder feet have been transmitted through three
generations; and I have heard of a race of six-toed cats. In several
breeds of the fowl the hinder toe is double, and is generally
transmitted truly, as is well shown when Dorkings are crossed with
common four-toed breeds.[30] With animals which have properly less than
five digits, the number is sometimes increased to five, especially in
the front legs, though rarely carried beyond that number; but this is
due to the development of a digit already existing in a more or less
rudimentary state. Thus the dog has properly four toes behind, but in
the larger breeds a fifth toe is commonly, though not perfectly,
developed. Horses, which properly have one toe alone fully developed
with rudiments of the others, have been described with each foot
bearing two or three small separate hoofs: analogous facts have been
noticed with sheep, goats, and pigs.[31]
The most interesting point with respect to supernumerary digits is
their occasional regrowth after amputation. Mr. White[32] describes a
child, three years old, with a thumb double from the first joint. He
removed the lesser thumb, which was furnished with a nail; but to his
astonishment it grew again, and reproduced a nail. The child was then
taken to an eminent London surgeon, and the newly-grown thumb was
wholly removed by its socket-joint, but again it grew and reproduced a
nail. Dr. Struthers mentions a case of partial regrowth of an
additional thumb, amputated when the child was three months old; and
the late Dr. Falconer communicated to me an analogous case which had
fallen under his own observation. A gentleman, who first called my
attention to this subject, has given me the following facts which
occurred in his own family. He himself, two brothers, and a sister were
born with an extra digit to each extremity. His parents were not
affected, and there was no tradition in the family, or in the village
in which the family had long resided, of any member having been thus
affected. Whilst a child, both additional toes, which were attached by
bones, were rudely cut off; but the stump of one grew again, and a
second operation was performed in his thirty-third year.
{15}
He has had fourteen children, of whom three have inherited additional
digits; and one of them, when about six weeks old, was operated on by
an eminent surgeon. The additional finger, which was attached by bone
to the outer side of the hand, was removed at the joint; the wound
healed, but immediately the digit began growing; and in about three
months' time the stump was removed for the second time by the root. But
it has since grown again, and is now fully a third of an inch in
length, including a bone; so that it will for the third time have to be
operated on.
Now the normal digits in adult man and other mammals, in birds, and, as
I believe, in true reptiles, have no power of regrowth. The nearest
approach to this power is exhibited by the occasional reappearance in
man of imperfect nails on the stumps of his fingers after
amputation.[33] But man in his embryonic condition has a considerable
power of reproduction, for Sir J. Simpson[34] has several times
observed arms which had been cut off in the womb by bands of false
membrane, and which had grown again to a certain extent. In one
instance, the extremity was "divided into three minute nodules, on two
of which small points of nails could be detected;" so that these
nodules clearly represented fingers in process of regrowth. When,
however, we descend to the lower vertebrate classes, which are
generally looked at as representing the higher classes in their
embryonic condition, we find ample powers of regrowth. Spallanzani[35]
cut off the legs and tail of a salamander six times, and Bonnet eight
times, successively, and they were reproduced. An additional digit
beyond the proper number was occasionally formed after Bonnet had cut
off or had divided longitudinally the hand or foot, and in one instance
three additional digits were thus formed.[36] These latter cases appear
at first sight quite distinct from the congenital production of
additional digits in the higher animals; but theoretically, as we shall
see in a future chapter, they probably present no real difference. The
larvae or tadpoles of the tailless Batrachians, but not the adults,[37]
are capable of reproducing lost members.[38] Lastly, as I have been
informed by Mr. J. J. Briggs and Mr. F. Buckland, when portions of the
pectoral and tail fins of various {16} fresh-water fish are cut off,
they are perfectly reproduced in about six weeks' time.
From these several facts we may infer that supernumerary digits in man
retain to a certain extent an embryonic condition, and that they resemble
in this respect the normal digits and limbs in the lower vertebrate
classes. They also resemble the digits of some of the lower animals in the
number exceeding five; for no mammal, bird, existing reptile, or amphibian
(unless the tubercle on the hind feet of the toad and other tailless
Batrachians be viewed as a digit) has more than five; whilst fishes
sometimes have in their pectoral fins as many as twenty metacarpal and
phalangeal bones, which, together with the bony filaments, apparently
represent our digits with their nails. So, again, in certain extinct
reptiles, namely, the Ichthyopterygia, "the digits may be seven, eight, or
nine in number, a significant mark," says Professor Owen, "of piscine
affinity."[39]
We encounter much difficulty in attempting to reduce these various facts to
any rule or law. The inconstant number of the additional digits--their
irregular attachment to either the inner or outer margin of the hand--the
gradation which can be traced from a mere loose rudiment of a single digit
to a completely double hand--the occasional appearance of additional digits
in the salamander after a limb has been amputated--these various facts
appear to indicate mere fluctuating monstrosity; and this perhaps is all
that can be safely said. Nevertheless, as supernumerary digits in the
higher animals, from their power of regrowth and from the number thus
acquired exceeding five, partake of the nature of the digits in the lower
vertebrate animals;--as they occur by no means rarely, and are transmitted
with remarkable strength, though perhaps not more strongly than some other
anomalies;--and as with animals which have fewer than five digits, when an
additional one appears it is generally due to the development of a visible
rudiment;--we are led in all cases to suspect, that, although no actual
rudiment can be detected, yet that a latent tendency to the formation of an
additional digit exists in all mammals, including man. On this view, as we
shall more plainly see in the {17} next chapter when discussing latent
tendencies, we should have to look at the whole case as one of reversion to
an enormously remote, lowly-organised, and multidigitate progenitor.
* * * * *
I may here allude to a class of facts closely allied to, but somewhat
different from, ordinary cases of inheritance. Sir H. Holland[40] states
that brothers and sisters of the same family are frequently affected, often
at about the same age, by the same peculiar disease, not known to have
previously occurred in the family. He specifies the occurrence of diabetes
in three brothers under ten years old; he also remarks that children of the
same family often exhibit in common infantile diseases the same peculiar
symptoms. My father mentioned to me the case of four brothers who died
between the ages of sixty and seventy, in the same highly peculiar comatose
state. An instance has been already given of supernumerary digits appearing
in four children out of six in a previously unaffected family. Dr. Devay
states[41] that two brothers married two sisters, their first-cousins, none
of the four nor any relation being an albino; but the seven children
produced from this double marriage were all perfect albinoes. Some of these
cases, as Mr. Sedgwick[42] has shown, are probably the result of reversion
to a remote ancestor, of whom no record had been preserved; and all these
cases are so far directly connected with inheritance that no doubt the
children inherited a similar constitution from their parents, and, from
being exposed to nearly similar conditions of life, it is not surprising
that they should be affected in the same manner and at the same period of
life.
* * * * *
Most of the facts hitherto given have served to illustrate the force of
inheritance, but we must now consider cases, grouped as well as the subject
allows into classes, showing how feeble, capricious, or deficient the power
of inheritance sometimes is. When a new peculiarity first appears, we can
never predict whether it will be inherited. If both parents from their
birth present {18} the same peculiarity, the probability is strong that it
will be transmitted to at least some of their offspring. We have seen that
variegation is transmitted much more feebly by seed from a branch which had
become variegated through bud-variation, than from plants which were
variegated as seedlings. With most plants the power of transmission
notoriously depends on some innate capacity in the individual: thus
Vilmorin[43] raised from a peculiarly coloured balsam some seedlings, which
all resembled their parent; but of these seedlings some failed to transmit
the new character, whilst others transmitted it to all their descendants
during several successive generations. So again with a variety of the rose,
two plants alone out of six were found by Vilmorin to be capable of
transmitting the desired character.
The weeping or pendulous growth of trees is strongly inherited in some
cases, and, without any assignable reason, feebly in other cases. I
have selected this character as an instance of capricious inheritance,
because it is certainly not proper to the parent-species, and because,
both sexes being borne on the same tree, both tend to transmit the same
character. Even supposing that there may have been in some instances
crossing with adjoining trees of the same species, it is not probable
that all the seedlings would have been thus affected. At Moccas Court
there is a famous weeping oak; many of its branches "are 30 feet long,
and no thicker in any part of this length than a common rope:" this
tree transmits its weeping character, in a greater or less degree, to
all its seedlings; some of the young oaks being so flexible that they
have to be supported by props; others not showing the weeping tendency
till about twenty years old.[44] Mr. Rivers fertilized, as he informs
me, the flowers of a new Belgian weeping thorn (_Crataegus oxyacantha_)
with pollen from a crimson not-weeping variety, and three young trees,
"now six or seven years old, show a decided tendency to be pendulous,
but as yet are not so much so as the mother-plant." According to Mr.
MacNab,[45] seedlings from a magnificent weeping birch (_Betula alba_),
in the Botanic Garden at Edinburgh, grew for the first ten or fifteen
years upright, but then all became weepers like their parent. A peach
with pendulous branches, like those of the weeping willow, has been
found capable of propagation by seed.[46] Lastly, a weeping and almost
prostrate yew (_Taxus baccata_) was found in a hedge in Shropshire; it
was a male, but one branch bore female flowers, and produced berries;
these, {19} being sown, produced seventeen trees, all of which had
exactly the same peculiar habit with the parent-tree.[47]
These facts, it might have been thought, would have been sufficient to
render it probable that a pendulous habit would in all cases be
strictly inherited. But let us look to the other side. Mr. MacNab[48]
sowed seeds of the weeping beech (_Fagus sylvanica_), but succeeded in
raising only common beeches. Mr. Rivers, at my request, raised a number
of seedlings from three distinct varieties of weeping elm; and at least
one of the parent-trees was so situated that it could not have been
crossed by any other elm; but none of the young trees, now about a foot
or two in height, show the least signs of weeping. Mr. Rivers formerly
sowed above twenty thousand seeds of the weeping ash (_Fraxinus
excelsior_), and not a single seedling was in the least degree
pendulous: in Germany, M. Borchmeyer raised a thousand seedlings, with
the same result. Nevertheless, Mr. Anderson, of the Chelsea Botanic
Garden, by sowing seed from a weeping ash, which was found before the
year 1780, in Cambridgeshire, raised several pendulous trees.[49]
Professor Henslow also informs me that some seedlings from a female
weeping ash in the Botanic Garden at Cambridge were at first a little
pendulous, but afterwards became quite upright: it is probable that
this latter tree, which transmits to a certain extent its pendulous
habit, was derived by a bud from the same original Cambridgeshire
stock; whilst other weeping ashes may have had a distinct origin. But
the crowning case, communicated to me by Mr. Rivers, which shows how
capricious is the inheritance of a pendulous habit, is that a variety
of another species of ash (_F. lentiscifolia_) which was formerly
pendulous, "now about twenty years old has long lost this habit, every
shoot being remarkably erect; but seedlings formerly raised from it
were perfectly prostrate, the stems not rising more than two inches
above the ground." Thus the weeping variety of the common ash, which
has been extensively propagated by buds during a long period, did not,
with Mr. Rivers, transmit its character to one seedling out of above
twenty thousand; whereas the weeping variety of a second species of
ash, which could not, whilst grown in the same garden, retain its own
weeping character, transmitted to its seedlings the pendulous habit in
excess!
Many analogous facts could be given, showing how apparently capricious
is the principle of inheritance. All the seedlings from a variety of
the Barberry (_B. vulgaris_) with red leaves inherited the same
character; only about one-third of the seedlings of the copper Beech
(_Fagus sylvatica_) had purple leaves. Not one out of a hundred
seedlings of a variety of the _Cerasus padus_, with yellow fruit, bore
yellow fruit: one-twelfth of the seedlings of the variety of _Cornus
mascula_, with yellow fruit, came true:[50] and lastly, all the trees
raised by my father from a yellow-berried holly (_Ilex aquifolium_),
{20} found wild, produced yellow berries. Vilmorin[51] observed in a
bed of _Saponaria calabrica_ an extremely dwarf variety, and raised
from it a large number of seedlings; some of these partially resembled
their parent, and he selected their seed; but the grandchildren were
not in the least dwarfed: on the other hand, he observed a stunted and
bushy variety of _Tagetes signata_ growing in the midst of the common
varieties by which it was probably crossed; for most of the seedlings
raised from this plant were intermediate in character, only two
perfectly resembling their parent; but seed saved from these two plants
reproduced the new variety so truly, that hardly any selection has
since been necessary.
Flowers transmit their colour truly, or most capriciously. Many annuals
come true: thus I purchased German seeds of thirty-four named
sub-varieties of one _race_ of ten-week stocks (_Matthiola annua_), and
raised a hundred and forty plants, all of which, with the exception of
a single plant, came true. In saying this, however, it must be
understood that I could distinguish only twenty kinds out of the
thirty-four named sub-varieties; nor did the colour of the flower
always correspond with the name affixed to the packet; but I say that
they came true, because in each of the thirty-six short rows every
plant was absolutely alike, with the one single exception. Again, I
procured packets of German seed of twenty-five named varieties of
common and quilled asters, and raised a hundred and twenty-four plants;
of these, all except ten were true in the above limited sense; and I
considered even a wrong shade of colour as false.
It is a singular circumstance that white varieties generally transmit
their colour much more truly than any other variety. This fact probably
stands in close relation with one observed by Verlot,[52] namely, that
flowers which are normally white rarely vary into any other colour. I
have found that the white varieties of _Delphinium consolida_ and of
the Stock are the truest. It is, indeed, sufficient to look through a
nurseryman's seed-list, to see the large number of white varieties
which can be propagated by seed. The several coloured varieties of the
sweet-pea (_Lathyrus odoratus_) are very true; but I hear from Mr.
Masters, of Canterbury, who has particularly attended to this plant,
that the white variety is the truest. The hyacinth, when propagated by
seed, is extremely inconstant in colour, but "white hyacinths almost
always give by seed white-flowered plants;"[53] and Mr. Masters informs
me that the yellow varieties also reproduce their colour, but of
different shades. On the other hand, pink and blue varieties, the
latter being the natural colour, are not nearly so true: hence, as Mr.
Masters has remarked to me, "we see that a garden variety may acquire a
more permanent habit than a natural species;" but it should have been
added, that this occurs under cultivation, and therefore under changed
conditions.
With many flowers, especially perennials, nothing can be more
fluctuating than the colour of the seedlings, as is notoriously the
case with verbenas, carnations, dahlias, cinerarias, and others.[54] I
sowed seed of twelve {21} named varieties of Snapdragon (_Antirrhinum
majus_), and utter confusion was the result. In most cases the
extremely fluctuating colour of seedling plants is probably in chief
part due to crosses between differently-coloured varieties during
previous generations. It is almost certain that this is the case with
the polyanthus and coloured primrose (_Primula veris_ and _vulgaris_),
from their reciprocally dimorphic structure;[55] and these are plants
which florists speak of as never come true by seed: but if care be
taken to prevent crossing, neither species is by any means very
inconstant in colour; thus I raised twenty-three plants from a purple
primrose, fertilised by Mr. J. Scott with its own pollen, and eighteen
came up purple of different shades, and only five reverted to the
ordinary yellow colour: again, I raised twenty plants from a bright-red
cowslip, similarly treated by Mr. Scott, and every one perfectly
resembled its parent in colour, as likewise did, with the exception of
a single plant, 73 grandchildren. Even with the most variable flowers,
it is probable that each delicate shade of colour might be permanently
fixed so as to be transmitted by seed, by cultivation in the same soil,
by long-continued selection, and especially by the prevention of
crosses. I infer this from certain annual larkspurs (_Delphinium
consolida_ and _ajacis_), of which common seedlings present a greater
diversity of colour than any other plant known to me; yet on procuring
seed of five named German varieties of _D. consolida_, only nine plants
out of ninety-four were false; and the seedlings of six varieties of
_D. ajacis_ were true in the same manner and degree as with the stocks
above described. A distinguished botanist maintains that the annual
species of Delphinium are always self-fertilised; therefore I may
mention that thirty-two flowers on a branch of _D. consolida_, enclosed
in a net, yielded twenty-seven capsules, with an average of 17.2 seed
in each; whilst five flowers, under the same net, which were
artificially fertilised, in the same manner as must be effected by bees
during their incessant visits, yielded five capsules with an average of
35.2 fine seed; and this shows that the agency of insects is necessary
for the full fertility of this plant. Analogous facts could be given
with respect to the crossing of many other flowers, such as carnations,
&c., of which the varieties fluctuate much in colour.
As with flowers, so with our domesticated animals, no character is more
variable than colour, and probably in no animal more so than with the
horse. Yet with a little care in breeding, it appears that races of any
colour might soon be formed. Hofacker gives the result of matching two
hundred and sixteen mares of four different colours with like-coloured
stallions, without regard to the colour of their ancestors; and of the
two hundred and sixteen colts born, eleven alone failed to inherit the
colour of their parents: Autenrieth and Ammon assert that, after two
generations, colts of a uniform colour are produced with certainty.[56]
In a few rare cases peculiarities fail to be inherited, apparently from the
force of inheritance being too strong. I have been assured by breeders of
the canary-bird that to get a good {22} jonquil-coloured bird it does not
answer to pair two jonquils, as the colour then comes out too strong, or is
even brown. So again, if two crested canaries are paired, the young birds
rarely inherit this character:[57] for in crested birds a narrow space of
bare skin is left on the back of the head, where the feathers are up-turned
to form the crest, and, when both parents are thus characterised, the
bareness becomes excessive, and the crest itself fails to be developed. Mr.
Hewitt, speaking of Laced Sebright Bantams, says[58] that, "why this should
be so, I know not, but I am confident that those that are best laced
frequently produce offspring very far from perfect in their markings,
whilst those exhibited by myself, which have so often proved successful,
were bred from the union of heavily-laced birds with those that were
scarcely sufficiently laced."
It is a singular fact that, although several deaf-mutes often occur in the
same family, and though their cousins and other relations are often in the
same condition, yet their parents are very rarely deaf-mutes. To give a
single instance: not one scholar out of 148, who were at the same time in
the London Institution, was the child of parents similarly afflicted. So
again, when a male or a female deaf-mute marries a sound person, their
children are most rarely affected: in Ireland out of 203 children thus
produced one alone was mute. Even when both parents have been deaf-mutes,
as in the case of forty-one marriages in the United States and of six in
Ireland, only two deaf and dumb children were produced. Mr. Sedgwick,[59]
in commenting on this remarkable and fortunate failure in the power of
transmission in the direct line, remarks that it may possibly be owing to
"excess having reversed the action of some natural law in development." But
it is safer in the present state of our knowledge to look at the whole case
as simply unintelligible.
* * * * *
With respect to the inheritance of structures mutilated by injuries or
altered by disease it is difficult to come to any {23} definite conclusion.
In some cases mutilations have been practised for a vast number of
generations without any inherited result. Godron has remarked[60] that
different races of man have from time immemorial knocked out their upper
incisors, cut off joints of their fingers, made holes of immense size
through the lobes of their ears or through their nostrils, made deep gashes
in various parts of their bodies, and there is no reason whatever to
suppose that these mutilations have ever been inherited. Adhesions due to
inflammation and pits from the small-pox (and formerly many consecutive
generations must have been thus pitted) are not inherited. With respect to
Jews, I have been assured by three medical men of the Jewish faith that
circumcision, which has been practised for so many ages, has produced no
inherited effect; Blumenbach, on the other hand, asserts[61] that in
Germany Jews are often born in a condition rendering circumcision
difficult, so that a name is here applied to them signifying "born
circumcised." The oak and other trees must have borne galls from primeval
times, yet they do not produce inherited excrescences; many other such
facts could be adduced.
On the other hand, various cases have been recorded of cats, dogs, and
horses, which have had their tails, legs, &c., amputated or injured,
producing offspring with the same parts ill-formed; but as it is not at all
rare for similar malformations to appear spontaneously, all such cases may
be due to mere coincidence. Nevertheless, Dr. Prosper Lucas has given, on
good authorities, such a long list of inherited injuries, that it is
difficult not to believe in them. Thus, a cow that had lost a horn from an
accident with consequent suppuration, produced three calves which were
hornless on the same side of the head. With the horse, there seems hardly a
doubt that bony exostoses on the legs, caused by too much travelling on
hard roads, are inherited. Blumenbach records the case of a man who had his
little finger on the right hand almost cut off, and which in consequence
grew crooked, and his sons had the same finger on the same hand similarly
crooked. A soldier, fifteen years before his marriage, lost his left eye
from purulent ophthalmia, and his {24} two sons were microphthalmic on the
same side.[62] In all such cases, if truthfully reported, in which the
parent has had an organ injured on one side, and more than one child has
been born with the same organ affected on the same side, the chances
against mere coincidence are enormous. But perhaps the most remarkable and
trustworthy fact is that given by Dr. Brown-Sequard,[63] namely, that many
young guinea-pigs inherited an epileptic tendency from parents which had
been subjected to a particular operation, inducing in the course of a few
weeks a convulsive disease like epilepsy: and it should be especially noted
that this eminent physiologist bred a large number of guinea-pigs from
animals which had not been operated on, and not one of these manifested the
epileptic tendency. On the whole, we can hardly avoid admitting, that
injuries and mutilations, especially when followed by disease, or perhaps
exclusively when thus followed, are occasionally inherited.
Although many congenital monstrosities are inherited, of which examples
have already been given, and to which may be added the lately recorded case
of the transmission during a century of hare-lip with a cleft-palate in the
writer's own family,[64] yet other malformations are rarely or never
inherited. Of these later cases, many are probably due to injuries in the
womb or egg, and would come under the head of non-inherited injuries or
mutilations. With plants, a long catalogue of inherited monstrosities of
the most serious and diversified nature could easily be given; and with
plants, there is no reason to suppose that monstrosities are caused by
direct injuries to the seed or embryo.
_Causes of Non-inheritance._
A large number of cases of non-inheritance are intelligible on the
principle, that a strong tendency to inheritance does exist, but {25} that
it is overborne by hostile or unfavourable conditions of life. No one would
expect that our improved pigs, if forced during several generations to
travel about and root in the ground for their own subsistence, would
transmit, as truly as they now do, their tendency to fatten, and their
short muzzles and legs. Dray-horses assuredly would not long transmit their
great size and massive limbs, if compelled to live on a cold, damp
mountainous region; we have indeed evidence of such deterioration in the
horses which have run wild on the Falkland Islands. European dogs in India
often fail to transmit their true character. Our sheep in tropical
countries lose their wool in a few generations. There seems also to be a
close relation between certain peculiar pastures and the inheritance of an
enlarged tail in fat-tailed sheep, which form one of the most ancient
breeds in the world. With plants, we have seen that the American varieties
of maize lose their proper character in the course of two or three
generations, when cultivated in Europe. Our cabbages, which here come so
true by seed, cannot form heads in hot countries. Under changed
circumstances, periodical habits of life soon fail to be transmitted, as
the period of maturity in summer and winter wheat, barley, and vetches. So
it is with animals; for instance, a person whose statement I can trust,
procured eggs of Aylesbury ducks from that town, where they are kept in
houses and are reared as early as possible for the London market; the ducks
bred from these eggs in a distant part of England, hatched their first
brood on January 24th, whilst common ducks, kept in the same yard and
treated in the same manner, did not hatch till the end of March; and this
shows that the period of hatching was inherited. But the grandchildren of
these Aylesbury ducks completely lost their early habit of incubation, and
hatched their eggs at the same time with the common ducks of the same
place.
Many cases of non-inheritance apparently result from the conditions of life
continually inducing fresh variability. We have seen that when the seeds of
pears, plums, apples, &c., are sown, the seedlings generally inherit some
degree of family likeness from the parent-variety. Mingled with these
seedlings, a few, and sometimes many, worthless, wild-looking plants
commonly appear; and their appearance may be attributed to the principle of
reversion. But scarcely a single seedling will be found {26} perfectly to
resemble the parent-form; and this, I believe, may be accounted for by
constantly recurring variability induced by the conditions of life. I
believe in this, because it has been observed that certain fruit-trees
truly propagate their kind whilst growing on their own roots, but when
grafted on other stocks, and by this process their natural state is
manifestly affected, they produce seedlings which vary greatly, departing
from the parental type in many characters.[65] Metzger, as stated in the
ninth chapter, found that certain kinds of wheat brought from Spain and
cultivated in Germany, failed during many years to reproduce themselves
truly; but that at last, when accustomed to their new conditions, they
ceased to be variable,--that is, they became amenable to the power of
inheritance. Nearly all the plants which cannot be propagated with any
approach to certainty by seed, are kinds which have long been propagated by
buds, cuttings, offsets, tubers, &c., and have in consequence been
frequently exposed during their individual lives to widely diversified
conditions of life. Plants thus propagated become so variable, that they
are subject, as we have seen in the last chapter, even to bud-variation.
Our domesticated animals, on the other hand, are not exposed during their
individual lives to such extremely diversified conditions, and are not
liable to such extreme variability; therefore they do not lose the power of
transmitting most of their characteristic features. In the foregoing
remarks on non-inheritance, crossed breeds are of course excluded, as their
diversity mainly depends on the unequal development of characters derived
from either parent, modified by the principles of reversion and prepotency.
_Conclusion._
It has, I think, been shown in the early part of this chapter how strongly
new characters of the most diversified nature, whether normal or abnormal,
injurious or beneficial, whether affecting organs of the highest or most
trifling importance, are inherited. Contrary to the common opinion, it is
often sufficient for the inheritance of some peculiar character, that one
parent alone should possess it, as in most cases in which the rarer {27}
anomalies have been transmitted. But the power of transmission is extremely
variable: in a number of individuals descended from the same parents, and
treated in the same manner, some display this power in a perfect manner,
and in some it is quite deficient; and for this difference no reason can be
assigned. In some cases the effects of injuries or mutilations apparently
are inherited; and we shall see in a future chapter that the effects of the
long-continued use and disuse of parts are certainly inherited. Even those
characters which are considered the most fluctuating, such as colour, are
with rare exceptions transmitted much more forcibly than is generally
supposed. The wonder, indeed, in all cases is not that any character should
be transmitted, but that the power of inheritance should ever fail. The
checks to inheritance, as far as we know them, are, firstly, circumstances
hostile to the particular character in question; secondly, conditions of
life incessantly inducing fresh variability; and lastly, the crossing of
distinct varieties during some previous generation, together with reversion
or atavism--that is, the tendency in the child to resemble its
grand-parents or more remote ancestors instead of its immediate parents.
This latter subject will be fully discussed in the following chapter.
* * * * *
{28}
CHAPTER XIII.
INHERITANCE _continued_--REVERSION OR ATAVISM.
DIFFERENT FORMS OF REVERSION--IN PURE OR UNCROSSED BREEDS, AS IN
PIGEONS, FOWLS, HORNLESS CATTLE AND SHEEP, IN CULTIVATED
PLANTS--REVERSION IN FERAL ANIMALS AND PLANTS--REVERSION IN CROSSED
VARIETIES AND SPECIES--REVERSION THROUGH BUD-PROPAGATION, AND BY
SEGMENTS IN THE SAME FLOWER OR FRUIT--IN DIFFERENT PARTS OF THE BODY IN
THE SAME ANIMAL--THE ACT OF CROSSING A DIRECT CAUSE OF REVERSION,
VARIOUS CASES OF, WITH INSTINCTS--OTHER PROXIMATE CAUSES OF
REVERSION--LATENT CHARACTERS--SECONDARY SEXUAL CHARACTERS--UNEQUAL
DEVELOPMENT OF THE TWO SIDES OF THE BODY--APPEARANCE WITH ADVANCING AGE
OF CHARACTERS DERIVED FROM A CROSS--THE GERM WITH ALL ITS LATENT
CHARACTERS A WONDERFUL OBJECT--MONSTROSITIES--PELORIC FLOWERS DUE IN
SOME CASES TO REVERSION.
The great principle of inheritance to be discussed in this chapter has been
recognised by agriculturists and authors of various nations, as shown by
the scientific term _Atavism_, derived from atavus, an ancestor; by the
English terms of _Reversion_, or _Throwing back_; by the French
_Pas-en-arriere_; and by the German _Rueck-schlag_, or _Rueck-schritt_.
When the child resembles either grandparent more closely than its immediate
parents, our attention is not much arrested, though in truth the fact is
highly remarkable; but when the child resembles some remote ancestor, or
some distant member in a collateral line,--and we must attribute the latter
case to the descent of all the members from a common progenitor,--we feel a
just degree of astonishment. When one parent alone displays some
newly-acquired and generally inheritable character, and the offspring do
not inherit it, the cause may lie in the other parent having the power of
prepotent transmission. But when both parents are similarly characterised,
and the child does not, whatever the cause may be, inherit the character in
question, but resembles its grandparents, we have one of the simplest cases
of reversion. We continually see another and even more simple case of
atavism, though not generally included under this head, namely, when {29}
the son more closely resembles his maternal than his paternal grandsire in
some male attribute, as in any peculiarity in the beard of man, the horns
of the bull, the hackles or comb of the cock, or, as in certain diseases
necessarily confined to the male sex; for the mother cannot possess or
exhibit such male attributes, yet the child has inherited them, through her
blood, from his maternal grandsire.
The cases of reversion may be divided into two main classes, which,
however, in some instances, blend into each other; namely, first, those
occurring in a variety or race which has not been crossed, but has lost by
variation some character that it formerly possessed, and which afterwards
reappears. The second class includes all cases in which a distinguishable
individual, sub-variety, race, or species, has at some former period been
crossed with a distinct form, and a character derived from this cross,
after having disappeared during one or several generations, suddenly
reappears. A third class, differing only in the manner of reproduction,
might be formed to include all cases of reversion effected by means of
buds, and therefore independent of true or seminal generation. Perhaps even
a fourth class might be instituted, to include reversions by segments in
the same individual flower or fruit, and in different parts of the body in
the same individual animal as it grows old. But the two first main classes
will be sufficient for our purpose.
* * * * *
_Reversion to lost Characters by pure or uncrossed forms._--Striking
instances of this first class of cases were given in the sixth chapter,
namely, of the occasional reappearance, in variously-coloured pure breeds
of the pigeon, of blue birds with all the marks which characterise the wild
_Columba livia_. Similar cases were given in the case of the fowl. With the
common ass, as we now know that the legs of the wild progenitor are
striped, we may feel assured that the occasional appearance of such stripes
in the domestic animal is a case of simple reversion. But I shall be
compelled to refer again to these cases, and therefore will here pass them
over.
The aboriginal species from which our domesticated cattle and sheep are
descended, no doubt possessed horns; but several hornless breeds are now
well established. Yet in these--for instance, {30} in Southdown sheep--"it
is not unusual to find among the male lambs some with small horns." The
horns, which thus occasionally reappear in other polled breeds, either
"grow to the full size, or are curiously attached to the skin alone and
hang loosely down, or drop off."[66] The Galloways and Suffolk cattle have
been hornless for the last 100 or 150 years, but a horned calf, with the
horn often loosely attached, is occasionally born.[67]
There is reason to believe that sheep in their early domesticated condition
were "brown or dingy black;" but even in the time of David certain flocks
were spoken of as white as snow. During the classical period the sheep of
Spain are described by several ancient authors as being black, red, or
tawny.[68] At the present day, notwithstanding the great care which is
taken to prevent it, particoloured lambs and some entirely black are
occasionally dropped by our most highly improved and valued breeds, such as
the Southdowns. Since the time of the famous Bakewell, during the last
century, the Leicester sheep have been bred with the most scrupulous care;
yet occasionally grey-faced, or black-spotted, or wholly black lambs
appear.[69] This occurs still more frequently with the less improved
breeds, such as the Norfolks.[70] As bearing on this tendency in sheep to
revert to dark colours, I may state (though in doing so I trench on the
reversion of crossed breeds, and likewise on the subject of prepotency)
that the Rev. W. D. Fox was informed that seven white Southdown ewes were
put to a so-called Spanish ram, which had two small black spots on his
sides, and they produced thirteen lambs, all perfectly black. Mr. Fox
believes that this ram belonged to a breed which he has himself kept, and
which is always spotted with black and white; and he finds that Leicester
sheep crossed by rams of this breed always produce black lambs: he has gone
on recrossing these crossed sheep with pure white Leicesters during three
successive {31} generations, but always with the same result. Mr. Fox was
also told by the friend from whom the spotted breed was procured, that he
likewise had gone on for six or seven generations crossing with white
sheep, but still black lambs were invariably produced.
Similar facts could be given with respect to tailless breeds of various
animals. For instance, Mr. Hewitt[71] states that chickens bred from some
Rumpless fowls, which were reckoned so good that they won a prize at an
exhibition, "in a considerable number of instances were furnished with
fully developed tail-feathers." On inquiry, the original breeder of these
fowls stated that, from the time when he had first kept them, they had
often produced fowls furnished with tails; but that these latter would
again reproduce rumpless chickens.
Analogous cases of reversion occur in the vegetable kingdom; thus "from
seeds gathered from the finest cultivated varieties of Heartsease (_Viola
tricolor_), plants perfectly wild both in their foliage and their flowers
are frequently produced;"[72] but the reversion in this instance is not to
a very ancient period, for the best existing varieties of the heartsease
are of comparatively modern origin. With most of our cultivated vegetables
there is some tendency to reversion to what is known to be, or may be
presumed to be, their aboriginal state; and this would be more evident if
gardeners did not generally look over their beds of seedlings, and pull up
the false plants or "rogues" as they are called. It has already been
remarked, that some few seedling apples and pears generally resemble, but
apparently are not identical with, the wild trees from which they are
descended. In our turnip[73] and carrot-beds a few plants often
"break"--that is, flower too soon; and their roots are generally found to
be hard and stringy, as in the parent-species. By the aid of a little
selection, carried on during a few generations, most of our cultivated
plants could probably be brought back, without any great change in their
conditions of life, to a wild or nearly wild condition: Mr. Buckman has
effected this with the parsnip;[74] {32} and Mr. Hewett C. Watson, as he
informs me, selected, during three generations, "the most diverging plants
of Scotch kail, perhaps one of the least modified varieties of the cabbage;
and in the third generation some of the plants came very close to the forms
now established in England about old castle-walls, and called indigenous."
* * * * *
_Reversion in Animals and Plants which have run wild._--In the cases
hitherto considered, the reverting animals and plants have not been exposed
to any great or abrupt change in their conditions of life which could have
induced this tendency; but it is very different with animals and plants
which have become feral or run wild. It has been repeatedly asserted in the
most positive manner by various authors, that feral animals and plants
invariably return to their primitive specific type. It is curious on what
little evidence this belief rests. Many of our domesticated animals could
not subsist in a wild state; thus, the more highly improved breeds of the
pigeon will not "field" or search for their own food. Sheep have never
become feral, and would be destroyed by almost every beast of prey. In
several cases we do not know the aboriginal parent-species, and cannot
possibly tell whether or not there has been any close degree of reversion.
It is not known in any instance what variety was first turned out; several
varieties have probably in some cases run wild, and their crossing alone
would tend to obliterate their proper character. Our domesticated animals
and plants, when they run wild, must always be exposed to new conditions of
life, for, as Mr. Wallace[75] has well remarked, they have to obtain their
own food, and are exposed to competition with the native productions. Under
these circumstances, if our domesticated animals did not undergo change of
some kind, the result would be quite opposed to the conclusions arrived at
in this work. Nevertheless, I do not doubt that the simple fact of animals
and plants becoming feral, does cause some tendency to reversion to the
primitive state; though this tendency has been much exaggerated by some
authors.
{33}
I will briefly run through the recorded cases. With neither horses nor
cattle is the primitive stock known; and it has been shown in former
chapters that they have assumed different colours in different
countries. Thus the horses which have run wild in South America are
generally brownish-bay, and in the East dun-coloured; their heads have
become larger and coarser, and this may be due to reversion. No careful
description has been given of the feral goat. Dogs which have run wild
in various countries have hardly anywhere assumed a uniform character;
but they are probably descended from several domestic races, and
aboriginally from several distinct species. Feral cats, both in Europe
and La Plata, are regularly striped; in some cases they have grown to
an unusually large size, but do not differ from the domestic animal in
any other character. When variously-coloured tame rabbits are turned
out in Europe, they generally reacquire the colouring of the wild
animal; there can be no doubt that this does really occur, but we
should remember that oddly-coloured and conspicuous animals would
suffer much from beasts of prey and from being easily shot; this at
least was the opinion of a gentleman who tried to stock his woods with
a nearly white variety; and when thus destroyed, they would in truth be
supplanted by, instead of being transformed into, the common rabbit. We
have seen that the feral rabbits of Jamaica, and especially of Porto
Santo, have assumed new colours and other new characters. The best
known case of reversion, and that on which the widely-spread belief in
its universality apparently rests, is that of pigs. These animals have
run wild in the West Indies, South America, and the Falkland Islands,
and have everywhere acquired the dark colour, the thick bristles, and
great tusks of the wild boar; and the young have reacquired
longitudinal stripes. But even in the case of the pig, Roulin describes
the half-wild animals in different parts of South America as differing
in several respects. In Louisiana the pig[76] has run wild, and is said
to differ a little in form, and much in colour, from the domestic
animal, yet does not closely resemble the wild boar of Europe. With
pigeons and fowls,[77] it is not known what variety was first turned
out, nor what character the feral birds have assumed. The guinea-fowl
in the West Indies, when feral, seems to vary more than in the
domesticated state.
With respect to plants run wild, Dr. Hooker[78] has strongly insisted
on what slight evidence the common belief in their power of reversion
rests. Godron[79] describes wild turnips, carrots, and celery; but
these plants in their cultivated state hardly differ from their wild
prototypes, except in the {34} succulency and enlargement of certain
parts,--characters which would be surely lost by plants growing in a
poor soil and struggling with other plants. No cultivated plant has run
wild on so enormous a scale as the cardoon (_Cynara cardunculus_) in La
Plata. Every botanist who has seen it growing there, in vast beds, as
high as a horse's back, has been struck with its peculiar appearance;
but whether it differs in any important point from the cultivated
Spanish form, which is said not to be prickly like its American
descendant, or whether it differs from he wild Mediterranean species,
which is said not to be social, I do not know.
* * * * *
_Reversion to Characters derived from a Cross, in the case of
Sub-varieties, Races, and Species._--When an individual having some
recognizable peculiarity unites with another of the same sub-variety, not
having the peculiarity in question, it often reappears in the descendants
after an interval of several generations. Every one must have noticed, or
heard from old people of children closely resembling in appearance or
mental disposition, or in so small and complex a character as expression,
one of their grandparents, or some more distant collateral relation. Very
many anomalies of structure and diseases,[80] of which instances have been
given in the last chapter, have come into a family from one parent, and
have reappeared in the progeny after passing over two or three generations.
The following case has been communicated to me on good authority, and may,
I believe, be fully trusted: a pointer-bitch produced seven puppies; four
were marked with blue and white, which is so unusual a colour with pointers
that she was thought to have played false with one of the greyhounds, and
the whole litter was condemned; but the gamekeeper was permitted to save
one as a curiosity. Two years afterwards a friend of the owner saw the
young dog, and declared that he was the image of his old pointer-bitch
Sappho, the only blue and white pointer of pure descent which he had ever
seen. This led to close inquiry, and it was proved that he was the
great-great-grandson of Sappho; so that, according to the common
expression, he had only 1-16th of her blood in his veins. Here it can
hardly be doubted that a character derived from a cross with an individual
of the same variety reappeared after passing over three generations.
{35}
When two distinct races are crossed, it is notorious that the tendency in
the offspring to revert to one or both parent-forms is strong, and endures
for many generations. I have myself seen the clearest evidence of this in
crossed pigeons and with various plants. Mr. Sidney[81] states that, in a
litter of Essex pigs, two young ones appeared which were the image of the
Berkshire boar that had been used twenty-eight years before in giving size
and constitution to the breed. I observed in the farmyard at Betley Hall
some fowls showing a strong likeness to the Malay breed, and was told by
Mr. Tollet that he had forty years before crossed his birds with Malays;
and that, though he had at first attempted to get rid of this strain, he
had subsequently given up the attempt in despair, as the Malay character
would reappear.
This strong tendency in crossed breeds to revert has given rise to endless
discussions in how many generations after a single cross, either with a
distinct breed or merely with an inferior animal, the breed may be
considered as pure, and free from all danger of reversion. No one supposes
that less than three generations suffices, and most breeders think that
six, seven, or eight are necessary, and some go to still greater
lengths.[82] But neither in the case of a breed which has been contaminated
by a single cross, nor when, in the attempt to form an intermediate breed,
half-bred animals have been matched together during many generations, can
any rule be laid down how soon the tendency to reversion will be
obliterated. It depends on the difference in the strength or prepotency of
transmission in the two parent-forms, on their actual amount of difference,
and on the nature of the conditions of life to which the crossed offspring
are exposed. But we must be careful not to confound these cases of
reversion to characters gained from a cross, with those given under the
first class, in which characters originally common to _both_ parents, but
lost at some former period, reappear; for such characters may recur after
an almost indefinite number of generations.
{36}
The law of reversion is equally powerful with hybrids, when they are
sufficiently fertile to breed together, or when they are repeatedly crossed
with either pure parent-form, as with mongrels. It is not necessary to give
instances, for in the case of plants almost every one who has worked on
this subject from the time of Koelreuter to the present day has insisted on
this tendency. Gaertner has recorded some good instances; but no one has
given more striking cases than Naudin.[83] The tendency differs in degree
or strength in different groups, and partly depends, as we shall presently
see, on the fact of the parent-plants having been long cultivated. Although
the tendency to reversion is extremely general with nearly all mongrels and
hybrids, it cannot be considered as invariably characteristic of them;
there is, also, reason to believe that it may be mastered by long-continued
selection; but these subjects will more properly be discussed in a future
chapter on Crossing. From what we see of the power and scope of reversion,
both in pure races and when varieties or species are crossed, we may infer
that characters of almost every kind are capable of reappearance after
having been lost for a great length of time. But it does not follow from
this that in each particular case certain characters will reappear: for
instance, this will not occur when a race is crossed with another endowed
with prepotency of transmission. In some few cases the power of reversion
wholly fails, without our being able to assign any cause for the failure:
thus it has been stated that in a French family in which 85 out of above
600 members, during six generations, had been subject to night-blindness,
"there has not been a single example of this affection in the children of
parents who were themselves free from it."[84]
* * * * *
_Reversion through Bud-propagation--Partial Reversion, by segments in the
same flower or fruit, or in different parts of the {37} body in the same
individual animal._--In the eleventh chapter, many cases of reversion by
buds, independently of seminal generation, were given--as when a leaf-bud
on a variegated, curled, or laciniated variety suddenly reassumes its
proper character; or as when a Provence-rose appears on a moss-rose, or a
peach on a nectarine-tree. In some of these cases only half the flower or
fruit, or a smaller segment, or mere stripes, reassumed their former
character; and here we have with buds reversion by segments. Vilmorin[85]
has also recorded several cases with plants derived from seed, of flowers
reverting by stripes or blotches to their primitive colours: he states that
in all such cases a white or pale-coloured variety must first be formed,
and, when this is propagated for a length of time by seed, striped
seedlings occasionally make their appearance; and these can afterwards by
care be multiplied by seed.
The stripes and segments just referred to are not due, as far as is known,
to reversion to characters derived from a cross, but to characters lost by
variation. These cases, however, as Naudin[86] insists in his discussion on
disjunction of character, are closely analogous with those given in the
eleventh chapter, in which crossed plants are known to have produced
half-and-half or striped flowers and fruit, or distinct kinds of flowers on
the same root resembling the two parent-forms. Many piebald animals
probably come under this same head. Such cases, as we shall see in the
chapter on Crossing, apparently result from certain characters not readily
blending together, and, as a consequence of this incapacity for fusion, the
offspring either perfectly resemble one of their two parents, or resemble
one parent in one part and the other parent in another part; or whilst
young are intermediate in character, but with advancing age revert wholly
or by segments to either parent-form, or to both. Thus young trees of the
_Cytisus adami_ are intermediate in foliage and flowers between the two
parent-forms; but when older the buds continually revert either partially
or wholly to both forms. The cases given in the eleventh chapter on the
changes which occurred during growth {38} in crossed plants of Tropaeolum,
Cereus, Datura, and Lathyrus are all analogous. As however these plants are
hybrids of the first generation, and as their buds after a time come to
resemble their parents and not their grandparents, these cases do not at
first appear to come under the law of reversion in the ordinary sense of
the word; nevertheless, as the change is effected through a succession of
bud-generations on the same plant, they may be thus included.
Analogous facts have been observed in the animal kingdom, and are more
remarkable, as they occur strictly in the same individual, and not as with
plants through a succession of bud-generations. With animals the act of
reversion, if it can be so designated, does not pass over a true
generation, but merely over the early stages of growth in the same
individual. For instance, I crossed several white hens with a black cock,
and many of the chickens were during the first year perfectly white, but
acquired during the second year black feathers; on the other hand, some of
the chickens which were at first black became during the second year
piebald with white. A great breeder[87] says, that a Pencilled Brahma hen
which has any of the blood of the Light Brahma in her, will "occasionally
produce a pullet well pencilled during the first year, but she will most
likely moult brown on the shoulders and become quite unlike her original
colours in the second year." The same thing occurs with Light Brahmas if of
impure blood. I have observed exactly similar cases with the crossed
offspring from differently coloured pigeons. But here is a more remarkable
fact: I crossed a turbit, which has a frill formed by the feathers being
reversed on its breast, with a trumpeter; and one of the young pigeons thus
raised showed at first not a trace of the frill, but, after moulting
thrice, a small yet unmistakably distinct frill appeared on its breast.
According to Girou,[88] calves produced from a red cow by a black bull, or
from a black cow by a red bull, are not rarely born red, and subsequently
become black.
In the foregoing cases, the characters which appear with advancing age are
the result of a cross in the previous or some {39} former generation; but
in the following cases, the characters which thus reappear formerly
appertained to the species, and were lost at a more or less remote epoch.
Thus, according to Azara,[89] the calves of a hornless race of cattle which
originated in Corrientes, though at first quite hornless, as they become
adult sometimes acquire small, crooked, and loose horns; and these in
succeeding years occasionally become attached to the skull. White and black
bantams, both of which generally breed true, sometimes assume as they grow
old a saffron or red plumage. For instance, a first-rate black bantam has
been described, which during three seasons was perfectly black, but then
annually became more and more red; and it deserves notice that this
tendency to change, whenever it occurs in a bantam, "is almost certain to
prove hereditary."[90] The cuckoo or blue-mottled Dorking cock, when old,
is liable to acquire yellow or orange hackles in place of his proper
bluish-grey hackles.[91] Now, as _Gallus bankiva_ is coloured red and
orange, and as Dorking fowls and both kinds of bantams are descended from
this species, we can hardly doubt that the change which occasionally occurs
in the plumage of these birds as their age advances, results from a
tendency in the individual to revert to the primitive type.
* * * * *
_Crossing as a direct cause of Reversion._--It has long been notorious that
hybrids and mongrels often revert to both or to one of their parent-forms,
after an interval of from two to seven or eight, or according to some
authorities even a greater number of generations. But that the act of
crossing in itself gives an impulse towards reversion, as shown by the
reappearance of long-lost characters, has never, I believe, been hitherto
proved. The proof lies in certain peculiarities, which do not characterise
the immediate parents, and therefore cannot have been derived from them,
frequently appearing in the offspring of two breeds when crossed, which
peculiarities never appear, or appear with extreme rarity, in these same
breeds, as long as they are {40} precluded from crossing. As this
conclusion seems to me highly curious and novel, I will give the evidence
in detail.
My attention was first called to this subject, and I was led to make
numerous experiments, by MM. Boitard and Corbie having stated that,
when they crossed certain breeds, pigeons coloured like the wild _C.
livia_, or the common dovecot, namely, slaty-blue, with double black
wing-bars, sometimes chequered with black, white loins, the tail barred
with black, with the outer feathers edged with white, were almost
invariably produced. The breeds which I crossed, and the remarkable
results attained, have been fully described in the sixth chapter. I
selected pigeons, belonging to true and ancient breeds, which had not a
trace of blue or any of the above specified marks; but when crossed,
and their mongrels recrossed, young birds were continually produced,
more or less plainly coloured slaty-blue, with some or all of the
proper characteristic marks. I may recall to the reader's memory one
case, namely, that of a pigeon, hardly distinguishable from the wild
Shetland species, the grandchild of a red-spot, white fantail, and two
black barbs, from any of which, when purely-bred, the production of a
pigeon coloured like the wild _C. livia_ would have been almost a
prodigy.
I was thus led to make the experiments, recorded in the seventh
chapter, on fowls. I selected long-established, pure breeds, in which
there was not a trace of red, yet in several of the mongrels feathers
of this colour appeared; and one magnificent bird, the offspring of a
black Spanish cock and white Silk hen, was coloured almost exactly like
the wild _Gallus bankiva_. All who know anything of the breeding of
poultry will admit that tens of thousands of pure Spanish and of pure
white Silk fowls might have been reared without the appearance of a red
feather. The fact, given on the authority of Mr. Tegetmeier, of the
frequent appearance, in mongrel fowls, of pencilled or
transversely-barred feathers, like those common to many gallinaceous
birds, is likewise apparently a case of reversion to a character
formerly possessed by some ancient progenitor of the family. I owe to
the kindness of this same excellent observer the inspection of some
neck-hackles and tail-feathers from a hybrid between the common fowl
and a very distinct species, the _Gallus varius_; and these feathers
are transversely striped in a conspicuous manner with dark metallic
blue and grey, a character which could not have been derived from
either immediate parent.
I have been informed by Mr. B. P. Brent, that he crossed a white
Aylesbury drake and a black so-called Labrador duck, both of which are
true breeds, and he obtained a young drake closely like the mallard
(_A. boschas_). Of the musk-duck (_A. moschata_, Linn.) there are two
sub-breeds, namely, white and slate-coloured; and these I am informed
breed true, or nearly true. But the Rev. W. D. Fox tells me that, by
putting a white drake to a slate-coloured duck, black birds, pied with
white, like the wild musk-duck, were always produced.
We have seen in the fourth chapter, that the so-called Himalayan
rabbit, with its snow-white body, black ears, nose, tail, and feet,
breeds {41} perfectly true. This race is known to have been formed by
the union of two varieties of silver-grey rabbits. Now, when a
Himalayan doe was crossed by a sandy-coloured buck, a silver-grey
rabbit was produced; and this is evidently a case of reversion to one
of the parent varieties. The young of the Himalayan rabbit are born
snow-white, and the dark marks do not appear until some time
subsequently; but occasionally young Himalayan rabbits are born of a
light silver-grey, which colour soon disappears; so that here we have a
trace of reversion, during an early period of life, to the
parent-varieties, independently of any recent cross.
In the third chapter is was shown that at an ancient period some breeds
of cattle in the wilder parts of Britain were white with dark ears, and
that the cattle now kept half wild in certain parks, and those which
have run quite wild in two distant parts of the world, are likewise
thus coloured. Now, an experienced breeder, Mr. J. Beasley, of
Northamptonshire,[92] crossed some carefully selected West Highland
cows with purely-bred shorthorn bulls. The bulls were red, red and
white, or dark roan; and the Highland cows were all of a red colour,
inclining to a light or yellow shade. But a considerable number of the
offspring--and Mr. Beasley calls attention to this as a remarkable
fact--were white, or white with red ears. Bearing in mind that none of
the parents were white, and that they were purely-bred animals, it is
highly probable that here the offspring reverted, in consequence of the
cross, to the colour either of the aboriginal parent-species or of some
ancient and half-wild parent-breed. The following case, perhaps, comes
under the same head: cows in their natural state have their udders but
little developed, and do not yield nearly so much milk as our
domesticated animals. Now there is some reason to believe[93] that
cross-bred animals between two kinds, both of which are good milkers,
such as Alderneys and Shorthorns, often turn out worthless in this
respect.
In the chapter on the Horse reasons were assigned for believing that
the primitive stock was striped and dun-coloured; and details were
given, showing that in all parts of the world stripes of a dark colour
frequently appear along the spine, across the legs, and on the
shoulders, where they are occasionally double or treble, and even
sometimes on the face and body of horses of all breeds and of all
colours. But the stripes appear most frequently on the various kinds of
duns. They may sometimes plainly be seen on foals, and subsequently
disappear. The dun-colour and the stripes are strongly transmitted when
a horse thus characterised is crossed with any other; but I was not
able to prove that striped duns are generally produced from the
crossing of two distinct breeds, neither of which are duns, though this
does sometimes occur.
The legs of the ass are often striped, and this may be considered as a
reversion to the wild parent-form, the _Asinus taeniopus_ of
Abyssinia,[94] which is thus striped. In the domestic animal the
stripes on the shoulder are occasionally double, or forked at the
extremity, as in certain zebrine {42} species. There is reason to
believe that the foal is frequently more plainly striped on the legs
than the adult animal. As with the horse, I have not acquired any
distinct evidence that the crossing of differently-coloured varieties
of the ass brings out the stripes.
But now let us turn to the result of crossing the horse and ass.
Although mules are not nearly so numerous in England as asses, I have
seen a much greater number with striped legs, and with the stripes far
more conspicuous than in either parent-form. Such mules are generally
light-coloured, and might be called fallow-duns. The shoulder-stripe in
one instance was deeply forked at the extremity, and in another
instance was double, though united in the middle. Mr. Martin gives a
figure of a Spanish mule with strong zebra-like marks on its legs,[95]
and remarks, that mules are particularly liable to be thus striped on
their legs. In South America, according to Roulin,[96] such stripes are
more frequent and conspicuous in the mule than in the ass. In the
United States, Mr. Gosse,[97] speaking of these animals, says, "that in
a great number, perhaps in nine out of every ten, the legs are banded
with transverse dark stripes."
Many years ago I saw in the Zoological Gardens a curious triple hybrid,
from a bay mare, by a hybrid from a male ass and female zebra. This
animal when old had hardly any stripes; but I was assured by the
superintendent, that when young it had shoulder-stripes, and faint
stripes on its flanks and legs. I mention this case more especially as
an instance of the stripes being much plainer during youth than in old
age.
As the zebra has such conspicuously striped legs, it might have been
expected that the hybrids from this animal and the common ass would
have had their legs in some degree striped; but it appears from the
figures given in Dr. Gray's 'Knowsley Gleanings,' and still more
plainly from that given by Geoffroy and F. Cuvier,[98] that the legs
are much more conspicuously striped than the rest of the body; and this
fact is intelligible only on the belief that the ass aids in giving,
through the power of reversion, this character to its hybrid offspring.
The quagga is banded over the whole front part of its body like a
zebra, but has no stripes on its legs, or mere traces of them. But in
the famous hybrid bred by Lord Morton,[99] from a chesnut, nearly
purely-bred, Arabian mare, by a male quagga, the stripes were "more
strongly defined and darker than those on the legs of the quagga." The
mare was subsequently put to a black Arabian horse, and bore two colts,
both of which, as formerly stated, were plainly striped on the legs,
and one of them likewise had stripes on the neck and body.
The _Asinus Indicus_[100] is characterised by a spinal stripe, without
shoulder {43} or leg stripes; but traces of these latter stripes may
occasionally be seen even in the adult;[101] and Colonel S. Poole, who
has had ample opportunities for observation, informs me that in the
foal, when first born, the head and legs are often striped, but the
shoulder-stripe is not so distinct as in the domestic ass; all these
stripes, excepting that along the spine, soon disappear. Now a hybrid,
raised at Knowsley[102] from a female of this species by a male
domestic ass, had all four legs transversely and conspicuously striped,
had three short stripes on each shoulder, and had even some zebra-like
stripes on its face! Dr. Gray informs me that he has seen a second
hybrid of the same parentage similarly striped.
From these facts we see that the crossing of the several equine species
tends in a marked manner to cause stripes to appear on various parts of the
body, especially on the legs. As we do not know whether the primordial
parent of the genus was striped, the appearance of the stripes can only
hypothetically be attributed to reversion. But most persons, after
considering the many undoubted cases of variously coloured marks
reappearing by reversion in crossed pigeons, fowls, ducks, &c., will come
to the same conclusion with respect to the horse-genus; and in this case we
must admit that the progenitor of the group was striped on the legs,
shoulders, face, and probably over the whole body, like a zebra. If we
reject this view, the frequent and almost regular appearance of stripes in
the several foregoing hybrids is left without any explanation.
* * * * *
It would appear that with crossed animals a similar tendency to the
recovery of lost characters holds good even with instincts. There are some
breeds of fowls which are called "everlasting layers," because they have
lost the instinct of incubation; and so rare is it for them to incubate
that I have seen notices published in works on poultry, when hens of such
breeds have taken to sit.[103] Yet the aboriginal species was of course a
good incubator; for with birds in a state of nature hardly any {44}
instinct is so strong as this. Now, so many cases have been recorded of the
crossed offspring from two races, neither of which are incubators, becoming
first-rate sitters, that the reappearance of this instinct must be
attributed to reversion from crossing. One author goes so far as to say,
"that a cross between two non-sitting varieties almost invariably produces
a mongrel that becomes broody, and sits with remarkable steadiness."[104]
Another author, after giving a striking example, remarks that the fact can
be explained only on the principle that "two negatives make a positive." It
cannot, however, be maintained that hens produced from a cross between two
non-sitting breeds invariably recover their lost instinct, any more than
that crossed fowls or pigeons invariably recover the red or blue plumage of
their prototypes. I raised several chickens from a Polish hen by a Spanish
cock,--breeds which do not incubate,--and none of the young hens at first
recovered their instinct, and this appeared to afford a well-marked
exception to the foregoing rule; but one of these hens, the only one which
was preserved, in the third year sat well on her eggs and reared a brood of
chickens. So that here we have the appearance with advancing age of a
primitive instinct, in the same manner as we have seen that the red plumage
of the _Gallus bankiva_ is sometimes reacquired by crossed and purely-bred
fowls of various kinds as they grow old.
The parents of all our domesticated animals were of course aboriginally
wild in disposition; and when a domesticated species is crossed with a
distinct species, whether this is a domesticated or only tamed animal, the
hybrids are often wild {45} to such a degree, that the fact is intelligible
only on the principle that the cross has caused a partial return to the
primitive disposition.
The Earl of Powis formerly imported some thoroughly domesticated humped
cattle from India, and crossed them with English breeds, which belong to a
distinct species; and his agent remarked to me, without any question having
been asked, how oddly wild the cross-bred animals were. The European wild
boar and the Chinese domesticated pig are almost certainly specifically
distinct: Sir F. Darwin crossed a sow of the latter breed with a wild
Alpine boar which had become extremely tame, but the young, though having
half-domesticated blood in their veins, were "extremely wild in
confinement, and would not eat swill like common English pigs." Mr. Hewitt,
who has had great experience in crossing tame cock-pheasants with fowls
belonging to five breeds, gives as the character of all "extraordinary
wildness;"[105] but I have myself seen one exception to this rule. Mr.
S. J. Salter,[106] who raised a large number of hybrids from a bantam-hen
by _Gallus Sonneratii_, states that "all were exceedingly wild." Mr.
Waterton[107] bred some wild ducks from eggs hatched under a common duck,
and the young were allowed to cross freely both amongst themselves and with
the tame ducks; they were "half wild and half tame; they came to the
windows to be fed, but still they had a wariness about them quite
remarkable."
On the other hand, mules from the horse and ass are certainly not in the
least wild, yet they are notorious for obstinacy and vice. Mr. Brent, who
has crossed canary-birds with many kinds of finches, has not observed, as
he informs me, that the hybrids were in any way remarkably wild. Hybrids
are often raised between the common and musk duck, and I have been assured
by three persons, who have kept these crossed birds, that they were not
wild; but Mr. Garnett[108] observed that his female hybrids exhibited
"migratory propensities," of which there is not a vestige in the common or
musk duck. No case is {46} known of this latter bird having escaped and
become wild in Europe or Asia, except, according to Pallas, on the Caspian
Sea; and the common domestic duck only occasionally becomes wild in
districts where large lakes and fens abound. Nevertheless, a large number
of cases have been recorded[109] of hybrids from these two ducks, although
so few are reared in comparison with purely-bred birds of either species,
having been shot in a completely wild state. It is improbable that any of
these hybrids could have acquired their wildness from the musk-duck having
paired with a truly wild duck; and this is known not to be the case in
North America; hence we must infer that they have reacquired, through
reversion, their wildness, as well as renewed powers of flight.
These latter facts remind us of the statements, so frequently made by
travellers in all parts of the world, on the degraded state and savage
disposition of crossed races of man. That many excellent and kind-hearted
mulattos have existed no one will dispute; and a more mild and gentle set
of men could hardly be found than the inhabitants of the island of Chiloe,
who consist of Indians commingled with Spaniards in various proportions. On
the other hand, many years ago, long before I had thought of the present
subject, I was struck with the fact that, in South America, men of
complicated descent between Negroes, Indians, and Spaniards, seldom had,
whatever the cause might be, a good expression.[110] Livingstone,--and a
more unimpeachable authority cannot be quoted,--after speaking of a
half-caste man on the Zambesi, described by the Portuguese as a rare
monster of inhumanity, remarks, "It is unaccountable why half-castes, such
as he, are so much more cruel than the Portuguese, but such is undoubtedly
the case." An inhabitant remarked to Livingstone, "God made white men, and
God made black men, but the Devil made half-castes."[111] When two races,
both {47} low in the scale, are crossed, the progeny seems to be eminently
bad. Thus the noble-hearted Humboldt, who felt none of that prejudice
against the inferior races now so current in England, speaks in strong
terms of the bad and savage disposition of Zambos, or half-castes between
Indians and Negroes; and this conclusion has been arrived at by various
observers.[112] From these facts we may perhaps infer that the degraded
state of so many half-castes is in part due to reversion to a primitive and
savage condition, induced by the act of crossing, as well as to the
unfavourable moral conditions under which they generally exist.
* * * * *
_Summary on the proximate causes leading to Reversion._--When purely-bred
animals or plants reassume long-lost characters,--when the common ass, for
instance, is born with striped legs, when a pure race of black or white
pigeons throws a slaty-blue bird, or when a cultivated heartsease with
large and rounded flowers produces a seedling with small and elongated
flowers,--we are quite unable to assign any proximate cause. When animals
run wild, the tendency to reversion, which, though it has been greatly
exaggerated, no doubt exists, is sometimes to a certain extent
intelligible. Thus, with feral pigs, exposure to the weather will probably
favour the growth of the bristles, as is known to be the case with the hair
of other domesticated animals, and through correlation the tusks will tend
to be redeveloped. But the reappearance of coloured longitudinal stripes on
young feral pigs cannot be attributed to the direct action of external
conditions. In this case, and in many others, we can only say that changed
habits of life apparently have favoured a tendency, inherent or latent in
the species, to return to the primitive state.
It will be shown in a future chapter that the position of flowers on the
summit of the axis, and the position of seeds within the capsule, sometimes
determine a tendency towards reversion; and this apparently depends on the
amount of sap or nutriment which the flower-buds and seeds receive. The
position, also, of buds, either on branches or on roots, sometimes
determines, as was formerly shown, the transmission of the {48} proper
character of the variety, or its reversion to a former state.
We have seen in the last section that when two races or species are crossed
there is the strongest tendency to the reappearance in the offspring of
long-lost characters, possessed by neither parent nor immediate progenitor.
When two white, or red, or black pigeons, of well-established breeds, are
united, the offspring are almost sure to inherit the same colours; but when
differently-coloured birds are crossed, the opposed forces of inheritance
apparently counteract each other, and the tendency which is inherent in
both parents to produce slaty-blue offspring becomes predominant. So it is
in several other cases. But when, for instance, the ass is crossed with _A.
Indicus_ or with the horse,--animals which have not striped legs,--and the
hybrids have conspicuous stripes on their legs and even on their faces, all
that can be said is, that an inherent tendency to reversion is evolved
through some disturbance in the organisation caused by the act of crossing.
Another form of reversion is far commoner, indeed is almost universal with
the offspring from a cross, namely, to the characters proper to either pure
parent-form. As a general rule, crossed offspring in the first generation
are nearly intermediate between their parents, but the grandchildren and
succeeding generations continually revert, in a greater or lesser degree,
to one or both of their progenitors. Several authors have maintained that
hybrids and mongrels include all the characters of both parents, not fused
together, but merely mingled in different proportions in different parts of
the body; or, as Naudin[113] has expressed it, a hybrid is a living
mosaic-work, in which the eye cannot distinguish the discordant elements,
so completely are they intermingled. We can hardly doubt that, in a certain
sense, this is true, as when we behold in a hybrid the elements of both
species segregating themselves into segments in the same flower or fruit,
by a process of self-attraction or self-affinity; this segregation taking
place either by seminal or by bud-propagation. Naudin further believes that
the segregation of the two specific elements or essences is eminently
liable to occur in the male and female reproductive matter; and he thus
explains the almost {49} universal tendency to reversion in successive
hybrid generations. For this would be the natural result of the union of
pollen and ovules, in both of which the elements of the same species had
been segregated by self-affinity. If, on the other hand, pollen which
included the elements of one species happened to unite with ovules
including the elements of the other species, the intermediate or hybrid
state would still be retained, and there would be no reversion. But it
would, as I suspect, be more correct to say that the elements of both
parent-species exist in every hybrid in a double state, namely, blended
together and completely separate. How this is possible, and what the term
specific essence or element may be supposed to express, I shall attempt to
show in the hypothetical chapter on pangenesis.
But Naudin's view, as propounded by him, is not applicable to the
reappearance of characters lost long ago by variation; and it is hardly
applicable to races or species which, after having been crossed at some
former period with a distinct form, and having since lost all traces of the
cross, nevertheless occasionally yield an individual which reverts (as in
the case of the great-great-grandchild of the pointer Sappho) to the
crossing form. The most simple case of reversion, namely, of a hybrid or
mongrel to its grandparents, is connected by an almost perfect series with
the extreme case of a purely-bred race recovering characters which had been
lost during many ages; and we are thus led to infer that all the cases must
be related by some common bond.
Gaertner believed that only those hybrid plants which are highly sterile
exhibit any tendency to reversion to their parent-forms. It is rash to
doubt so good an observer, but this conclusion must I think be an error;
and it may perhaps be accounted for by the nature of the genera observed by
him, for he admits that the tendency differs in different genera. The
statement is also directly contradicted by Naudin's observations, and by
the notorious fact that perfectly fertile mongrels exhibit the tendency in
a high degree,--even in a higher degree, according to Gaertner himself,
than hybrids.[114]
Gaertner further states that reversions rarely occur with {50} hybrid
plants raised from species which have not been cultivated, whilst, with
those which have been long cultivated, they are of frequent occurrence.
This conclusion explains a curious discrepancy: Max Wichura,[115] who
worked exclusively on willows, which had not been subjected to culture,
never saw an instance of reversion; and he goes so far as to suspect that
the careful Gaertner had not sufficiently protected his hybrids from the
pollen of the parent-species: Naudin, on the other hand, who chiefly
experimented on cucurbitaceous and other cultivated plants, insists more
strenuously than any other author on the tendency to reversion in all
hybrids. The conclusion that the condition of the parent-species, as
affected by culture, is one of the proximate causes leading to reversion,
agrees fairly well with the converse case of domesticated animals and
cultivated plants being liable to reversion when they become feral; for in
both cases the organisation or constitution must be disturbed, though in a
very different way.
Finally, we have seen that characters often reappear in purely-bred races
without our being able to assign any proximate cause; but when they become
feral this is either indirectly or directly induced by the change in their
conditions of life. With crossed breeds, the act of crossing in itself
certainly leads to the recovery of long-lost characters, as well as of
those derived from either parent-form. Changed conditions, consequent on
cultivation, and the relative position of buds, flowers, and seeds on the
plant, all apparently aid in giving this same tendency. Reversion may occur
either through seminal or bud generation, generally at birth, but sometimes
only with an advance of age. Segments or portions of the individual may
alone be thus affected. That a being should be born resembling in certain
characters an ancestor removed by two or three, and in some cases by
hundreds or even thousands of generations, is assuredly a wonderful fact.
In these cases the child is commonly said to inherit such characters
directly from its grandparents or more remote ancestors. But this view is
hardly conceivable. If, however, we suppose that every character is derived
{51} exclusively from the father or mother, but that many characters lie
latent in both parents during a long succession of generations, the
foregoing facts are intelligible. In what manner characters may be
conceived to lie latent, will be considered in a future chapter to which I
have lately alluded.
* * * * *
_Latent Characters._--But I must explain what is meant by characters lying
latent. The most obvious illustration is afforded by secondary sexual
characters. In every female all the secondary male characters, and in every
male all the secondary female characters, apparently exist in a latent
state, ready to be evolved under certain conditions. It is well known that
a large number of female birds, such as fowls, various pheasants,
partridges, peahens, ducks, &c., when old or diseased, or when operated on,
partly assume the secondary male characters of their species. In the case
of the hen-pheasant this has been observed to occur far more frequently
during certain seasons than during others.[116] A duck ten years old has
been known to assume both the perfect winter and summer plumage of the
drake.[117] Waterton[118] gives a curious case of a hen which had ceased
laying, and had assumed the plumage, voice, spurs, and warlike disposition
of the cock; when opposed to an enemy she would erect her hackles and show
fight. Thus every character, even to the instinct and manner of fighting,
must have lain dormant in this hen as long as her ovaria continued to act.
The females of two kinds of deer, when old, have been known to acquire
horns; and, as Hunter has remarked, we see something of an analogous nature
in the human species.
On the other hand, with male animals, it is notorious that the secondary
sexual characters are more or less completely lost when they are subjected
to castration. Thus, if the operation be performed on a young cock, he
never, as Yarrell states, crows {52} again; the comb, wattles, and spurs do
not grow to their full size, and the hackles assume an intermediate
appearance between true hackles and the feathers of the hen. Cases are
recorded of confinement alone causing analogous results. But characters
properly confined to the female are likewise acquired; the capon takes to
sitting on eggs, and will bring up chickens; and what is more curious, the
utterly sterile male hybrids from the pheasant and the fowl act in the same
manner, "their delight being to watch when the hens leave their nests, and
to take on themselves the office of a sitter."[119] That admirable observer
Reaumur[120] asserts that a cock, by being long confined in solitude and
darkness, can be taught to take charge of young chickens; he then utters a
peculiar cry, and retains during his whole life this newly acquired
maternal instinct. The many well-ascertained cases of various male mammals
giving milk, show that their rudimentary mammary glands retain this
capacity in a latent condition.
We thus see that in many, probably in all cases, the secondary characters
of each sex lie dormant or latent in the opposite sex, ready to be evolved
under peculiar circumstances. We can thus understand how, for instance, it
is possible for a good milking cow to transmit her good qualities through
her male offspring to future generations; for we may confidently believe
that these qualities are present, though latent, in the males of each
generation. So it is with the game-cock, who can transmit his superiority
in courage and vigour through his female to his male offspring; and with
man it is known [121] that diseases, such as hydrocele, necessarily
confined to the male sex, can be transmitted through the female to the
grandson. Such cases as these offer, as was remarked at the commencement of
this chapter, the simplest possible examples of reversion; and they are
intelligible on the belief that characters common to the grandparent and
grandchild of the same sex are present, though latent, in the intermediate
parent of the opposite sex.
The subject of latent characters is so important, as we shall see in a
future chapter, that I will give another illustration. {53} Many animals
have the right and left sides of their body unequally developed: this is
well known to be the case with flat-fish, in which the one side differs in
thickness and colour, and in the shape of the fins, from the other; and
during the growth of the young fish one eye actually travels, as shown by
Steenstrup, from the lower to the upper surface.[122] In most flat-fishes
the left is the blind side, but in some it is the right; though in both
cases "wrong fishes," which are developed in a reversed manner to what is
usual, occasionally occur, and in _Platessa flesus_ the right or left side
is indifferently developed, the one as often as the other. With gasteropods
or shell-fish, the right and left sides are extremely unequal; the far
greater number of species are dextral, with rare and occasional reversals
of development, and some few are normally sinistral; but certain species of
Bulimus, and, many Achatinellae,[123] are as often sinistral as dextral. I
will give an analogous case in the great Articulate kingdom: the two sides
of Verruca[124] are so wonderfully unlike, that without careful dissection
it is extremely difficult to recognise the corresponding parts on the
opposite sides of the body; yet it is apparently a mere matter of chance
whether it be the right or the left side that undergoes so singular an
amount of change. One plant is known to me[125] in which the flower,
according as it stands on the one or other side of the spike, is unequally
developed. In all the foregoing cases the two sides of the animal are
perfectly symmetrical at an early period of growth. Now, whenever a species
is as liable to be unequally developed on the one as on the other side, we
may infer that the capacity for such development is present, though latent,
in the undeveloped side. And as a reversal of development occasionally
occurs in animals of many kinds, this latent capacity is probably very
common.
The best yet simplest instances of characters lying dormant are, perhaps,
those previously given, in which chickens and {54} young pigeons, raised
from a cross between differently coloured birds, are at first of one
colour, but in a year or two acquire feathers of the colour of the other
parent; for in this case the tendency to a change of plumage is clearly
latent in the young bird. So it is with hornless breeds of cattle, some of
which acquire, as they grow old, small horns. Purely bred black and white
bantams, and some other fowls, occasionally assume, with advancing years,
the red feathers of the parent-species. I will here add a somewhat
different case, as it connects in a striking manner latent characters of
two classes. Mr. Hewitt[126] possessed an excellent Sebright gold-laced hen
bantam, which, as she became old, grew diseased in her ovaria, and assumed
male characters. In this breed the males resemble the females in all
respects except in their combs, wattles, spurs, and instincts; hence it
might have been expected that the diseased hen would have assumed only
those masculine characters which are proper to the breed, but she acquired,
in addition, well-arched tail sickle-feathers quite a foot in length,
saddle-feathers on the loins, and hackles on the neck,--ornaments which, as
Mr. Hewitt remarks, "would be held as abominable in this breed." The
Sebright bantam is known[127] to have originated about the year 1800 from a
cross between a common bantam and a Polish fowl, recrossed by a hen-tailed
bantam, and carefully selected; hence there can hardly be a doubt that the
sickle-feathers and hackles which appeared in the old hen were derived from
the Polish fowl or common bantam; and we thus see that not only certain
masculine characters proper to the Sebright bantam, but other masculine
characters derived from the first progenitors of the breed, removed by a
period of above sixty years, were lying latent in this hen-bird, ready to
be evolved as soon as her ovaria became diseased.
From these several facts it must be admitted that certain characters,
capacities, and instincts may lie latent in an individual, and even in a
succession of individuals, without our being able to detect the least signs
of their presence. We have {55} already seen that the transmission of a
character from the grandparent to the grandchild, with its apparent
omission in the intermediate parent of the opposite sex, becomes simple on
this view. When fowls, pigeons, or cattle of different colours are crossed,
and their offspring change colour as they grow old, or when the crossed
turbit acquired the characteristic frill after its third moult, or when
purely-bred bantams partially assume the red plumage of their prototype, we
cannot doubt that these qualities were from the first present, though
latent, in the individual animal, like the characters of a moth in the
caterpillar. Now, if these animals had produced offspring before they had
acquired with advancing age their new characters, nothing is more probable
than that they would have transmitted them to some of their offspring,
which in this case would in appearance have received such characters from
their grandparents or more distant progenitors. We should then have had a
case of reversion, that is, of the reappearance in the child of an
ancestral character, actually present, though during youth completely
latent, in the parent; and this we may safely conclude is what occurs with
reversions of all kinds to progenitors however remote.
This view of the latency in each generation of all the characters which
appear through reversion, is also supported by their actual presence in
some cases during early youth alone, or by their more frequent appearance
and greater distinctness at this age than during maturity. We have seen
that this is often the case with the stripes on the legs and faces of the
several species of the horse-genus. The Himalayan rabbit, when crossed,
sometimes produces offspring which revert to the parent silver-grey breed,
and we have seen that in purely bred animals pale-grey fur occasionally
reappears during early youth. Black cats, we may feel assured, would
occasionally produce by reversion tabbies; and on young black kittens, with
a pedigree[128] known to have been long pure, faint traces of stripes may
almost always be seen which afterwards disappear. Hornless Suffolk cattle
occasionally produce by reversion horned animals; and Youatt[129] asserts
that even in hornless individuals {56} "the rudiment of a horn may be often
felt at an early age."
No doubt it appears at first sight in the highest degree improbable that in
every horse of every generation there should be a latent capacity and
tendency to produce stripes, though these may not appear once in a thousand
generations; that in every white, black, or other coloured pigeon, which
may have transmitted its proper colour during centuries, there should be a
latent capacity in the plumage to become blue and to be marked with certain
characteristic bars; that in every child in a six-fingered family there
should be the capacity for the production of an additional digit; and so in
other cases. Nevertheless there is no more inherent improbability in this
being the case than in a useless and rudimentary organ, or even in only a
tendency to the production of a rudimentary organ, being inherited during
millions of generations, as is well known to occur with a multitude of
organic beings. There is no more inherent improbability in each domestic
pig, during a thousand generations, retaining the capacity and tendency to
develop great tusks under fitting conditions, than in the young calf having
retained for an indefinite number of generations rudimentary incisor teeth,
which never protrude through the gums.