CHARACTER—PREPOTENCY—SEXUAL LIMITATION—CORRESPONDENCE OF AGE.
FIXEDNESS OF CHARACTER APPARENTLY NOT DUE TO ANTIQUITY OF
INITANCE—PREPOTENCY OF TRANSMISSION IN INDIVIDUALS OF THE SAME FAMILY,
IN CROSSED BREEDS AND SPECIES; OFTEN STRONGER IN ONE SEX THAN THE
OTHER; SOMETIMES DUE TO THE SAME CHARACTER BEING PRESENT AND VISIBLE IN
ONE BREED AND LATENT IN THE OTHER—INHERITANCE AS LIMITED BY
SEX—NEWLY-ACQUIRED CHARACTERS IN OUR DOMESTICATED ANIMALS OFTEN
TRANSMITTED BY ONE SEX ALONE, SOMETIMES LOST BY ONE SEX
ALONE—INHERITANCE AT CORRESPONDING PERIODS OF LIFE—THE IMPORTANCE OF
THE PRINCIPLE WITH RESPECT TO EMBRYOLOGY; AS EXHIBITED IN DOMESTICATED
ANIMALS: AS EXHIBITED IN THE APPEARANCE AND DISAPPEARANCE OF INHERITED
DISEASES; SOMETIMES SUPERVENING EARLIER IN THE CHILD THAN IN THE
PARENT—SUMMARY OF THE THREE PRECEDING CHAPTERS.
In the last two chapters the nature and force of Inheritance, the
circumstances which interfere with its power, and the tendency to
Reversion, with its many remarkable contingencies, were discussed. In
the present chapter some other related phenomena will be treated of, as
fully as my materials permit.
_Fixedness of Character._
It is a general belief amongst breeders that the longer any character
has been transmitted by a breed, the more fully it will continue to be
transmitted. I do not wish to dispute the truth of the proposition that
inheritance gains strength simply through long continuance, but I doubt
whether it can be proved. In one sense the proposition is little better
than a truism; if any character has remained constant during many
generations, it will be likely to continue so, if the conditions of
life remain the same. So, again, in improving a breed, if care be taken
for a length of time to exclude all inferior individuals, the breed
will obviously tend to become truer, as it will not have been crossed
during many generations by an inferior animal. We have previously seen,
but without being able to assign any cause, that, when a new character
appears, it is occasionally from the first constant, or fluctuates
much, or wholly fails to be transmitted. So it is with the aggregate of
slight differences which characterise a new variety, for some propagate
their kind from the first much truer than others. Even with plants
multiplied by bulbs, layers, etc., which may in one sense be said to
form parts of the same individual, it is well known that certain
varieties retain and transmit through successive bud-generations their
newly-acquired characters more truly than others. In none of these, nor
in the following cases, does there appear to be any relation between
the force with which a character is transmitted and the length of time
during which it has been transmitted. Some varieties, such as white and
yellow hyacinths and white sweet-peas, transmit their colours more
faithfully than do the varieties which have retained their natural
colour. In the Irish family, mentioned in the twelfth chapter, the
peculiar tortoiseshell-like colouring of the eyes was transmitted far
more faithfully than any ordinary colour. Ancon and Mauchamp sheep and
niata cattle, which are all comparatively modern breeds, exhibit
remarkably strong powers of inheritance. Many similar cases could be
adduced.
As all domesticated animals and cultivated plants have varied, and yet
are descended from aboriginally wild forms, which no doubt had retained
the same character from an immensely remote epoch, we see that scarcely
any degree of antiquity ensures a character being transmitted perfectly
true. In this case, however, it may be said that changed conditions of
life induce certain modifications, and not that the power of
inheritance fails; but in every case of failure, some cause, either
internal or external, must interfere. It will generally be found that
the organs or parts which in our domesticated productions have varied,
or which still continue to vary,—that is, which fail to retain their
former state,—are the same with the parts which differ in the natural
species of the same genus. As, on the theory of descent with
modification, the species of the same genus have been modified since
they branched off from a common progenitor, it follows that the
characters by which they differ from one another have varied, whilst
other parts of the organisation have remained unchanged; and it might
be argued that these same characters now vary under domestication, or
fail to be inherited, from their lesser antiquity. But variation in a
state of nature seems to stand in some close relation with changed
conditions of life, and characters which have already varied under such
conditions would be apt to vary under the still greater changes
consequent on domestication, independently of their greater or less
antiquity.
Fixedness of character, or the strength of inheritance, has often been
judged of by the preponderance of certain characters in the crossed
offspring between distinct races; but prepotency of transmission here
comes into play, and this, as we shall immediately see, is a very
different consideration from the strength or weakness of
inheritance.[1] It has often been observed that breeds of animals
inhabiting wild and mountainous countries cannot be permanently
modified by our improved breeds; and as these latter are of modern
origin, it has been thought that the greater antiquity of the wilder
breeds has been the cause of their resistance to improvement by
crossing; but it is more probably due to their structure and
constitution being better adapted to the surrounding conditions. When
plants are first subjected to culture, it has been found that, during
several generations, they transmit their characters truly, that is, do
not vary, and this has been attributed to ancient characters being
strongly inherited: but it may with equal or greater probability be
consequent on changed conditions of life requiring a long time for
their cumulative action. Notwithstanding these considerations, it would
perhaps be rash to deny that characters become more strongly fixed the
longer they are transmitted; but I believe that the proposition
resolves itself into this,—that characters of all kinds, whether new or
old, tend to be inherited, and that those which have already withstood
all counteracting influences and been truly transmitted, will, as a
general rule, continue to withstand them, and consequently be
faithfully inherited.
_Prepotency in the Transmission of Character._
When individuals, belonging to the same family, but distinct enough to
be recognised, or when two well-marked races, or two species, are
crossed, the usual result, as stated in the previous chapter, is, that
the offspring in the first generation are intermediate between their
parents, or resemble one parent in one part and the other parent in
another part. But this is by no means the invariable rule; for in many
cases it is found that certain individuals, races, and species, are
prepotent in transmitting their likeness. This subject has been ably
discussed by Prosper Lucas,[2] but is rendered extremely complex by the
prepotency sometimes running equally in both sexes, and sometimes more
strongly in one sex than in the other; it is likewise complicated by
the presence of secondary sexual characters, which render the
comparison of crossed breeds with their parents difficult.
It would appear that in certain families some one ancestor, and after
him others in the same family, have had great power in transmitting
their likeness through the male line; for we cannot otherwise
understand how the same features should so often be transmitted after
marriages with many females, as in the case of the Austrian Emperors;
and so it was, according to Niebuhr, with the mental qualities of
certain Roman families.[3] The famous bull Favourite is believed[4] to
have had a prepotent influence on the shorthorn race. It has also been
observed[5] with English racehorses that certain mares have generally
transmitted their own character, whilst other mares of equally pure
blood have allowed the character of the sire to prevail. A famous black
greyhound, Bedlamite, as I hear from Mr. C. M. Brown “invariably got
all his puppies black, no matter what was the colour of the bitch;” but
then Bedlamite “had a preponderance of black in his blood, both on the
sire and dam side.”
The truth of the principle of prepotency comes out more clearly when
distinct races are crossed. The improved Shorthorns, notwithstanding
that the breed is comparatively modern, are generally acknowledged to
possess great power in impressing their likeness on all other breeds;
and it is chiefly in consequence of this power that they are so highly
valued for exportation.[6] Godine has given a curious case of a ram of
a goat-like breed of sheep from the Cape of Good Hope, which produced
offspring hardly to be distinguished from himself, when crossed with
ewes of twelve other breeds. But two of these half-bred ewes, when put
to a merino ram, produced lambs closely resembling the merino breed.
Girou de Buzareingues[7] found that of two races of French sheep the
ewes of one, when crossed during successive generations with merino
rams, yielded up their character far sooner than the ewes of the other
race. Sturm and Girou have given analogous cases with other breeds of
sheep and with cattle, the prepotency running in these cases through
the male side; but I was assured on good authority in South America,
that when niata cattle are crossed with common cattle, though the niata
breed is prepotent whether males or females are used, yet that the
prepotency is strongest through the female line. The Manx cat is
tailless and has long hind legs; Dr. Wilson crossed a male Manx with
common cats, and, out of twenty-three kittens, seventeen were destitute
of tails; but when the female Manx was crossed by common male cats all
the kittens had tails, though they were generally short and
imperfect.[8]
In making reciprocal crosses between pouter and fantail pigeons, the
pouter-race seemed to be prepotent through both sexes over the fantail.
But this is probably due to weak power in the fantail rather than to
any unusually strong power in the pouter, for I have observed that
barbs also preponderate over fantails. This weakness of transmission in
the fantail, though the breed is an ancient one, is said[9] to be
general; but I have observed one exception to the rule, namely, in a
cross between a fantail and laugher. The most curious instance known to
me of weak power in both sexes is in the trumpeter pigeon. This breed
has been well known for at least 130 years: it breeds perfectly true,
as I have been assured by those who have long kept many birds: it is
characterised by a peculiar tuft of feathers over the beak, by a crest
on the head, by a singular coo quite unlike that of any other breed,
and by much-feathered feet. I have crossed both sexes with turbits of
two sub-breeds, with almond tumblers, spots, and runts, and reared many
mongrels and recrossed them; and though the crest on the head and
feathered feet were inherited (as is generally the case with most
breeds), I have never seen a vestige of the tuft over the beak or heard
the peculiar coo. Boitard and Corbié[10] assert that this is the
invariable result of crossing trumpeters with other breeds:
Neumeister,[11] however, states that in Germany mongrels have been
obtained, though very rarely, which were furnished with the tuft and
would trumpet: but a pair of these mongrels with a tuft, which I
imported, never trumpeted. Mr. Brent states[12] that the crossed
offspring of a trumpeter were crossed with trumpeters for three
generations, by which time the mongrels had 7/8ths of this blood in
their veins, yet the tuft over the beak did not appear. At the fourth
generation the tuft appeared, but the birds though now having 15-16ths
trumpeter’s blood still did not trumpet. This case well shows the wide
difference between inheritance and prepotency; for here we have a
well-established old race which transmits its characters faithfully,
but which, when crossed with any other race, has the feeblest power of
transmitting its two chief characteristic qualities.
I will give one other instance with fowls and pigeons of weakness and
strength in the transmission of the same character to their crossed
offspring. The Silk fowl breeds true, and there is reason to believe is
a very ancient race; but when I reared a large number of mongrels from
a Silk hen by a Spanish cock, not one exhibited even a trace of the
so-called silkiness. Mr. Hewitt also asserts that in no instance are
the silky feathers transmitted by this breed when crossed with any
other variety. But three birds out of many raised by Mr. Orton from a
cross between a silk cock and a bantam hen had silky feathers.[13] So
that it is certain that this breed very seldom has the power of
transmitting its peculiar plumage to its crossed progeny. On the other
hand, there is a silk sub-variety of the fantail pigeon, which has its
feathers in nearly the same state as in the Silk fowl: now we have
already seen that fantails, when crossed, possess singularly weak power
in transmitting their general qualities; but the silk sub-variety when
crossed with any other small-sized race invariably transmits its silky
feathers![14]
The well-known horticulturist, Mr. Paul, informs me that he fertilised
the Black Prince hollyhock with pollen of the White Globe and the
Lemonade and Black Prince hollyhocks reciprocally; but not one seedling
from these three crosses inherited the black colour of the Black
Prince. So, again, Mr. Laxton, who has had such great experience in
crossing peas, writes to me that “whenever a cross has been effected
between a white-blossomed and a purple-blossomed pea, or between a
white-seeded and a purple-spotted, brown or maple-seeded pea, the
offspring seems to lose nearly all the characteristics of the
white-flowered and white-seeded varieties; and this result follows
whether these varieties have been used as the pollen-bearing or
seed-producing parents.”
The law of prepotency comes into action when species are crossed, as
with races and individuals. Gärtner has unequivocally shown[15] that
this is the case with plants. To give one instance: when _Nicotiana
paniculata_ and _ vincæflora_ are crossed, the character of _N.
paniculata_ is almost completely lost in the hybrid; but if _ N.
quadrivalvis_ be crossed with _N. vincæflora,_ this latter species,
which was before so prepotent, now in its turn almost disappears under
the power of _N. quadrivalvis._ It is remarkable that the prepotency of
one species over another in transmission is quite independent, as shown
by Gärtner, of the greater or less facility with which the one
fertilises the other.
With animals, the jackal is prepotent over the dog, as is stated by
Flourens, who made many crosses between these animals; and this was
likewise the case with a hybrid which I once saw between a jackal and a
terrier. I cannot doubt, from the observations of Colin and others,
that the ass is prepotent over the horse; the prepotency in this
instance running more strongly through the male than through the female
ass; so that the mule resembles the ass more closely than does the
hinny.[16] The male pheasant, judging from Mr. Hewitt’s
descriptions,[17] and from the hybrids which I have seen, preponderates
over the domestic fowl; but the latter, as far as colour is concerned,
has considerable power of transmission, for hybrids raised from five
differently coloured hens differed greatly in plumage. I formerly
examined some curious hybrids in the Zoological Gardens, between the
Penguin variety of the common duck and the Egyptian goose (_Anser
ægyptiacus_); and although I will not assert that the domesticated
variety preponderated over the natural species, yet it had strongly
impressed its unnatural upright figure on these hybrids.
I am aware that such cases as the foregoing have been ascribed by
various authors, not to one species, race, or individual being
prepotent over the other in impressing its character on its crossed
offspring, but to such rules as that the father influences the external
characters and the mother the internal or vital organs. But the great
diversity of the rules given by various authors almost proves their
falseness. Dr. Prosper Lucas has fully discussed this point, and has
shown[18] that none of the rules (and I could add others to those
quoted by him) apply to all animals. Similar rules have been announced
for plants, and have been proved by Gärtner[19] to be all erroneous. If
we confine our view to the domesticated races of a single species, or
perhaps even to the species of the same genus, some such rules may hold
good; for instance, it seems that in reciprocally crossing various
breeds of fowls the male generally gives colour;[20] but conspicuous
exceptions have passed under my own eyes. It seems that the ram usually
gives its peculiar horns and fleece to its crossed offspring, and the
bull the presence or absence of horns.
In the following chapter on Crossing I shall have occasion to show that
certain characters are rarely or never blended by crossing, but are
transmitted in an unmodified state from either parent-form; I refer to
this fact here because it is sometimes accompanied on the one side by
prepotency, which thus acquires the false appearance of unusual
strength. In the same chapter I shall show that the rate at which a
species or breed absorbs and obliterates another by repeated crosses,
depends in chief part on prepotency in transmission.
In conclusion, some of the cases above given,—for instance, that of the
trumpeter pigeon,—prove that there is a wide difference between mere
inheritance and prepotency. This latter power seems to us, in our
ignorance, to act in most cases quite capriciously. The very same
character, even though it be an abnormal or monstrous one, such as
silky feathers, may be transmitted by different species, when crossed,
either with prepotent force or singular feebleness. It is obvious, that
a purely-bred form of either sex, in all cases in which prepotency does
not run more strongly in one sex than the other, will transmit its
character with prepotent force over a mongrelised and already variable
form.[21] From several of the above-given cases we may conclude that
mere antiquity of character does not by any means necessarily make it
prepotent. In some cases prepotency apparently depends on the same
character being present and visible in one of the two breeds which are
crossed, and latent or invisible in the other breed; and in this case
it is natural that the character which is potentially present in both
breeds should be prepotent. Thus, we have reason to believe that there
is a latent tendency in all horses to be dun-coloured and striped; and
when a horse of this kind is crossed with one of any other colour, it
is said that the offspring are almost sure to be striped. Sheep have a
similar latent tendency to become dark-coloured, and we have seen with
what prepotent force a ram with a few black spots, when crossed with
white sheep of various breeds, coloured its offspring. All pigeons have
a latent tendency to become slaty-blue, with certain characteristic
marks, and it is known that, when a bird thus coloured is crossed with
one of any other colour, it is most difficult afterwards to eradicate
the blue tint. A nearly parallel case is offered by those black bantams
which, as they grow old, develop a latent tendency to acquire red
feathers. But there are exceptions to the rule: hornless breeds of
cattle possess a latent capacity to reproduce horns, yet when crossed
with horned breeds they do not invariably produce offspring bearing
horns.
We meet with analogous cases with plants. Striped flowers, though they
can be propagated truly by seed, have a latent tendency to become
uniformly coloured, but when once crossed by a uniformly coloured
variety, they ever afterwards fail to produce striped seedlings.[22]
Another case is in some respects more curious: plants bearing peloric
flowers have so strong a latent tendency to reproduce their normally
irregular flowers, that this often occurs by buds when a plant is
transplanted into poorer or richer soil.[23] Now I crossed the peloric
snapdragon (_Antirrhinum majus_), described in the last chapter, with
pollen of the common form; and the latter, reciprocally, with peloric
pollen. I thus raised two great beds of seedlings, and not one was
peloric. Naudin[24] obtained the same result from crossing a peloric
Linaria with the common form. I carefully examined the flowers of
ninety plants of the crossed Antirrhinum in the two beds, and their
structure had not been in the least affected by the cross, except that
in a few instances the minute rudiment of the fifth stamen, which is
always present, was more fully or even completely developed. It must
not be supposed that this entire obliteration of the peloric structure
in the crossed plants can be accounted for by any incapacity of
transmission; for I raised a large bed of plants from the peloric
Antirrhinum, artificially fertilised by its own pollen, and sixteen
plants, which alone survived the winter, were all as perfectly peloric
as the parent-plant. Here we have a good instance of the wide
difference between the inheritance of a character and the power of
transmitting it to crossed offspring. The crossed plants, which
perfectly resembled the common snapdragon, were allowed to sow
themselves, and out of a hundred and twenty-seven seedlings,
eighty-eight proved to be common snapdragons, two were in an
intermediate condition between the peloric and normal state, and
thirty-seven were perfectly peloric, having reverted to the structure
of their one grand-parent. This case seems at first sight to offer an
exception to the rule just given, namely, that a character which is
present in one form and latent in the other is generally transmitted
with prepotent force when the two forms are crossed. For in all the
Scrophulariaceæ, and especially in the genera Antirrhinum and Linaria,
there is, as was shown in the last chapter, a strong latent tendency to
become peloric; but there is also, as we have seen, a still stronger
tendency in all peloric plants to reacquire their normal irregular
structure. So that we have two opposed latent tendencies in the same
plants. Now, with the crossed Antirrhinums the tendency to produce
normal or irregular flowers, like those of the common Snapdragon,
prevailed in the first generation; whilst the tendency to pelorism,
appearing to gain strength by the intermission of a generation,
prevailed to a large extent in the second set of seedlings. How it is
possible for a character to gain strength by the intermission of a
generation, will be considered in the chapter on pangenesis.
On the whole, the subject of prepotency is extremely intricate,—from
its varying so much in strength, even in regard to the same character,
in different animals,—from its running either equally in both sexes,
or, as frequently is the case with animals, but not with plants, much
stronger in one sex than the other,—from the existence of secondary
sexual characters,—from the transmission of certain characters being
limited, as we shall immediately see, by sex,—from certain characters
not blending together,—and, perhaps, occasionally from the effects of a
previous fertilisation on the mother. It is therefore not surprising
that no one has hitherto succeeded in drawing up general rules on the
subject of prepotency.
_Inheritance as limited by Sex._
New characters often appear in one sex, and are afterwards transmitted
to the same sex, either exclusively or in a much greater degree than to
the other. This subject is important, because with animals of many
kinds in a state of nature, both high and low in the scale, secondary
sexual characters, not directly connected with the organs of
reproduction, are conspicuously present. With our domesticated animals,
characters of this kind often differ widely from those distinguishing
the two sexes of the parent species; and the principle of inheritance,
as limited by sex, explains how this is possible.
Dr. P. Lucas has shown[25] that when a peculiarity, in no manner
connected with the reproductive organs, appears in either parent, it is
often transmitted exclusively to the offspring of the same sex, or to a
much greater number of them than of the opposite sex. Thus, in the
family of Lambert, the horn-like projections on the skin were
transmitted from the father to his sons and grandsons alone; so it has
been with other cases of ichthyosis, with supernumerary digits, with a
deficiency of digits and phalanges, and in a lesser degree with various
diseases, especially with colour-blindness and the hæmorrhagic
diathesis, that is, an extreme liability to profuse and uncontrollable
bleeding from trifling wounds. On the other hand, mothers have
transmitted, during several generations, to their daughters alone,
supernumerary and deficient digits, colour-blindness and other
peculiarities. So that the very same peculiarity may become attached to
either sex, and be long inherited by that sex alone; but the attachment
in certain cases is much more frequent to one than the other sex. The
same peculiarities also may be promiscuously transmitted to either sex.
Dr. Lucas gives other cases, showing that the male occasionally
transmits his peculiarities to his daughters alone, and the mother to
her sons alone; but even in this case we see that inheritance is to a
certain extent, though inversely, regulated by sex. Dr. Lucas, after
weighing the whole evidence, comes to the conclusion that every
peculiarity tends to be transmitted in a greater or lesser degree to
that sex in which it first appears. But a more definite rule, as I have
elsewhere shown[26] generally holds good, namely, that variations which
first appear in either sex at a late period of life, when the
reproductive functions are active, tend to be developed in that sex
alone; whilst variations which first appear early in life in either sex
are commonly transmitted to both sexes. I am, however, far from
supposing that this is the sole determining cause.
A few details from the many cases collected by Mr. Sedgwick,[27] may be
here given. Colour-blindness, from some unknown cause, shows itself
much oftener in males than in females; in upwards of two hundred cases
collected by Mr. Sedgwick, nine-tenths related to men; but it is
eminently liable to be transmitted through women. In the case given by
Dr. Earle, members of eight related families were affected during five
generations: these families consisted of sixty-one individuals, namely,
of thirty-two males, of whom nine-sixteenths were incapable of
distinguishing colour, and of twenty-nine females, of whom only
one-fifteenth were thus affected. Although colour-blindness thus
generally clings to the male sex, nevertheless, in one instance in
which it first appeared in a female, it was transmitted during five
generations to thirteen individuals, all of whom were females. The
hæmorrhagic diathesis, often accompanied by rheumatism, has been known
to affect the males alone during five generations, being transmitted,
however, through the females. It is said that deficient phalanges in
the fingers have been inherited by the females alone during ten
generations. In another case, a man thus deficient in both hands and
feet, transmitted the peculiarity to his two sons and one daughter; but
in the third generation,—out of nineteen grandchildren, twelve sons had
the family defect, whilst the seven daughters were free. In ordinary
cases of sexual limitation, the sons or daughters inherit the
peculiarity, whatever it may be, from their father or mother, and
transmit it to their children of the same sex; but generally with the
hæmorrhagic diathesis, and often with colour-blindness, and in some
other cases, the sons never inherit the peculiarity directly from their
fathers, but the daughters alone transmit the latent tendency, so that
the sons of the daughters alone exhibit it. Thus the father, grandson,
and great-great-grandson will exhibit a peculiarity,—the grandmother,
daughter, and great-grand-daughter having transmitted it in a latent
state. Hence we have, as Mr. Sedgwick remarks, a double kind of atavism
or reversion; each grandson apparently receiving and developing the
peculiarity from his grandfather, and each daughter apparently
receiving the latent tendency from her grandmother.
From the various facts recorded by Dr. Prosper Lucas, Mr. Sedgwick, and
others, there can be no doubt that peculiarities first appearing in
either sex, though not in any way necessarily or invariably connected
with that sex, strongly tend to be inherited by the offspring of the
same sex, but are often transmitted in a latent state through the
opposite sex.
Turning now to domesticated animals, we find that certain characters
not proper to the parent species are often confined to, and inherited
by, one sex alone; but we do not know the history of the first
appearance of such characters. In the chapter on Sheep, we have seen
that the males of certain races differ greatly from the females in the
shape of their horns, these being absent in the ewes of some breeds;
they differ also in the development of fat in the tail and in the
outline of the forehead. These differences, judging from the character
of the allied wild species, cannot be accounted for by supposing that
they have been derived from distinct parent forms. There is, also, a
great difference between the horns of the two sexes in one Indian breed
of goats. The bull zebu is said to have a larger hump than the cow. In
the Scotch deer-hound the two sexes differ in size more than in any
other variety of the dog[28] and, judging from analogy, more than in
the aboriginal parent-species. The peculiar colour called
tortoise-shell is very rarely seen in a male cat; the males of this
variety being of a rusty tint.
In various breeds of the fowl the males and females often differ
greatly; and these differences are far from being the same with those
which distinguish the two sexes of the parent-species, the _Gallus
bankiva_; and consequently have originated under domestication. In
certain sub-varieties of the Game race we have the unusual case of the
hens differing from each other more than the cocks. In an Indian breed
of a white colour shaded with black, the hens invariably have black
skins, and their bones are covered by a black periosteum, whilst the
cocks are never or most rarely thus characterised. Pigeons offer a more
interesting case; for throughout the whole great family the two sexes
do not often differ much; and the males and females of the parent-form,
the _C. livia,_ are undistinguishable: yet we have seen that with
pouters the male has the characteristic quality of pouting more
strongly developed than the female; and in certain sub-varieties the
males alone are spotted or striated with black, or otherwise differ in
colour. When male and female English carrier-pigeons are exhibited in
separate pens, the difference in the development of the wattle over the
beak and round the eyes is conspicuous. So that here we have instances
of the appearance of secondary sexual characters in the domesticated
races of a species in which such differences are naturally quite
absent.
On the other hand, secondary sexual characters which belong to the
species in a state of nature are sometimes quite lost, or greatly
diminished, under domestication. We see this in the small size of the
tusks in our improved breeds of the pig, in comparison with those of
the wild boar. There are sub-breeds of fowls, in which the males have
lost the fine-flowing tail-feathers and hackles; and others in which
there is no difference in colour between the two sexes. In some cases
the barred plumage, which in gallinaceous birds is commonly the
attribute of the hen, has been transferred to the cock, as in the
cuckoo sub-breeds. In other cases masculine characters have been partly
transferred to the female, as with the splendid plumage of the
golden-spangled Hamburgh hen, the enlarged comb of the Spanish hen, the
pugnacious disposition of the Game hen, and as in the well-developed
spurs which occasionally appear in the hens of various breeds. In
Polish fowls both sexes are ornamented with a topknot, that of the male
being formed of hackle-like feathers, and this is a new male character
in the genus Gallus. On the whole, as far as I can judge, new
characters are more apt to appear in the males of our domesticated
animals than in the females,[29] and afterwards to be inherited
exclusively or more strongly by the males. Finally, in accordance with
the principle of inheritance as limited by sex, the preservation and
augmentation of secondary sexual characters in natural species offers
no especial difficulty, as this would follow through that form of
selection which I have called sexual selection.
_Inheritance at corresponding periods of Life._
This is an important subject. Since the publication of my ‘Origin of
Species’ I have seen no reason to doubt the truth of the explanation
there given of one of the most remarkable facts in biology, namely, the
difference between the embryo and the adult animal. The explanation is,
that variations do not necessarily or generally occur at a very early
period of embryonic growth, and that such variations are inherited at a
corresponding age. As a consequence of this the embryo, even after the
parent-form has undergone great modification, is left only slightly
modified; and the embryos of widely-different animals which are
descended from a common progenitor remain in many important respects
like one another and probably like their common progenitor. We can thus
understand why embryology throws a flood of light on the natural system
of classification, as this ought to be as far as possible genealogical.
When the embryo leads an independent life, that is, becomes a larva, it
has to be adapted to the surrounding conditions in its structure and
instincts, independently of those of its parents; and the principle of
inheritance at corresponding periods of life renders this possible.
This principle is, indeed, in one way so obvious that it escapes
attention. We possess a number of races of animals and plants, which,
when compared with one another and with their parent-forms, present
conspicuous differences, both in their immature and mature states. Look
at the seeds of the several kinds of peas, beans, maize, which can be
propagated truly, and see how they differ in size, colour, and shape,
whilst the full-grown plants differ but little. Cabbages, on the other
hand, differ greatly in foliage and manner of growth, but hardly at all
in their seeds; and generally it will be found that the differences
between cultivated plants at different periods of growth are not
necessarily closely connected together, for plants may differ much in
their seeds and little when full-grown, and conversely may yield seeds
hardly distinguishable, yet differ much when full-grown. In the several
breeds of poultry, descended from a single species, differences in the
eggs and chickens whilst covered with down, in the plumage at the first
and subsequent moults, as well as in the comb and wattles, are all
inherited. With man peculiarities in the milk and second teeth (of
which I have received the details) are inheritable, and longevity is
often transmitted. So again with our improved breeds of cattle and
sheep, early maturity, including the early development of the teeth,
and with certain breeds of fowl the early appearance of secondary
sexual characters, all come under the same head of inheritance at
corresponding periods.
Numerous analogous facts could be given. The silk-moth, perhaps, offers
the best instance; for in the breeds which transmit their characters
truly, the eggs differ in size, colour, and shape: the caterpillars
differ, in moulting three or four times, in colour, even in having a
dark-coloured mark like an eyebrow, and in the loss of certain
instincts;—the cocoons differ in size, shape, and in the colour and
quality of the silk; these several differences being followed by slight
or barely distinguishable differences in the mature moth.
But it may be said that, if in the above cases a new peculiarity is
inherited, it must be at the corresponding stage of development; for an
egg or seed can resemble only an egg or seed, and the horn in a
full-grown ox can resemble only a horn. The following cases show
inheritance at corresponding periods more plainly, because they refer
to peculiarities which might have supervened, as far as we can see,
earlier or later in life, yet are inherited at the same period at which
they first appeared.
In the Lambert family the porcupine-like excrescences appeared in the
father and sons at the same age, namely, about nine weeks after
birth.[30] In the extraordinary hairy family described by Mr.
Crawfurd,[31] children were produced during three generations with
hairy ears; in the father the hair began to grow over his body at six
years old; in his daughter somewhat earlier, namely, at one year; and
in both generations the milk teeth appeared late in life, the permanent
teeth being afterwards singularly deficient. Greyness of hair at an
unusually early age has been transmitted in some families. These cases
border on diseases inherited at corresponding periods of life, to which
I shall immediately refer.
It is a well-known peculiarity with almond-tumbler pigeons, that the
full beauty and peculiar character of the plumage does not appear until
the bird has moulted two or three times. Neumeister describes and
figures a brace of pigeons in which the whole body is white except the
breast, neck, and head; but in their first plumage all the white
feathers have coloured edges. Another breed is more remarkable: its
first plumage is black, with rusty-red wing-bars and a crescent-shaped
mark on the breast; these marks then become white, and remain so during
three or four moults; but after this period the white spreads over the
body, and the bird loses its beauty.[32] Prize canary-birds have their
wings and tail black: “this colour, however, is only retained until the
first moult, so that they must be exhibited ere the change takes place.
Once moulted, the peculiarity has ceased. Of course all the birds
emanating from this stock have black wings and tails the first
year.”[33] A curious and somewhat analogous account has been given[34]
of a family of wild pied rooks which were first observed in 1798, near
Chalfont, and which every year from that date up to the period of the
published notice, viz., 1837 “have several of their brood
particoloured, black and white. This variegation of the plumage,
however, disappears with the first moult; but among the next young
families there are always a few pied ones.” These changes of plumage,
which are inherited at various corresponding periods of life in the
pigeon, canary-bird, and rook, are remarkable, because the
parent-species passes through no such change.
Inherited diseases afford evidence in some respects of less value than
the foregoing cases, because diseases are not necessarily connected
with any change in structure; but in other respects of more value,
because the periods have been more carefully observed. Certain diseases
are communicated to the child apparently by a process like inoculation,
and the child is from the first affected; such cases may be here passed
over. Large classes of diseases usually appear at certain ages, such as
St. Vitus’s dance in youth, consumption in early mid-life, gout later,
and apoplexy still later; and these are naturally inherited at the same
period. But even in diseases of this class, instances have been
recorded, as with St. Vitus’s dance, showing that an unusually early or
late tendency to the disease is inheritable.[35] In most cases the
appearance of any inherited disease is largely determined by certain
critical periods in each person’s life, as well as by unfavourable
conditions. There are many other diseases, which are not attached to
any particular period, but which certainly tend to appear in the child
at about the same age at which the parent was first attacked. An array
of high authorities, ancient and modern, could be given in support of
this proposition. The illustrious Hunter believed in it; and Piorry[36]
cautions the physician to look closely to the child at the period when
any grave inheritable disease attacked the parent. Dr. Prosper
Lucas,[37] after collecting facts from every source, asserts that
affections of all kinds, though not related to any particular period of
life, tend to reappear in the offspring at whatever period of life they
first appeared in the progenitor.
As the subject is important, it may be well to give a few instances,
simply as illustrations, not as proof; for proof, recourse must be had
to the authorities above quoted. Some of the following cases have been
selected for the sake of showing that, when a slight departure from the
rule occurs, the child is affected somewhat earlier in life than the
parent. In the family of Le Compte blindness was inherited through
three generations, and no less than twenty-seven children and
grandchildren were all affected at about the same age; their blindness
in general began to advance about the fifteenth or sixteenth year, and
ended in total deprivation of sight at the age of about twenty-two.[38]
In another case a father and his four children all became blind at
twenty-one years old; in another, a grandmother grew blind at
thirty-five, her daughter at nineteen, and three grandchildren at the
ages of thirteen and eleven.[39] So with deafness, two brothers, their
father and paternal grandfather, all became deaf at the age of
forty.[40]
Esquirol gives several striking instances of insanity coming on at the
same age, as that of a grandfather, father, and son, who all committed
suicide near their fiftieth year. Many other cases could be given, as
of a whole family who became insane at the age of forty.[41] Other
cerebral affections sometimes follow the same rule,—for instance,
epilepsy and apoplexy. A woman died of the latter disease when
sixty-three years old; one of her daughters at forty-three, and the
other at sixty-seven: the latter had twelve children, who all died from
tubercular meningitis.[42] I mention this latter case because it
illustrates a frequent occurrence, namely, a change in the precise
nature of an inherited disease, though still affecting the same organ.
Asthma has attacked several members of the same family when forty years
old, and other families during infancy. The most different diseases,
such as angina pectoris, stone in the bladder, and various affections
of the skin, have appeared in successive generations at nearly the same
age. The little finger of a man began from some unknown cause to grow
inwards, and the same finger in his two sons began at the same age to
bend inwards in a similar manner. Strange and inexplicable neuralgic
affections have caused parents and children to suffer agonies at about
the same period of life.[43]
I will give only two other cases, which are interesting as illustrating
the disappearance as well as the appearance of disease at the same age.
Two brothers, their father, their paternal uncles, seven cousins, and
their paternal grandfather, were all similarly affected by a
skin-disease, called pityriasis versicolor; “the disease, strictly
limited to the males of the family (though transmitted through the
females), usually appeared at puberty, and disappeared at about the age
of forty or forty-five years.” The second case is that of four
brothers, who when about twelve years old suffered almost every week
from severe headaches, which were relieved only by a recumbent position
in a dark room. Their father, paternal uncles, paternal grandfather,
and granduncles all suffered in the same way from headaches, which
ceased at the age of fifty-four or fifty-five in all those who lived so
long. None of the females of the family were affected.[44]
It is impossible to read the foregoing accounts, and the many others
which have been recorded, of diseases coming on during three or even
more generations in several members of the same family at the same age,
especially in the case of rare affections in which the coincidence
cannot be attributed to chance, and to doubt that there is a strong
tendency to inheritance in disease at corresponding periods of life.
When the rule fails, the disease is apt to come on earlier in the child
than in the parent; the exceptions in the other direction being very
much rarer. Dr. Lucas[45] alludes to several cases of inherited
diseases coming on at an earlier period. I have already given one
striking instance with blindness during three generations; and Mr.
Bowman remarks that this frequently occurs with cataract. With cancer
there seems to be a peculiar liability to earlier inheritance: Sir J.
Paget, who has particularly attended to this subject, and tabulated a
large number of cases, informs me that he believes that in nine cases
out of ten the later generation suffers from the disease at an earlier
period than the previous generation. He adds, “In the instances in
which the opposite relation holds, and the members of later generations
have cancer at a later age than their predecessors, I think it will be
found that the non-cancerous parents have lived to extreme old ages.”
So that the longevity of a non-affected parent seems to have the power
of influencing the fatal period in the offspring; and we thus
apparently get another element of complexity in inheritance.
The facts, showing that with certain diseases the period of inheritance
occasionally or even frequently advances, are important with respect to
the general descent-theory, for they render it probable that the same
thing would occur with ordinary modifications of structure. The final
result of a long series of such advances would be the gradual
obliteration of characters proper to the embryo and larva, which would
thus come to resemble more and more closely the mature parent-form. But
any structure which was of service to the embryo or larva would be
preserved by the destruction at this stage of growth of each individual
which manifested any tendency to lose its proper character at too early
an age.
Finally, from the numerous races of cultivated plants and domestic
animals, in which the seeds or eggs, the young or old, differ from one
another and from those of the parent-species;—from the cases in which
new characters have appeared at a particular period, and afterwards
been inherited at the same period;—and from what we know with respect
to disease, we must believe in the truth of the great principle of
inheritance at corresponding periods of life.
_Summary of the three preceding Chapters._—Strong as is the force of
inheritance, it allows the incessant appearance of new characters.
These, whether beneficial or injurious,—of the most trifling
importance, such as a shade of colour in a flower, a coloured lock of
hair, or a mere gesture,—or of the highest importance, as when
affecting the brain, or an organ so perfect and complex as the eye,—or
of so grave a nature as to deserve to be called a monstrosity,—or so
peculiar as not to occur normally in any member of the same natural
class,—often inherited by man, by the lower animals, and plants. In
numberless cases it suffices for the inheritance of a peculiarity that
one parent alone should be thus characterised. Inequalities in the two
sides of the body, though opposed to the law of symmetry, may be
transmitted. There is ample evidence that the effects of mutilations
and of accidents, especially or perhaps exclusively when followed by
disease, are occasionally inherited. There can be no doubt that the
evil effects of the long-continued exposure of the parent to injurious
conditions are sometimes transmitted to the offspring. So it is, as we
shall see in a future chapter, with the effects of the use and disuse
of parts, and of mental habits. Periodical habits are likewise
transmitted, but generally, as it would appear, with little force.
Hence we are led to look at inheritance as the rule, and
non-inheritance as the anomaly. But this power often appears to us in
our ignorance to act capriciously, transmitting a character with
inexplicable strength or feebleness. The very same peculiarity, as the
weeping habit of trees, silky feathers, etc., may be inherited either
firmly or not at all by different members of the same group, and even
by different individuals of the same species, though treated in the
same manner. In this latter case we see that the power of transmission
is a quality which is merely individual in its attachment. As with
single characters, so it is with the several concurrent slight
differences which distinguish sub-varieties or races; for of these,
some can be propagated almost as truly as species, whilst others cannot
be relied on. The same rule holds good with plants, when propagated by
bulbs, offsets, etc., which in one sense still form parts of the same
individual, for some varieties retain or inherit through successive
bud-generations their character far more truly than others.
Some characters not proper to the parent-species have certainly been
inherited from an extremely remote epoch, and may therefore be
considered as firmly fixed. But it is doubtful whether length of
inheritance in itself gives fixedness of character; though the chances
are obviously in favour of any character which has long been
transmitted true or unaltered still being transmitted true as long as
the conditions of life remain the same. We know that many species,
after having retained the same character for countless ages, whilst
living under their natural conditions, when domesticated have varied in
the most diversified manner, that is, have failed to transmit their
original form; so that no character appears to be absolutely fixed. We
can sometimes account for the failure of inheritance by the conditions
of life being opposed to the development of certain characters; and
still oftener, as with plants cultivated by grafts and buds, by the
conditions causing new and slight modifications incessantly to appear.
In this latter case it is not that inheritance wholly fails, but that
new characters are continually superadded. In some few cases, in which
both parents are similarly characterised, inheritance seems to gain so
much force by the combined action of the two parents, that it
counteracts its own power, and a new modification is the result.
In many cases the failure of the parents to transmit their likeness is
due to the breed having been at some former period crossed; and the
child takes after his grandparent or more remote ancestor of foreign
blood. In other cases, in which the breed has not been crossed, but
some ancient character has been lost through variation, it occasionally
reappears through reversion, so that the parents apparently fail to
transmit their own likeness. In all cases, however, we may safely
conclude that the child inherits all its characters from its parents,
in whom certain characters are latent, like the secondary sexual
characters of one sex in the other. When, after a long succession of
bud-generations, a flower or fruit becomes separated into distinct
segments, having the colours or other attributes of both parent-forms,
we cannot doubt that these characters were latent in the earlier buds,
though they could not then be detected, or could be detected only in an
intimately commingled state. So it is with animals of crossed
parentage, which with advancing years occasionally exhibit characters
derived from one of their two parents, of which not a trace could at
first be perceived. Certain monstrosities, which resemble what
naturalists call the typical form of the group in question, apparently
come under the same law of reversion. It is assuredly an astonishing
fact that the male and female sexual elements, that buds, and even
full-grown animals, should retain characters, during several
generations in the case of crossed breeds, and during thousands of
generations in the case of pure breeds, written as it were in invisible
ink, yet ready at any time to be evolved under certain conditions.
What these conditions precisely are, we do not know. But any cause
which disturbs the organisation or constitution seems to be sufficient.
A cross certainly gives a strong tendency to the reappearance of
long-lost characters, both corporeal and mental. In the case of plants,
this tendency is much stronger with those species which have been
crossed after long cultivation and which therefore have had their
constitutions disturbed by this cause as well as by crossing, than with
species which have always lived under their natural conditions and have
then been crossed. A return, also, of domesticated animals and
cultivated plants to a wild state favours reversion; but the tendency
under these circumstances has been much exaggerated.
When individuals of the same family which differ somewhat, and when
races or species are crossed, the one is often prepotent over the other
in transmitting its character. A race may possess a strong power of
inheritance, and yet when crossed, as we have seen with
trumpeter-pigeons, yield to the prepotency of every other race.
Prepotency of transmission may be equal in the two sexes of the same
species, but often runs more strongly in one sex. It plays an important
part in determining the rate at which one race can be modified or
wholly absorbed by repeated crosses with another. We can seldom tell
what makes one race or species prepotent over another; but it sometimes
depends on the same character being present and visible in one parent,
and latent or potentially present in the other.
Characters may first appear in either sex, but oftener in the male than
in the female, and afterwards be transmitted to the offspring of the
same sex. In this case we may feel confident that the peculiarity in
question is really present though latent in the opposite sex! hence the
father may transmit through his daughter any character to his grandson;
and the mother conversely to her granddaughter. We thus learn, and the
fact is an important one, that transmission and development are
distinct powers. Occasionally these two powers seem to be antagonistic,
or incapable of combination in the same individual; for several cases
have been recorded in which the son has not directly inherited a
character from his father, or directly transmitted it to his son, but
has received it by transmission through his non-affected mother, and
transmitted it through his non-affected daughter. Owing to inheritance
being limited by sex, we see how secondary sexual characters may have
arisen under nature; their preservation and accumulation being
dependent on their service to either sex.
At whatever period of life a new character first appears, it generally
remains latent in the offspring until a corresponding age is attained,
and then is developed. When this rule fails, the child generally
exhibits the character at an earlier period than the parent. On this
principle of inheritance at corresponding periods, we can understand
how it is that most animals display from the germ to maturity such a
marvellous succession of characters.
Finally, though much remains obscure with respect to Inheritance, we
may look at the following laws as fairly well established. Firstly, a
tendency in every character, new and old, to be transmitted by seminal
and bud generation, though often counteracted by various known and
unknown causes. Secondly, reversion or atavism, which depends on
transmission and development being distinct powers: it acts in various
degrees and manners through both seminal and bud generation. Thirdly,
prepotency of transmission, which may be confined to one sex, or be
common to both sexes. Fourthly, transmission, as limited by sex,
generally to the same sex in which the inherited character first
appeared; and this in many, probably most cases, depends on the new
character having first appeared at a rather late period of life.
Fifthly, inheritance at corresponding periods of life, with some
tendency to the earlier development of the inherited character. In
these laws of Inheritance, as displayed under domestication, we see an
ample provision for the production, through variability and natural
selection, of new specific forms.
REFERENCES
[1] _See_ Youatt on Cattle, pp. 92, 69, 78, 88, 163; and Youatt on
Sheep, p. 325. Also Dr. Lucas ‘L’Héréd. Nat.,’ tom. ii. p. 310.
[2] ‘Héréd. Nat.,’ tom. ii. pp. 112-120.
[3] Sir H. Holland, ‘Chapters on Mental Physiology,’ 1852, p. 234.
[4] ‘Gardener’s Chronicle,’ 1860, p. 270.
[5] Mr. N. H. Smith, ‘Observations on Breeding,’ quoted in ‘Encyclop.
of Rural Sports,’ p. 278.
[6] Quoted by Bronn, ‘Geshichte der Natur,’ b. ii. s. 170. _ See_
Sturm, ‘Ueber Racen,’ 1825, s. 104-107. For the niata cattle, _see_ my
‘Journal of Researches,’ 1845, p. 146.
[7] Lucas, ‘L’Hérédite Nat.,’ tom. ii. p. 112.
[8] Mr. Orton, ‘Physiology of Breeding,’ 1855, p. 9.
[9] Boitard and Corbié, ‘Les Pigeons,’ 1824, p. 224.
[10] ‘Les Pigeons,’ pp. 168, 198.
[11] ‘Das Ganze,’ etc., 1837, s. 39.
[12] ‘The Pigeon Book,’ p. 46.
[13] ‘Physiology of Breeding,’ p. 22; Mr. Hewitt, in ‘The Poultry
Book,’ by Tegetmeier, 1866, p. 224.
[14] Boitard and Corbié, ‘Les Pigeons,’ 1824, p. 226.
[15] ‘Bastarderzeugung,’ s. 256, 290, etc. Naudin (‘Nouvelles Archives
du Muséum,’ tom. i. p. 149) gives a striking instance of prepotency in
_Datura stramonium_ when crossed with two other species.
[16] Flourens, ‘Longévité Humaine,’ p. 144, on crossed jackals. With
respect to the difference between the mule and the hinny I am aware
that this has generally been attributed to the sire and dam
transmitting their characters differently; but Colin, who has given in
his ‘Traité Phys. Comp.,’ tom. ii. pp. 537-539, the fullest
description which I have met with of these reciprocal hybrids, is
strongly of opinion that the ass preponderates in both crosses, but in
an unequal degree. This is likewise the conclusion of Flourens, and of
Bechstein in his ‘Naturgeschichte Deutschlands,’ b. i. s. 294. The
tail of the hinny is much more like that of the horse than is the tail
of the mule, and this is generally accounted for by the males of both
species transmitting with greater power this part of their structure;
but a compound hybrid which I saw in the Zoological Gardens, from a
mare by a hybrid ass-zebra, closely resembled its mother in its tail.
[17] Mr. Hewitt, who has had such great experience in raising these
hybrids says (‘Poultry Book,’ by Mr. Tegetmeier, 1866, pp. 165-167)
that in all, the head was destitute of wattles, comb, and ear-lappets;
and all closely resembled the pheasant in the shape of the tail and
general contour of the body. These hybrids were raised from hens of
several breeds by a cock-pheasant; but another hybrid, described by
Mr. Hewitt, was raised from a hen-pheasant, by a silver-laced Bantam
cock, and this possessed a rudimental comb and wattles.
[18] ‘L’Héréd. Nat.’ tom. ii. 2 book ii. ch. i.
[19] ‘Bastarderzeugung,’ s. 264-266. Naudin (‘Nouvelles Archives du
Muséum,’ tom. i. p. 148) has arrived at a similar conclusion.
[20] ‘Cottage Gardener,’ 1856, pp. 101, 137.
[21] _See_ some remarks on this head with respect to sheep by Mr.
Wilson, in ‘Gardener’s Chronicle,’ 1863, p. 15. Many striking
instances of this result are given by M. Malingié-Nouel (‘Journ. R.
Agricult. Soc.,’ vol. xiv. 1853, p. 220) with respect to crosses
between English and French sheep. He found that he obtained the
desired influence of the English breeds by crossing intentionally
mongrelised French breeds with pure English breeds.
[22] Verlot, ‘Des Variétés,’ 1865, p. 66.
[23] Moquin-Tandon, ‘Tératologie,’ p. 191.
[24] ‘Nouvelles Archives du Muséum,’ tom. i. p. 137.
[25] ‘L’Héréd. Nat.,’ tom. ii. pp. 137-165. _See also_ Mr. Sedgwick’s
four memoirs, immediately to be referred to.
[26] ‘Descent of Man,’ 2nd edit., p. 32.
[27] On Sexual Limitation in Hereditary Diseases, ‘Brit. and For.
Med.-Chirurg. Review,’ April 1861, p. 477; July, p. 198; April 1863,
p. 445; and July, p. 159. Also in 1867, ‘On the influence of Age in
Hereditary Disease.’
[28] W. Scrope, ‘Art of Deer Stalking,’ p. 354.
[29] I have given in my ‘Descent of Man’ (2nd edit. p. 223) sufficient
evidence that male animals are usually more variable than the females.
[30] Prichard, ‘Phys. Hist. of Mankind,’ 1851, vol. i. p. 349.
[31] ‘Embassy to the Court of Ava,’ vol. i. p. 320. The third
generation is described by Capt. Yule in his ‘Narrative of the Mission
to the Court of Ava,’ 1855, p. 94.
[32] ‘Das Ganze der Taubenzucht,’ 1837, s. 24, tab. iv., fig. 2; s.
21, tab. i., fig. 4.
[33] Kidd’s ‘Treatise on the Canary,’ p. 18.
[34] Charlesworth, ‘Mag. of Nat. Hist.,’ vol. i. 1837, p. 167.
[35] Dr. Prosper Lucas, ‘Héréd. Nat.,’ tom. ii. p. 713.
[36] ‘L’Héréd. dans les Maladies,’ 1840, p. 135. For Hunter, _see_
Harlan’s ‘Med. Researches,’ p. 530.
[37] ‘L’Héréd. Nat.,’ tom. ii. p. 850.
[38] Sedgwick, ‘Brit. and For. Med.-Chirurg. Review,’ April, 1861, p.
485. In some accounts the number of children and grandchildren is
given as 37; but this seems to be an error judging from the paper
first published in the ‘Baltimore Med. and Phys. Reg.’ 1809, of which
Mr. Sedgwick has been so kind as to send me a copy.
[39] Prosper Lucas, ‘Héréd. Nat.,’ tom. i. p. 400.
[40] Sedgwick, ibid., July, 1861, p. 202.
[41] Piorry, p. 109; Prosper Lucas, tom. ii. p. 759.
[42] Prosper Lucas, tom. ii. p. 748.
[43] Prosper Lucas, tom. iii. pp. 678, 700, 702; Sedgwick, ibid.,
April, 1863, p. 449, and July, 1863, p. 162. Dr. J. Steinan ‘Essay on
Hereditary Disease,’ 1843, pp. 27, 34.
[44] These cases are given by Mr. Sedgwick on the authority of Dr. H.
Stewart, in ‘Med.-Chirurg. Review,’ April, 1863, pp. 449, 477.
[45] ‘Héréd. Nat.,’ tom. ii. p. 852.