The Variation of Animals and Plants under Domestication
by
Charles Darwin
Part 10 out of 10
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." (12/21. These
various statements are taken from the following works and papers:--Youatt
on 'The Horse' pages 35, 220. Lawrence 'The Horse' page 30. Karkeek in an
excellent paper in 'Gard. Chronicle' 1853 page 92. Mr. Burke in 'Journal of
R. Agricul. Soc. of England' volume 5 page 511. 'Encyclop. of Rural Sports'
page 279. Girou de Buzareingues 'Philosoph. Phys.' page 215. See following
papers in 'The Veterinary;' Roberts in volume 2 page 144; M. Marrimpoey
volume 2 page 387; Mr. Karkeek volume 4 page 5; Youatt on Goitre in 'Dogs'
volume 5 page 483: Youatt in volume 6 pages 66, 348, 412; M. Bernard volume
11 page 539; Dr. Samesreuther on 'Cattle' in volume 12 page 181; Percivall
in volume 13 page 47. With respect to blindness in horses see also a whole
row of authorities in Dr. P. Lucas's great work, tome 1 page 399. Mr. Baker
in 'The Veterinary' volume 13 page 721, gives a strong case of hereditary
imperfect vision and of jibbing.) So it is in regard to cattle, with
consumption, good and bad teeth, fine skin, etc. etc. 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. (12/22. Knight on 'The Culture of the
Apple and Pear' page 34. Lindley's 'Horticulture' page 180.)
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 life-time
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. (12/23. These statements are taken from the following works
in order:--Youatt on 'The Horse' page 48; Mr. Darvill in 'The Veterinary'
volume 8 page 50. With respect to Robson see 'The Veterinary' volume 3 page
580; Mr. Lawrence on 'The Horse' 1829 page 9; 'The Stud Farm' by Cecil
1851; Baron Cameronn quoted in 'The Veterinary' volume 10 page 500.)
With respect to the transmission of the many slight, but 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 (12/24. 'Recreations in Agriculture and Nat.
Hist.' volume 1 page 68.) 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 (12/25. 'Ueber die Eigenschaften' etc. 1828 s.
107.) 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 (12/26. Bronn 'Geschichte der Natur' b. 2 s. 132.), 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 encroach
on the subject of inherited mutilations. A man who is left-handed, and a
shell in which the spire turns in the wrong directions, are departures from
the normal asymmetrical condition, and they are well-known to be inherited.
[POLYDACTYLISM.
Supernumerary fingers and toes are eminently liable, as various authors
have insisted, to be inherited. Polydactylism graduates (12/27. Vrolik has
discussed this point at full length in a work published in Dutch, from
which Sir J. Paget has kindly translated for me passages. See, also,
Isidore Geoffroy St. Hilaire 'Hist. des Anomalies' 1832 tome 1 page 684.)
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 be attached to
the inner or outer margin of the hand, representing either a thumb or
little finger, the latter being the more frequent. Generally, through the
law of correlation, both hands and both feet are similarly affected. Dr.
Burt Wilder has tabulated (12/28. 'Massachusetts Medical Society' volume 2
No. 3; and 'Proc. Boston Soc. of Nat. Hist.' volume 14 1871 page 154.) a
large number of cases, and finds that supernumerary digits are more common
on the hands than on the feet, and that men are affected oftener than
women. Both these facts can be explained on two principles which seem
generally to hold good; firstly, that of two parts, the more specialised
one is the more variable, and the arm is more highly specialised than the
leg; and secondly that male animals are more variable than females.
The presence of a greater number of digits than five is a great anomaly,
for this number is not normally exceeded by any existing mammal, bird, or
reptile. 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 has 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 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 (12/29. Dr. J.W. Ogle gives a case of the inheritance of
deficient phalanges during four generations. He adds references to various
recent papers on inheritance 'Brit. and For. Med.-Chirurg. Review' April
1872.), thickened joints, crooked fingers, etc., are, in like manner,
strongly inherited, and are equally subject to intermission, together with
reversion, though in such cases there is no reason to suppose that both
parents had been similarly affected. (12/30. For these several statements
see Dr. Struthers 'Edinburgh New Phil. Journal' July 1863 especially on
intermissions in the line of descent. Prof. Huxley 'Lectures on our
Knowledge of Organic Nature' 1863 page 97. With respect to inheritance, see
Dr. Prosper Lucas 'L'Heredite Nat.' tome 1 page 325. Isid. Geoffroy 'Anom.'
tome 1 page 701. Sir A. Carlisle in 'Phil. Transact.' 1814 page 94. A.
Walker on 'Intermarriage' 1838 page 140 gives a case of five generations;
as does Mr. Sedgwick in 'Brit. and Foreign Medico-Chirurg. Review' April
1863 page 462. On the inheritance of other anomalies in the extremities see
Dr. H. Dobell in volume 46 of Medico-Chirurg. Transactions 1863; also Mr.
Sedgwick in op. cit. April 1863 page 460. With respect to additional digits
in the negro see Prichard 'Physical History of Mankind.' Dr. Dieffenbach
'Jour. Royal Geograph. Soc.' 1841 page 208 says this anomaly is not
uncommon with the Polynesians of the Chatham Islands; and I have heard of
several cases with Hindus and Arabs.)
Additional digits have been observed in negroes as well as in other races
of man, and in several of the lower animals, and have been inherited. Six
toes have been described on the hind feet of the newt (Salamandra
cristata), and are said to have occurred with the frog. It deserves notice,
that the six-toed newt, though adult, preserved some of its larval
characters; for part of the hyoidal apparatus, which is properly absorbed
during the act of metamorphosis, was retained. It is also remarkable that
in the case of man various structures in an embryonic or arrested state of
development, such as a cleft-palate, bifid uterus, etc., are often
accompanied by polydactylism. (12/31. Meckel and Isid G. St. Hilaire insist
on this fact. See also M. A. Roujou 'Sur quelques Analogies du Type Humain'
page 61 published, I believe, in the 'Journal of the Anthropolog. Soc. of
Paris' January 1872.) Six toes on the hinder feet are known to have been
inherited for three generations of 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 (12/32. 'The Poultry
Chronicle' 1854 page 559.) With animals which have properly less than five
digits, the number is sometimes increased to five, especially on 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 cows, sheep, goats,
and pigs. (12/33. The statements in this paragraph are taken from Isidore
Geoffroy St. Hilaire 'Hist. des Anomalies' tome 1 pages 688-693. Mr.
Goodman gives, 'Phil. Soc. of Cambridge' November 25, 1872 the case of a
cow with three well developed toes on each hind limb, besides the ordinary
rudiments; and her calf by an ordinary bull had extra digits. This calf
also bore two calves having extra digits.)
There is a famous case described by Mr. White of 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 removed by its socket-joint, but again it
grew and reproduced a nail. Dr. Struthers mentions a case of the partial
regrowth of an additional thumb, amputated when a child was three months
old; and the late Dr. Falconer communicated to me an analogous instance. In
the last edition of this work I also gave a case of the regrowth of a
supernumerary little-finger after amputation; but having been informed by
Dr. Bachmaier that several eminent surgeons expressed, at a meeting of the
Anthropological Society of Munich, great doubt about my statements, I have
made more particular inquiries. The full information thus gained, together
with a tracing of the hand in its present state, has been laid before Sir
J. Paget, and he has come to the conclusion that the degree of regrowth in
this case is not greater than sometimes occurs with normal bones,
especially with the humerus, when amputated at an early age. He further
does not feel fully satisfied about the facts recorded by Mr. White. This
being so, it is necessary for me to withdraw the view which I formerly
advanced, with much hesitation, chiefly on the ground of the supposed
regrowth of additional digits, namely, that their occasional development in
man is a case of reversion to a lowly, organised progenitor provided with
more than five digits.]
I may here allude to a class of facts closely allied to, but somewhat
different from, ordinary cases of inheritance. Sir H. Holland (12/34.
'Medical Notes and Reflections' 1839 pages 24, 34. See also Dr. P. Lucas
'L'Hered. Nat.' tome 2 page 33.) 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
already been given of supernumerary digits appearing in four children out
of six in a previously unaffected family. Dr. Devay states (12/35. 'Du
Danger des Mariages Consanguins' 2nd edition 1862 page 103.) 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
(12/36. 'British and Foreign Medico-Chirurg. Review' July 1863 pages 183,
189.) 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 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, taken 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 (12/37. Verlot 'La Product. des Varietes' 1865 page 32.) 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; numerous analogous cases could be given.
[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.
(12/38. Loudon's 'Gardener's Mag.' volume 12 1836 page 368.) Mr. Rivers
fertilised, 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 (12/39. Verlot 'La Product. des Varietes' 1865 page 94.),
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. (12/40. Bronn 'Geschichte der Natur' b. 2 s. 121. Mr. Meehan makes a
similar statement in 'Proc. Nat. of Philadelphia' 1872 page 235.) Lastly, a
weeping or rather a 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, being sown, produced seventeen trees all of which had
exactly the same peculiar habit with the parent-tree. (12/41. Rev. W.A.
Leighton 'Flora of Shropshire' page 497; and Charlesworth 'Mag. of Nat.
Hist.' volume 1 1837 page 30. I possess prostrate trees produced from these
seeds.)
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 (12/42. Verlot op.
cit. page 93.) sowed seeds of the weeping beech (Fagus sylvatica), 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. (12/43. For these several statements see Loudon's
'Gard. Magazine' volume 10 1834 pages 408, 180; and volume 9 1833 page
597.) 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), now about twenty years old, which was formerly
pendulous, "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 character 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
(12/44. These statements are taken from Alph. De Candolle 'Bot. Geograph.'
page 1083.): and lastly, all the trees raised by my father from a yellow-
berried holly (Ilex aquifolium), found wild, produced yellow berries.
Vilmorin (12/45. Verlot op. cit. page 38.) 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 (12/46. Op. cit. page 59.),
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" (12/47. Alph. De Candolle 'Geograph. Bot.' page
1082.); 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. (12/48. See 'Cottage Gardener'
April 10, 1860 page 18 and September 10, 1861 page 456; 'Gardener's
Chronicle' 1845 page 102.) I sowed seed of twelve 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 (12/49. Darwin in 'Journal of Proc. Linn.
Soc. Bot.' 1862 page 94.); and these are plants which florists speak of as
never coming 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 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, 72 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, etc., 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. (12/50. Hofacker
'Ueber die Eigenschaften' etc. s. 10.)]
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 jonquil-coloured bird it does not answer
to pair two jonquils, as the colour then comes out too strong, or is even
brown; but this statement is disputed by other breeders. So again, if two
crested canaries are paired, the young birds rarely inherit this character
(12/51. Bechstein 'Naturgesch. Deutschlands' b. 4 s. 462. Mr. Brent, a
great breeder of canaries, informs me that he believes that these
statements are correct.): 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 (12/52. 'The Poultry Book' by W.B.
Tegetmeier 1866 page 245.) 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 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 affected. So again,
when a male or 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 (12/53. 'British and
Foreign Med.-Chirurg. Review' July 1861 pages 200-204. Mr. Sedgwick has
given such full details on this subject, with ample references, that I need
refer to no other authorities), 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.
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 (12/54. Mr. Sproule in 'British Medical Journal' April
18, 1863.) yet other malformations are rarely or never inherited. Of these
latter 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.
With respect to the inheritance of structures mutilated by injuries or
altered by disease, it was until lately difficult to come to any definite
conclusion. Some mutilations have been practised for a vast number of
generations without any inherited result. Godron remarks (12/55. 'De
l'Espece' tome 2 1859 page 299.) 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, tatooed themselves, made deep gashes in various
parts of their bodies, and there is no reason to suppose that these
mutilations have ever been inherited. (12/56. Nevertheless Mr. Wetherell
states, 'Nature' December 1870 page 168, that when he visited fifteen years
ago the Sioux Indians, he was informed "by a physician, who has passed much
of his time with these tribes, that some times a child was born with these
marks. This was confirmed by the U.S. Government Indian Agent.") 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, however, asserts (12/57.
'Philosoph. Mag.' volume 4 1799 page 5.) that Jews are often born in
Germany in a condition rendering circumcision difficult, so that a name is
given them signifying "born circumcised;" and Professor Preyer informs me
that this is the case in Bonn, such children being considered the special
favourites of Jehovah. I have also heard from Dr. A. Newman, of Guy's
Hospital, of the grandson of a circumcised Jew, the father not having been
circumcised, in a similar condition. But it is possible that all these
cases may be accidental coincidence, for Sir J. Paget has seen five sons of
a lady and one son of her sister with adherent prepuces; and one of these
boys was affected in a manner "which might be considered like that commonly
produced by circumcision;" yet there was no suspicion of Jewish blood in
the family of these two sisters. Circumcision is practised by Mahomedans,
but at a much later age than by Jews; and Dr. Riedel, Assistant Resident in
North Celebes, writes to me that the boys there go naked until from six to
ten years old; and he has observed that many of them, though not all, have
their prepuces much reduced in length, and this he attributes to the
inherited effects of the operation. In the vegetable kingdom oaks and other
trees have borne galls from primeval times, yet they do not produce
inherited excrescences; and many other such facts could be adduced.
Notwithstanding the above several negative cases, we now possess conclusive
evidence that the effects of operations are sometimes inherited. Dr. Brown-
Sequard (12/58. 'Proc. Royal Soc.' volume 10 page 297. 'Communication to
the Brit. Assoc.' 1870. 'The Lancet' January 1875 page 7. The extracts are
from this last paper. It appears that Obersteiner 'Stricker's Med.
Jahrbucher' 1875 No. 2 has confirmed Brown-Sequard's observations.) gives
the following summary of his observations on guinea-pigs; and this summary
is so important that I will quote the whole:--
["1st. Appearance of epilepsy in animals born of parents having been
rendered epileptic by an injury to the spinal cord.
"2nd. Appearance of epilepsy also in animals born of parents having been
rendered epileptic by the section of the sciatic nerve.
"3rd. A change in the shape of the ear in animals born of parents in which
such a change was the effect of a division of the cervical sympathetic
nerve.
"4th. Partial closure of the eyelids in animals born of parents in which
that state of the eyelids had been caused either by the section of the
cervical sympathetic nerve or the removal of the superior cervical
ganglion.
"5th. Exophthalmia in animals born of parents in which an injury to the
restiform body had produced that protrusion of the eyeball. This
interesting fact I have witnessed a good many times, and I have seen the
transmission of the morbid state of the eye continue through four
generations. In these animals, modified by heredity, the two eyes generally
protruded, although in the parents usually only one showed exophthalmia,
the lesion having been made in most cases only on one of the corpora
restiformia.
"6th. Haematoma and dry gangrene of the ears in animals born of parents in
which these ear-alterations had been caused by an injury to the restiform
body near the nib of the calamus.
"7th. Absence of two toes out of the three of the hind leg, and sometimes
of the three, in animals whose parents had eaten up their hind-leg toes
which had become anaesthetic from a section of the sciatic nerve alone, or
of that nerve and also of the crural. Sometimes, instead of complete
absence of the toes, only a part of one or two or three was missing in the
young, although in the parent not only the toes but the whole foot was
absent (partly eaten off, partly destroyed by inflammation, ulceration, or
gangrene).
"8th. Appearance of various morbid states of the skin and hair of the neck
and face in animals born of parents having had similar alterations in the
same parts, as effects of an injury to the sciatic nerve."]
It should be especially observed that Brown-Sequard has bred during thirty
years many thousand guinea-pigs from animals which had not been operated
upon, and not one of these manifested the epileptic tendency. Nor has he
ever seen a guinea-pig born without toes, which was not the offspring of
parents which had gnawed off their own toes owing to the sciatic nerve
having been divided. Of this latter fact thirteen instances were carefully
recorded, and a greater number were seen; yet Brown-Sequard speaks of such
cases as one of the rarer forms of inheritance. It is a still more
interesting fact--
["That the sciatic nerve in the congenitally toeless animal has inherited
the power of passing through all the different morbid states which have
occurred in one of its parents from the time of the division till after its
reunion with the peripheric end. It is not therefore simply the power of
performing an action which is inherited, but the power of performing a
whole series of actions, in a certain order."]
In most of the cases of inheritance recorded by Brown-Sequard only one of
the two parents had been operated upon and was affected. He concludes by
expressing his belief that "what is transmitted is the morbid state of the
nervous system," due to the operation performed on the parents.
With the lower animals Dr. Prosper Lucas has collected a long list of
inherited injuries. A few instances will suffice. A cow lost a horn from an
accident with consequent suppuration, and she produced three calves which
were hornless on the same side of the head. With the horse, there seems
hardly a doubt that 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 two sons were microphthalmic on the same
side. (12/59. This last case is quoted by Mr. Sedgwick in 'British and
Foreign Medico-Chirurg. Review' April, 1861 page 484. For Blumenbach see
above-cited paper. See also Dr. P. Lucas 'Traite de l'Hered. Nat.' tome 2
page 492. Also 'Transact. Linn. Soc.' volume 9 page 323. Some curious cases
are given by Mr. Baker in the 'Veterinary' volume 13 page 723. Another
curious case is given in the 'Annales des Scienc. Nat.' 1st series, tome 11
page 324.) In all cases in which a parent has had an organ injured on one
side, and two or more of the offspring are born with the same organ
affected on the same side, the chances against mere coincidence are almost
infinitely great. Even when only a single child is born having exactly the
same part of the body affected as that of his injured parent, the chances
against coincidence are great; and Professor Rolleston has given me two
such cases which have fallen under his own observation,--namely of two men,
one of whom had his knee and the other his cheek severely cut, and both had
children born with exactly the same spot marked or scarred. Many instances
have been recorded of cats, dogs, and horses, which have had their tails,
legs, etc., amputated or injured, producing offspring with the same parts
ill-formed; but as it is not very rare for similar malformations to appear
spontaneously, all such cases may be due to coincidence. It is, however, an
argument on the other side that "under the old excise laws the shepherd-dog
was only exempt from tax when without a tail, and for this reason it was
always removed" (12/60. 'The Dog' by Stonehenge 1867 page 118.); and there
still exist breeds of the shepherd-dog which are always born destitute of a
tail. Finally, it must be admitted, more especially since the publication
of Brown-Sequard's observations, that the effects of injuries, especially
when followed by disease, or perhaps exclusively when thus followed, are
occasionally inherited. (12/61. The Mot-mot habitually bites the barbs off
the middle part of the two central tail-feathers, and as the barbs are
congenitally somewhat reduced on the same part of these feathers, it seems
extremely probable, as Mr. Salvin remarks, 'Proc. Zoolog. Soc.' 1873 page
429, that this is due to the inherited effects of long-continued
mutilation.)
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 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 short muzzles and legs, and their
tendency to fatten. 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 tropical varieties of
maize lose their proper character in the course of two or three
generations, when cultivated in Europe; and conversely so it is with
European varieties cultivated in Brazil. Our cabbages, which here come so
true by seed, cannot form heads in hot countries. According to Carriere
(12/62. 'Production et Fixation des Varietes' 1865 page 72.) the purple-
leafed beech and barberry transmit their character by seed far less truly
in certain districts than in others. 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 habit of early 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, etc., are sown, the seedlings generally inherit some
degree of family likeness. 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 perfectly to resemble the parent-form; and
thus 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. (12/63.
Downing 'Fruits of America' page 5: Sageret 'Pom. Phys.' pages 43, 72.)
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 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 been long
propagated by buds, cuttings, offsets, tubers, etc., and have in
consequence been frequently exposed during what may be called 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 commonly exposed during the life of the individual 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 character derived from either parent or their
ancestors.
CONCLUSION.
It has been shown in the early part of this chapter how commonly 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. 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 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. The
effects of injuries or mutilations are occasionally inherited; and we shall
see in a future chapter that the long-continued use and disuse of parts
produces an inherited effect. 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
discussed in the following chapter.
END OF VOLUME I.
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