EXPLANATION OF TERM CORRELATION—CONNECTED WITH
DEVELOPMENT—MODIFICATIONS CORRELATED WITH THE INCREASED OR DECREASED
SIZE OF PARTS—CORRELATED VARIATION OF HOMOLOGOUS PARTS—FEATHERED FEET
IN BIRDS ASSUMING THE STRUCTURE OF THE WINGS—CORRELATION BETWEEN THE
HEAD AND THE EXTREMITIES—BETWEEN THE SKIN AND DERMAL APPENDAGES—BETWEEN
THE ORGANS OF SIGHT AND HEARING—CORRELATED MODIFICATIONS IN THE ORGANS
OF PLANTS—CORRELATED MONSTROSITIES—CORRELATION BETWEEN THE SKULL AND
EARS—SKULL AND CREST OF FEATHERS—SKULL AND HORNS—CORRELATION OF GROWTH
COMPLICATED BY THE ACCUMULATED EFFECTS OF NATURAL SELECTION—COLOUR AS
CORRELATED WITH CONSTITUTIONAL PECULIARITIES.
All parts of the organisation are to a certain extent connected
together; but the connection may be so slight that it hardly exists, as
with compound animals or the buds on the same tree. Even in the higher
animals various parts are not at all closely related; for one part may
be wholly suppressed or rendered monstrous without any other part of
the body being affected. But in some cases, when one part varies,
certain other parts always, or nearly always, simultaneously vary; they
are then subject to the law of correlated variation. The whole body is
admirably co-ordinated for the peculiar habits of life of each organic
being, and may be said, as the Duke of Argyll insists in his ‘Reign of
Law’ to be correlated for this purpose. Again, in large groups of
animals certain structures always co-exist: for instance, a peculiar
form of stomach with teeth of peculiar form, and such structures may in
one sense be said to be correlated. But these cases have no necessary
connection with the law to be discussed in the present chapter; for we
do not know that the initial or primary variations of the several parts
were in any way related: slight modifications or individual differences
may have been preserved, first in one and then in another part, until
the final and perfectly co-adapted structure was acquired; but to this
subject I shall presently recur. Again, in many groups of animals the
males alone are furnished with weapons, or are ornamented with gay
colours; and these characters manifestly stand in some sort of
correlation with the male reproductive organs, for when the latter are
destroyed these characters disappear. But it was shown in the twelfth
chapter that the very same peculiarity may become attached at any age
to either sex, and afterwards be exclusively transmitted to the same
sex at a corresponding age. In these cases we have inheritance limited
by both sex and age; but we have no reason for supposing that the
original cause of the variation was necessarily connected with the
reproductive organs, or with the age of the affected being.
In cases of true correlated variation, we are sometimes able to see the
nature of the connection; but in most cases it is hidden from us, and
certainly differs in different cases. We can seldom say which of two
correlated parts first varies, and induces a change in the other; or
whether the two are the effects of some common cause. Correlated
variation is an important subject for us; for when one part is modified
through continued selection, either by man or under nature, other parts
of the organisation will be unavoidably modified. From this correlation
it apparently follows that with our domesticated animals and plants,
varieties rarely or never differ from one another by a single character
alone.
One of the simplest cases of correlation is that a modification which
arises during an early stage of growth tends to influence the
subsequent development of the same part, as well as of other and
intimately connected parts. Isidore Geoffroy Saint-Hilaire states[1]
that this may constantly be observed with monstrosities in the animal
kingdom; and Moquin-Tandon[2] remarks, that, as with plants the axis
cannot become monstrous without in some way affecting the organs
subsequently produced from it, so axial anomalies are almost always
accompanied by deviations of structure in the appended parts. We shall
presently see that with short-muzzled races of the dog certain
histological changes in the basal elements of the bones arrest their
development and shorten them, and this affects the position of the
subsequently developed molar teeth. It is probable that certain
modifications in the larvæ of insects would affect the structure of the
mature insects. But we must be careful not to extend this view too far,
for during the normal course of development, certain species pass
through an extraordinary course of change, whilst other and closely
allied species arrive at maturity with little change of structure.
Another simple case of correlation is that with the increased or
decreased dimensions of the whole body, or of any particular part,
certain organs are increased or diminished in number, or are otherwise
modified. Thus pigeon fanciers have gone on selecting pouters for
length of body, and we have seen that their vertebrae are generally
increased not only in size but in number, and their ribs in breadth.
Tumblers have been selected for their small bodies, and their ribs and
primary wing-feathers are generally lessened in number. Fantails have
been selected for their large widely-expanded tails, with numerous
tail-feathers, and the caudal vertebrae are increased in size and
number. Carriers have been selected for length of beak, and their
tongues have become longer, but not in strict accordance with the
length of beak. In this latter breed and in others having large feet,
the number of the scutellae on the toes is greater than in the breeds
with small feet. Many similar cases could be given. In Germany it has
been observed that the period of gestation is longer in large than in
small breeds of cattle. With our highly-improved breeds of all kinds,
the periods of maturity and of reproduction have advanced with respect
to the age of the animal; and, in correspondence with this, the teeth
are now developed earlier than formerly, so that, to the surprise of
agriculturists, the ancient rules for judging of the age of an animal
by the state of its teeth are no longer trustworthy.[3]
_Correlated Variation of Homologous Parts._—Parts which are homologous
tend to vary in the same manner; and this is what might have been
expected, for such parts are identical in form and structure during an
early period of embryonic development, and are exposed in the egg or
womb to similar conditions. The symmetry, in most kinds of animals, of
the corresponding or homologous organs on the right and left sides of
the body, is the simplest case in point; but this symmetry sometimes
fails, as with rabbits having only one ear, or stags with one horn, or
with many-horned sheep which sometimes carry an additional horn on one
side of their heads. With flowers which have regular corollas, all the
petals generally vary in the same manner, as we see in the complicated
and symmetrical pattern, on the flowers, for instance, of the Chinese
pink; but with irregular flowers, though the petals are of course
homologous, this symmetry often fails, as with the varieties of the
_Antirrhinum_ or snapdragon, or that variety of the kidney-bean
(_Phaseolus_) which has a white standard-petal.
In the Vertebrata the front and hind limbs are homologous, and they
tend to vary in the same manner, as we see in long and short legged, or
in thick and thin legged races of the horse and dog. Isidore
Geoffroy[4] has remarked on the tendency of supernumerary digits in man
to appear, not only on the right and left sides, but on the upper and
lower extremities. Meckel has insisted[5] that, when the muscles of the
arm depart in number or arrangement from their proper type, they almost
always imitate those of the leg; and so conversely the varying muscles
of the leg imitate the normal muscles of the arm.
In several distinct breeds of the pigeon and fowl, the legs and the two
outer toes are heavily feathered, so that in the trumpeter pigeon they
appear like little wings. In the feather-legged bantam the “boots” or
feathers, which grow from the outside of the leg and generally from the
two outer toes, have, according to the excellent authority of Mr.
Hewitt,[6] been seen to exceed the wing-feathers in length, and in one
case were actually nine and a half inches long! As Mr. Blyth has
remarked to me, these leg-feathers resemble the primary wing-feathers,
and are totally unlike the fine down which naturally grows on the legs
of some birds, such as grouse and owls. Hence it may be suspected that
excess of food has first given redundancy to the plumage, and then that
the law of homologous variation has led to the development of feathers
on the legs, in a position corresponding with those on the wing,
namely, on the outside of the tarsi and toes. I am strengthened in this
belief by the following curious case of correlation, which for a long
time seemed to me utterly inexplicable, namely, that in pigeons of any
breed, if the legs are feathered, the two outer toes are partially
connected by skin. These two outer toes correspond with our third and
fourth toes.[7] Now, in the wing of the pigeon or of any other bird,
the first and fifth digits are aborted; the second is rudimentary and
carries the so-called “bastard-wing;” whilst the third and fourth
digits are completely united and enclosed by skin, together forming the
extremity of the wing. So that in feather-footed pigeons, not only does
the exterior surface support a row of long feathers, like
wing-feathers, but the very same digits which in the wing are
completely united by skin become partially united by skin in the feet;
and thus by the law of the correlated variation of homologous parts we
can understand the curious connection of feathered legs and membrane
between the two outer toes.
Andrew Knight[8] has remarked that the face or head and the limbs
usually vary together in general proportions. Compare, for instance,
the limbs of a dray and race horse, or of a greyhound and mastiff. What
a monster a greyhound would appear with the head of a mastiff! The
modern bulldog, however, has fine limbs, but this is a
recently-selected character. From the measurements given in the sixth
chapter, we see that in several breeds of the pigeon the length of the
beak and the size of the feet are correlated. The view which, as before
explained, seems the most probable is, that disuse in all cases tends
to diminish the feet, the beak becoming at the same time shorter
through correlation; but that in some few breeds in which length of
beak has been a selected point, the feet, notwithstanding disuse, have
increased in size through correlation. In the following case some kind
of correlation is seen to exist between the feet and beak: several
specimens have been sent to Mr. Bartlett at different times, as hybrids
between ducks and fowls, and I have seen one; these were, as might be
expected, ordinary ducks in a semi-monstrous condition, and in all of
them the swimming-web between the toes was quite deficient or much
reduced, and in all the beak was narrow and ill-shaped.
With the increased length of the beak in pigeons, not only the tongue
increases in length, but likewise the orifice of the nostrils. But the
increased length of the orifice of the nostrils perhaps stands in
closer correlation with the development of the corrugated skin or
wattle at the base of the beak, for when there is much wattle round the
eyes, the eyelids are greatly increased or even doubled in length.
There is apparently some correlation even in colour between the head
and the extremities. Thus with horses a large white star or blaze on
the forehead is generally accompanied by white feet.[9] With white
rabbits and cattle, dark marks often co-exist on the tips of the ears
and on the feet. In black and tan dogs of different breeds,
tan-coloured spots over the eyes and tan-coloured feet almost
invariably go together. These latter cases of connected colouring may
be due either to reversion or to analogous variation,—subjects to which
I shall hereafter return,—but this does not necessarily determine the
question of their original correlation. Mr. H. W. Jackson informs me
that he has observed many hundred white-footed cats, and he finds that
all are more or less conspicuously marked with white on the front of
the neck or chest.
The lopping forwards and downwards of the immense ears of fancy rabbits
seems partly due to the disuse of the muscles, and partly to the weight
and length of the ears, which have been increased by selection during
many generations. Now, with the increased size and changed direction of
the ears not only has the bony auditory meatus become changed in
outline, direction, and greatly in size, but the whole skull has been
slightly modified. This could be clearly seen in “half-lops”—that is,
in rabbits with only one ear lopping forward— for the opposite sides of
their skulls were not strictly symmetrical. This seems to me a curious
instance of correlation, between hard bones and organs so soft and
flexible, as well as so unimportant under a physiological point of
view, as the external ears. The result no doubt is largely due to mere
mechanical action, that is, to the weight of the ears, on the same
principle that the skull of a human infant is easily modified by
pressure.
The skin and the appendages of hair, feathers, hoofs, horns, and teeth,
are homologous over the whole body. Every one knows that the colour of
the skin and that of the hair usually vary together; so that Virgil
advises the shepherd to look whether the mouth and tongue of the ram
are black, lest the lambs should not be purely white. The colour of the
skin and hair, and the odour emitted by the glands of the skin, are
said[10] to be connected, even in the same race of men. Generally the
hair varies in the same way all over the body in length, fineness, and
curliness. The same rule holds good with feathers, as we see with the
laced and frizzled breeds both of fowls and pigeons. In the common cock
the feathers on the neck and loins are always of a particular shape,
called hackles: now in the Polish breed, both sexes are characterised
by a tuft of feathers on the head, and through correlation these
feathers in the male always assume the form of hackles. The wing and
tail-feathers, though arising from parts not homologous, vary in length
together; so that long or short winged pigeons generally have long or
short tails. The case of the Jacobin-pigeon is more curious, for the
wing and tail feathers are remarkably long; and this apparently has
arisen in correlation with the elongated and reversed feathers on the
back of the neck, which form the hood.
The hoofs and hair are homologous appendages; and a careful observer,
namely Azara,[11] states that in Paraguay horses of various colours are
often born with their hair curled and twisted like that on the head of
a negro. This peculiarity is strongly inherited. But what is remarkable
is that the hoofs of these horses “are absolutely like those of a
mule.” The hair also of their manes and tails is invariably much
shorter than usual, being only from four to twelve inches in length; so
that curliness and shortness of the hair are here, as with the negro,
apparently correlated.
With respect to the horns of sheep, Youatt[12] remarks that
“multiplicity of horns is not found in any breed of much value; it is
generally accompanied by great length and coarseness of the fleece.”
Several tropical breeds of sheep which are clothed with hair instead of
wool, have horns almost like those of a goat. Sturm[13] expressly
declares that in different races the more the wool is curled the more
the horns are spirally twisted. We have seen in the third chapter,
where other analogous facts have been given, that the parent of the
Mauchamp breed, so famous for its fleece, had peculiarly shaped horns.
The inhabitants of Angora assert[14] that “only the white goats which
have horns wear the fleece in the long curly locks that are so much
admired; those which are not horned having a comparatively close coat.”
From these cases we may infer that the hair or wool and the horns tend
to vary in a correlated manner.[15] Those who have tried hydropathy are
aware that the frequent application of cold water stimulates the skin;
and whatever stimulates the skin tends to increase the growth of the
hair, as is well shown in the abnormal growth of hair near old inflamed
surfaces. Now, Professor Low[16] is convinced that with the different
races of British cattle thick skin and long hair depend on the humidity
of the climate which they inhabit. We can thus see how a humid climate
might act on the horns—in the first place directly on the skin and
hair, and secondly by correlation on the horns. The presence or absence
of horns, moreover, both in the case of sheep and cattle, acts, as will
presently be shown, by some sort of correlation on the skull.
With respect to hair and teeth, Mr. Yarrell[17] found many of the teeth
deficient in three hairless “Egyptian dogs,” and in a hairless terrier.
The incisors, canines, and the premolars suffered most, but in one case
all the teeth, except the large tubercular molar on each side, were
deficient. With man several striking cases have been recorded[18] of
inherited baldness with inherited deficiency, either complete or
partial, of the teeth. I may give an analogous case, communicated to me
by Mr. W. Wedderburn, of a Hindoo family in Scinde, in which ten men,
in the course of four generations, were furnished, in both jaws taken
together, with only four small and weak incisor teeth and with eight
posterior molars. The men thus affected have very little hair on the
body, and become bald early in life. They also suffer much during hot
weather from excessive dryness of the skin. It is remarkable that no
instance has occurred of a daughter being thus affected; and this fact
reminds us how much more liable men are in England to become bald than
women. Though the daughters in the above family are never affected,
they transmit the tendency to their sons; and no case has occurred of a
son transmitting it to his sons. The affection thus appears only in
alternate generations, or after longer intervals. There is a similar
connection between hair and teeth, according to Mr. Sedgwick, in those
rare cases in which the hair has been renewed in old age, for this has
“usually been accompanied by a renewal of the teeth.” I have remarked
in a former part of this volume that the great reduction in the size of
the tusks in domestic boars probably stands in close relation with
their diminished bristles, due to a certain amount of protection; and
that the reappearance of the tusks in boars, which have become feral
and are fully exposed to the weather, probably depends on the
reappearance of the bristles. I may add, though not strictly connected
with our present point, that an agriculturist[19] asserts that “pigs
with little hair on their bodies are most liable to lose their tails,
showing a weakness of the tegumental structure. It may be prevented by
crossing with a more hairy breed.”
In the previous cases deficient hair, and teeth deficient in number or
size, are apparently connected. In the following cases abnormally
redundant hair, and teeth either deficient or redundant, are likewise
connected. Mr. Crawfurd[20] saw at the Burmese Court a man, thirty
years old, with his whole body, except the hands and feet, covered with
straight silky hair, which on the shoulders and spine was five inches
in length. At birth the ears alone were covered. He did not arrive at
puberty, or shed his milk teeth, until twenty years old; and at this
period he acquired five teeth in the upper jaw, namely, four incisors
and one canine, and four incisor teeth in the lower jaw; all the teeth
were small. This man had a daughter who was born with hair within her
ears; and the hair soon extended over her body. When Captain Yule[21]
visited the Court, he found this girl grown up; and she presented a
strange appearance with even her nose densely covered with soft hair.
Like her father, she was furnished with incisor teeth alone. The King
had with difficulty bribed a man to marry her, and of her two children,
one, a boy fourteen months old, had hair growing out of his ears, with
a beard and moustache. This strange peculiarity has, therefore, been
inherited for three generations, with the molar teeth deficient in the
grandfather and mother; whether these teeth would likewise fail in the
infant could not then be told.
A parallel case of a man fifty-five years old, and of his son, with
their faces covered with hair, has recently occurred in Russia. Dr.
Alex. Brandt has sent me an account of this case, together with
specimens of the extremely fine hair from the cheeks. The man is
deficient in teeth, possessing only four incisors in the lower and two
in the upper jaw. His son, about three years old, has no teeth except
four lower incisors. The case, as Dr. Brandt remarks in his letter, no
doubt is due to an arrest of development in the hair and teeth. We here
see how independent of the ordinary conditions of existence such
arrests must be, for the lives of a Russian peasant and of a native of
Burmah are as different as possible.[22]
Here is another and somewhat different case communicated to me by Mr.
Wallace on the authority of Dr. Purland, a dentist: Julia Pastrana, a
Spanish dancer, was a remarkably fine woman, but she had a thick
masculine beard and a hairy forehead; she was photographed, and her
stuffed skin was exhibited as a show; but what concerns us is, that she
had in both the upper and lower jaw an irregular double set of teeth,
one row being placed within the other, of which Dr. Purland took a
cast. From the redundancy of teeth her mouth projected, and her face
had a gorilla-like appearance. These cases and those of the hairless
dogs forcibly call to mind the fact, that the two orders of
mammals—namely, the Edentata and Cetacea—which are the most abnormal in
their dermal covering, are likewise the most abnormal either by
deficiency or redundancy of teeth.
The organs of sight and hearing are generally admitted to be homologous
with one another and with various dermal appendages; hence these parts
are liable to be abnormally affected in conjunction. Mr. White Cowper
says “that in all cases of double microphthalmia brought under his
notice he has at the same time met with defective development of the
dental system.” Certain forms of blindness seem to be associated with
the colour of the hair; a man with black hair and a woman with
light-coloured hair, both of sound constitution, married and had nine
children, all of whom were born blind; of these children, five “with
dark hair and brown iris were afflicted with amaurosis; the four
others, with light-coloured hair and blue iris, had amaurosis and
cataract conjoined.” Several cases could be given, showing that some
relation exists between various affections of the eyes and ears; thus
Liebreich states that out of 241 deaf-mutes in Berlin, no less than
fourteen suffered from the rare disease called pigmentary retinitis.
Mr. White Cowper and Dr. Earle have remarked that inability to
distinguish different colours, or colour-blindness, “is often
associated with a corresponding inability to distinguish musical
sounds.”[23]
Here is a more curious case: white cats, if they have blue eyes, are
almost always deaf. I formerly thought that the rule was invariable,
but I have heard of a few authentic exceptions. The first two notices
were published in 1829 and relate to English and Persian cats: of the
latter, the Rev. W. T. Bree possessed a female, and he states, “that of
the offspring produced at one and the same birth, such as, like the
mother, were entirely white (with blue eyes) were, like her, invariably
deaf; while those that had the least speck of colour on their fur, as
invariably possessed the usual faculty of hearing.”[24] The Rev. W.
Darwin Fox informs me that he has seen more than a dozen instances of
this correlation in English, Persian, and Danish cats; but he adds
“that, if one eye, as I have several times observed, be not blue, the
cat hears. On the other hand, I have never seen a white cat with eyes
of the common colour that was deaf.” In France Dr. Sichel[25] has
observed during twenty years similar facts; he adds the remarkable case
of the iris beginning, at the end of four months, to grow
dark-coloured, and then the cat first began to hear.
This case of correlation in cats has struck many persons as marvellous.
There is nothing unusual in the relation between blue eyes and white
fur; and we have already seen that the organs of sight and hearing are
often simultaneously affected. In the present instance the cause
probably lies in a slight arrest of development in the nervous system
in connection with the sense-organs. Kittens during the first nine
days, whilst their eyes are closed, appear to be completely deaf; I
have made a great clanging noise with a poker and shovel close to their
heads, both when they were asleep and awake, without producing any
effect. The trial must not be made by shouting close to their ears, for
they are, even when asleep, extremely sensitive to a breath of air.
Now, as long as the eyes continue closed, the iris is no doubt blue,
for in all the kittens which I have seen this colour remains for some
time after the eyelids open. Hence, if we suppose the development of
the organs of sight and hearing to be arrested at the stage of the
closed eyelids, the eyes would remain permanently blue and the ears
would be incapable of perceiving sound; and we should thus understand
this curious case. As, however, the colour of the fur is determined
long before birth, and as the blueness of the eyes and the whiteness of
the fur are obviously connected, we must believe that some primary
cause acts at a much earlier period.
The instances of correlated variability hitherto given have been
chiefly drawn from the animal kingdom, and we will now turn to plants.
Leaves, sepals, petals, stamens, and pistils are all homologous. In
double flowers we see that the stamens and pistils vary in the same
manner, and assume the form and colour of the petals. In the double
columbine (_Aquilegia vulgaris_), the successive whorls of stamens are
converted into cornucopias, which are enclosed within one another and
resemble the true petals. In hose-in-hose flowers the sepals mock the
petals. In some cases the flowers and leaves vary together in tint: in
all the varieties of the common pea, which have purple flowers, a
purple mark may be seen on the stipules.
M. Faivre states that with the varieties of _ Primula sinensis_ the
colour of the flower is evidently correlated with the colour of the
under side of the leaves; and he adds that the varieties with
fimbriated flowers almost always have voluminous, balloon-like
calyces.[26] With other plants the leaves and fruit or seeds vary
together in colour, as in a curious pale-leaved variety of the
sycamore, which has recently been described in France,[27] and as in
the purple-leaved hazel, in which the leaves, the husk of the nut, and
the pellicle round the kernel are all coloured purple.[28] Pomologists
can predict to a certain extent, from the size and appearance of the
leaves of their seedlings, the probable nature of the fruit; for, as
Van Mons remarks[29] variations in the leaves are generally accompanied
by some modification in the flower, and consequently in the fruit. In
the Serpent melon, which has a narrow tortuous fruit above a yard in
length, the stem of the plant, the peduncle of the female flower, and
the middle lobe of the leaf, are all elongated in a remarkable manner.
On the other hand, several varieties of Cucurbita, which have dwarfed
stems, all produce, as Naudin remarks, leaves of the same peculiar
shape. Mr. G. Maw informs me that all the varieties of the scarlet
Pelargoniums which have contracted or imperfect leaves have contracted
flowers: the difference between “Brilliant” and its parent “Tom Thumb”
is a good instance of this. It may be suspected that the curious case
described by Risso,[30] of a variety of the Orange which produces on
the young shoots rounded leaves with winged petioles, and afterwards
elongated leaves on long but wingless petioles, is connected with the
remarkable change in form and nature which the fruit undergoes during
its development.
In the following instance we have the colour and the form of the petals
apparently correlated, and both dependent on the nature of the season.
An observer, skilled in the subject, writes,[31] “I noticed, during the
year 1842, that every Dahlia of which the colour had any tendency to
scarlet, was deeply notched—indeed, to so great an extent as to give
the petals the appearance of a saw; the indentures were, in some
instances, more than a quarter of an inch deep.” Again, Dahlias which
have their petals tipped with a different colour from the rest of the
flower are very inconstant, and during certain years some, or even all
the flowers, become uniformly coloured; and it has been observed with
several varieties[32] that when this happens the petals grow much
elongated and lose their proper shape. This, however, may be due to
reversion, both in colour and form, to the aboriginal species.
In this discussion on correlation, we have hitherto treated of cases in
which we can partly understand the bond of connection; but I will now
give cases in which we cannot even conjecture, or can only very
obscurely see, the nature of the bond. Isidore Geoffroy Saint-Hilaire,
in his work on Monstrosities, insists,[33] “que certaines anomalies
coexistent rarement entr’elles, d’autres fréquemment, d’autres enfin
presque constamment, malgré la différence très-grande de leur nature,
et quoiqu’elles puissent paraître _complètement indépendantes_ les unes
des autres.” We see something analogous in certain diseases: thus in a
rare affection of the renal capsules (of which the functions are
unknown), the skin becomes bronzed; and in hereditary syphilis, as I
hear from Sir J. Paget, both the milk and the second teeth assume a
peculiar and characteristic form. Professor Rolleston, also, informs me
that the incisor teeth are sometimes furnished with a vascular rim in
correlation with intra-pulmonary deposition of tubercles. In other
cases of phthisis and of cyanosis the nails and finger-ends become
clubbed like acorns. I believe that no explanation has been offered of
these and of many other cases of correlated disease.
What can be more curious and less intelligible than the fact previously
given, on the authority of Mr. Tegetmeier, that young pigeons of all
breeds, which when mature have white, yellow, silver-blue, or
dun-coloured plumage, come out of the egg almost naked; whereas pigeons
of other colours when first born are clothed with plenty of down? White
Pea-fowls, as has been observed both in England and France,[34] and as
I have myself seen, are inferior in size to the common coloured kind;
and this cannot be accounted for by the belief that albinism is always
accompanied by constitutional weakness; for white or albino moles are
generally larger than the common kind.
To turn to more important characters: the niata cattle of the Pampas
are remarkable from their short foreheads, upturned muzzles, and curved
lower jaws. In the skull the nasal and premaxillary bones are much
shortened, the maxillaries are excluded from any junction with the
nasals, and all the bones are slightly modified, even to the plane of
the occiput. From the analogous case of the dog, hereafter to be given,
it is probable that the shortening of the nasal and adjoining bones is
the proximate cause of the other modifications in the skull, including
the upward curvature of the lower jaw, though we cannot follow out the
steps by which these changes have been effected.
Polish fowls have a large tuft of feathers on their heads; and their
skulls are perforated by numerous holes, so that a pin can be driven
into the brain without touching any bone. That this deficiency of bone
is in some way connected with the tuft of feathers is clear from tufted
ducks and geese likewise having perforated skulls. The case would
probably be considered by some authors as one of balancement or
compensation. In the chapter on Fowls, I have shown that with Polish
fowls the tuft of feathers was probably at first small; by continued
selection it became larger, and then rested on a fibrous mass; and
finally, as it became still larger, the skull itself became more and
more protuberant until it acquired its present extraordinary structure.
Through correlation with the protuberance of the skull, the shape and
even the relative connection of the premaxillary and nasal bones, the
shape of the orifice of the nostrils, the breadth of the frontal bone,
the shape of the post-lateral processes of the frontal and squamosal
bones, and the direction of the bony cavity of the ear, have all been
modified. The internal configuration of the skull and the whole shape
of the brain have likewise been altered in a truly marvellous manner.
After this case of the Polish fowl it would be superfluous to do more
than refer to the details previously given on the manner in which the
changed form of the comb has affected the skull, in various breeds of
the fowl, causing by correlation crests, protuberances, and depressions
on its surface.
With our cattle and sheep the horns stand in close connection with the
size of the skull, and with the shape of the frontal bones; thus
Cline[35] found that the skull of a horned ram weighed five times as
much as that of a hornless ram of the same age. When cattle become
hornless, the frontal bones are “materially diminished in breadth
towards the poll;” and the cavities between the bony plates “are not so
deep, nor do they extend beyond the frontals.”[36] It may be well here
to pause and observe how the effects of correlated variability, of the
increased use of parts, and of the accumulation of so-called
spontaneous variations through natural selection, are in many cases
inextricably commingled. We may borrow an illustration from Mr. Herbert
Spencer, who remarks that, when the Irish elk acquired its gigantic
horns, weighing above one hundred pounds, numerous co-ordinated changes
of structure would have been indispensable,—namely, a thickened skull
to carry the horns; strengthened cervical vertebrae, with strengthened
ligaments; enlarged dorsal vertebrae to support the neck, with powerful
fore-legs and feet; all these parts being supplied with proper muscles,
blood-vessels, and nerves. How then could these admirably co-ordinated
modifications of structure have been acquired? According to the
doctrine which I maintain, the horns of the male elk were slowly gained
through sexual selection,—that is, by the best-armed males conquering
the worse-armed, and leaving a greater number of descendants. But it is
not at all necessary that the several parts of the body should have
simultaneously varied. Each stag presents individual characteristics,
and in the same district those which had slightly heavier horns, or
stronger necks, or stronger bodies, or were the most courageous, would
secure the greater number of does, and consequently have a greater
number of offspring. The offspring would inherit, in a greater or less
degree, these same qualities, would occasionally intercross with one
another, or with other individuals varying in some favourable manner;
and of their offspring, those which were the best endowed in any
respect would continue multiplying; and so onwards, always progressing,
sometimes in one direction, and sometimes in another, towards the
excellently co-ordinated structure of the male elk. To make this clear,
let us reflect on the probable steps, as shown in the twentieth
chapter, by which our race and dray horses have arrived at their
present state of excellence; if we could view the whole series of
intermediate forms between one of these animals and an early unimproved
progenitor, we should behold a vast number of animals, not equally
improved in each generation throughout their entire structure, but
sometimes a little more in one point, and sometimes in another, yet on
the whole gradually approaching in character to our present race or
dray horses, which are so admirably fitted in the one case for
fleetness and in the other for draught.
Although natural selection would thus[37] tend to give to the male elk
its present structure, yet it is probable that the inherited effects of
use, and of the mutual action of part on part, have been equally or
more important. As the horns gradually increased in weight the muscles
of the neck, with the bones to which they are attached, would increase
in size and strength; and these parts would react on the body and legs.
Nor must we overlook the fact that certain parts of the skull and the
extremities would, judging by analogy, tend from the first to vary in a
correlated manner. The increased weight of the horns would also act
directly on the skull, in the same manner as when one bone is removed
in the leg of a dog, the other bone, which has to carry the whole
weight of the body, increases in thickness. But from the fact given
with respect to horned and hornless cattle, it is probable that the
horns and skull would immediately act on each other through the
principle of correlation. Lastly, the growth and subsequent wear and
tear of the augmented muscles and bones would require an increased
supply of blood, and consequently increased supply of food; and this
again would require increased powers of mastication, digestion,
respiration, and excretion.
_Colour as Correlated with Constitutional Peculiarities._
It is an old belief that with man there is a connection between
complexions and constitution; and I find that some of the best
authorities believe in this to the present day.[38] Thus Dr. Beddoe by
his tables shows[39] that a relation exists between liability to
consumption and the colour of the hair, eyes, and skin. It has been
affirmed[40] that, in the French army which invaded Russia, soldiers
having a dark complexion from the southern parts of Europe, withstood
the intense cold better than those with lighter complexions from the
north; but no doubt such statements are liable to error.
In the second chapter on Selection I have given several cases proving
that with animals and plants differences in colour are correlated with
constitutional differences, as shown by greater or less immunity from
certain diseases, from the attacks of parasitic plants and animals,
from scorching by the sun, and from the action of certain poisons. When
all the individuals of any one variety possess an immunity of this
nature, we do not know that it stands in any sort of correlation with
their colour; but when several similarly coloured varieties of the same
species are thus characterised, whilst other coloured varieties are not
thus favoured, we must believe in the existence of a correlation of
this kind. Thus, in the United States purple-fruited plums of many
kinds are far more affected by a certain disease than green or
yellow-fruited varieties. On the other hand, yellow-fleshed peaches of
various kinds suffer from another disease much more than the
white-fleshed varieties. In the Mauritius red sugar-canes are much less
affected by a particular disease than the white canes. White onions and
verbenas are the most liable to mildew; and in Spain the green-fruited
grapes suffered from the vine-disease more than other coloured
varieties. Dark-coloured pelargoniums and verbenas are more scorched by
the sun than varieties of other colours. Red wheats are believed to be
hardier than white; and red-flowered hyacinths were more injured during
one particular winter in Holland than other coloured varieties. With
animals, white terriers suffer most from the distemper, white chickens
from a parasitic worm in their tracheae, white pigs from scorching by
the sun, and white cattle from flies; but the caterpillars of the
silk-moth which yield white cocoons suffered in France less from the
deadly parasitic fungus than those producing yellow silk.
The cases of immunity from the action of certain vegetable poisons, in
connexion with colour, are more interesting, and are at present wholly
inexplicable. I have already given a remarkable instance, on the
authority of Professor Wyman, of all the hogs, excepting those of a
black colour, suffering severely in Virginia from eating the root of
the _Lachnanthes tinctoria._ According to Spinola and others,[41]
buckwheat (_Po1ygonum fagopyrum_), when in flower, is highly injurious
to white or white-spotted pigs, if they are exposed to the heat of the
sun, but is quite innocuous to black pigs. According to two accounts,
the _Hypericum crispum_ in Sicily is poisonous to white sheep alone;
their heads swell, their wool falls off, and they often die; but this
plant, according to Lecce, is poisonous only when it grows in swamps;
nor is this improbable, as we know how readily the poisonous principle
in plants is influenced by the conditions under which they grow.
Three accounts have been published in Eastern Prussia, of white and
white-spotted horses being greatly injured by eating mildewed and
honeydewed vetches; every spot of skin bearing white hairs becoming
inflamed and gangrenous. The Rev. J. Rodwell informs me that his father
turned out about fifteen cart-horses into a field of tares which in
parts swarmed with black aphides, and which no doubt were honeydewed,
and probably mildewed; the horses, with two exceptions, were chestnuts
and bays with white marks on their faces and pasterns, and the white
parts alone swelled and became angry scabs. The two bay horses with no
white marks entirely escaped all injury. In Guernsey, when horses eat
fool’s parsley (_Æthusa cynapium_) they are sometimes violently purged;
and this plant “has a peculiar effect on the nose and lips, causing
deep cracks and ulcers, particularly on horses with white muzzles.”[42]
With cattle, independently of the action of any poison, cases have been
published by Youatt and Erdt of cutaneous diseases with much
constitutional disturbance (in one instance after exposure to a hot
sun) affecting every single point which bore a white hair, but
completely passing over other parts of the body. Similar cases have
been observed with horses.[43]
We thus see that not only do those parts of the skin which bear white
hair differ in a remarkable manner from those bearing hair of any other
colour, but that some great constitutional difference must be
correlated with the colour of the hair; for in the above-mentioned
cases, vegetable poisons caused fever, swelling of the head, as well as
other symptoms, and even death, to all the white, or white-spotted
animals.
REFERENCES
[1] ‘Hist. des Anomalies,’ tom. iii. p. 392. Prof. Huxley applies the
same principle in accounting for the remarkable, though normal,
differences in the arrangement of the nervous system in the Mollusca,
in his paper on the Morphology of the Cephalous Mollusca in ‘Phil.
Transact.,’ 1853, p. 56.
[2] ‘Eléments de Tératologie Veg.,’ 1841, p. 13.
[3] Prof. J. B. Simonds on the Age of the Ox, Sheep, etc., quoted in
‘Gardener’s Chronicle,’ 1854, p. 588.
[4] ‘Hist. des Anomalies,’ tom. i. p. 674.
[5] Quoted by Isid. Geoffroy, ibid., tom. i. p. 635.
[6] ‘The Poultry Book,’ by W. B. Tegetmeier, 1866, p. 250.
[7] Naturalists differ with respect to the homologies of the digits of
birds; but several uphold the view above advanced. _ See_ on this
subject Dr. E. S. Morse in ‘Annals of the Lyceum of Nat. Hist. of New
York,’ vol. x. 1872, p. 16.
[8] A. Walker on Intermarriage, 1838, p. 160.
[9] ‘The Farrier and Naturalist,’ vol. i. 1828, p. 456. A gentleman
who has attended to this point, tells me that about three-fourths of
white-faced horses have white legs.
[10] Godron, ‘Sur l’Espèce,’ tom. ii. p. 217.
[11] ‘Quadrupèdes du Paraguay,’ tom. ii. p. 333.
[12] On Sheep, p. 142.
[13] ‘Ueber Racen, Kreuzungen,’ etc., 1825, s. 24.
[14] Quoted from Conolly, in ‘The Indian Field,’ Feb. 1859, vol. ii.
p. 266.
[15] In the third chapter I have said that “the hair and horns are so
closely related to each other, that they are apt to vary together.”
Dr. Wilckens (“Darwin’s Theorie,” ‘Jahrbuch der Deutschen Viehzucht,’
1866, 1. Heft) translates my words into “lang-und grobhaarige Thiere
sollen geneigter sein, lange und viele Hörner zu bekommen” and he then
justly disputes this proposition; but what I have really said, in
accordance with the authorities just quoted, may, I think, be trusted.
[16] ‘Domesticated Animals of the British Islands,’ pp. 307, 368. Dr.
Wilckens argues (‘Landwirth. Wochenblatt,’ Nr. 10, 1869) to the same
effect with respect to domestic animals in Germany.
[17] ‘Proceedings Zoolog. Soc.,’ 1833, p. 113.
[18] Sedgwick, ‘Brit. and Foreign Medico-Chirurg. Review,’ April 1863,
p. 453.
[19] ‘Gardener’s Chronicle,’ 1849, p. 205.
[20] ‘Embassy to the Court of Ava,’ vol. i. p. 320.
[21] ‘Narrative of a Mission to the Court of Ava in 1855,’ p. 94.
[22] I owe to the kindness of M. Chauman, of St. Petersburg, excellent
photographs of this man and his son, both of whom have since been
exhibited in Paris and London.
[23] These statements are taken from Mr. Sedgwick in the
‘Medico-Chirurg. Review,’ July, 1861, p. 198; April, 1863, pp. 455 and
458. Liebreich is quoted by Professor Devay, in his ‘Mariages
Consanguins,’ 1862, p. 116.
[24] Loudon’s ‘Mag. of Nat. Hist.,’ vol. i. 1829, pp. 66, 178. _ See
also_ Dr. P. Lucas, ‘L’Héréd. Nat.,’ tom. i. p. 428, on the
inheritance of deafness in cats. Mr. Lawson Tait states (‘Nature,’
1873, p. 323) that only male cats are thus affected; but this must be
a hasty generalisation. The first case recorded in England by Mr. Bree
related to a female, and Mr. Fox informs me that he has bred kittens
from a white female with blue eyes, which was completely deaf; he has
also observed other females in the same condition.
[25] ‘Annales des Sc. Nat.’ Zoolog., 3rd series, 1847, tom. viii. p.
239.
[26] ‘Revue des Cours Scientifiques,’ June 5th, 1869, p. 430.
[27] ‘Gardener’s Chronicle,’ 1864, p. 1202.
[28] Verlot gives several other instances, ‘Des Variétés,’ 1865, p.
72.
[29] ‘Arbres Fruitiers,’ 1836, tom. ii. pp. 204, 226.
[30] ‘Annales du Muséum,’ tom. xx. p. 188.
[31] ‘Gardener’s Chronicle,’ 1843, p. 877.
[32] Ibid., 1845, p. 102.
[33] ‘Hist. des Anomalies,’ tom. iii. p. 402. _See also_ M. Camille
Dareste, ‘Recherches sur les Conditions,’ etc., 1863, pp. 16, 48.
[34] Rev. E. S. Dixon, ‘Ornamental Poultry,’ 1848, p. 111; Isidore
Geoffroy, ‘Hist. Anomalies,’ tom. i. p. 211.
[35] ‘On the Breeding of Domestic Animals,’ 1829, p. 6.
[36] Youatt on Cattle, 1834, p. 283.
[37] Mr. Herbert Spencer (‘Principles of Biology,’ 1864, vol. i. pp.
452, 468) takes a different view; and in one place remarks: “We have
seen reason to think that, as fast as essential faculties multiply,
and as fast as the number of organs that co-operate in any given
function increases, indirect equilibration through natural selection
becomes less and less capable of producing specific adaptations; and
remains fully capable only of maintaining the general fitness of
constitution to conditions.” This view that natural selection can do
little in modifying the higher animals surprises me, seeing that man’s
selection has undoubtedly effected much with our domesticated
quadrupeds and birds.
[38] Dr. Prosper Lucas apparently disbelieves in any such connection;
‘L’Héréd. Nat.,’ tom. ii. pp. 88-94.
[39] ‘British Medical Journal,’ 1862, p. 433.
[40] Boudin, ‘Géograph. Médicale,’ tom. i. p. 406.
[41] This fact and the following cases, when not stated to the
contrary, are taken from a very curious paper by Prof. Heusinger, in
‘Wochenschrift fur Heilkunde,’ May, 1846, s. 277. Settegast (‘Die
Thierzucht,’ 1868, p. 39) says that white or white-spotted sheep
suffer like pigs, or even die from eating buckwheat; whilst black or
dark-woolled individuals are not in the least affected.
[42] Mr. Mogford, in the ‘Veterinarian,’ quoted in ‘The Field,’ Jan.
22nd, 1861, p. 545.
[43] ‘Edinburgh Veterinary Journal,’ Oct. 1860, p. 347.