LAWS OF VARIATION, continued.--CORRELATED VARIABILITY.
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 (25/1. 'Hist. des Anomalies' tome 3 page 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 page 56.) that this may constantly be observed with
monstrosities in the animal kingdom; and Moquin-Tandon (25/2. 'Elements de
Teratologie Veg.' 1841 page 13.) 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 larvae 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. (25/3. Prof. J.B. Simonds on the
Age of the Ox, Sheep, etc. quoted in 'Gardener's Chronicle' 1854 page 588.)
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 (25/4. 'Hist. des
Anomalies' tome 1 page 674.) 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 (25/5. Quoted by Isid.
Geoffroy ibid tome 1 page 635.) 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 (25/6. 'The Poultry Book'
by W.B. Tegetmeier 1866 page 250.), 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. (25/7. 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' volume 10 1872 page 16.) 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 (24/8. A. Walker on 'Intermarriage' 1838 page 160.) 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. (25/9. 'The Farrier and Naturalist'
volume 1 1828 page 456. A gentleman who has attended to this point, tells me
that about three-fourths of white-faced horses have white legs.) 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 (25/10. Godron 'Sur
l'Espece' tome 2 page 217.) 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 (25/11. 'Quadrupedes du Paraguay' tome 2 page 333.), 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 (25/12. 'On Sheep' page 142.)
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 (25/13. 'Ueber Racen,
Kreuzungen' etc. 1825 s. 24.) 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 (25/14. Quoted from Conolly in 'The
Indian Field' February 1859 volume 2 page 266.) 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. (25/15. 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 Horner 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.) 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 (25/16. 'Domesticated Animals of the British
Islands' pages 307, 368. Dr. Wilckens argues ('Landwirth. Wochenblatt' Nr. 10
1869) to the same effect with respect to domestic animals in Germany.) 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 (25/17. 'Proceedings Zoolog. Soc.'
1833 page 113.) 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 (25/18. Sedgwick 'Brit. and Foreign Medico-Chirurg. Review' April
1863 page 453.) 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 (25/19. 'Gardener's
Chronicle' 1849 page 205.) 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
(25/20. 'Embassy to the Court of Ava' volume 1 page 320.) 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 (25/21. 'Narrative of a Mission to
the Court of Ava in 1855' page 94.) 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. (25/22. 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.)
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."
(25/23. These statements are taken from Mr. Sedgwick in the 'Medico-Chirurg.
Review' July 1861 page 198; April 1863 pages 455 and 458. Liebreich is quoted
by Professor Devay in his 'Mariages Consanguins' 1862 page 116.)
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."
(25/24. Loudon's 'Mag. of Nat. Hist.' volume 1 1829 pages 66, 178. See also
Dr. P. Lucas 'L'Hered. Nat.' tome 1 page 428 on the inheritance of deafness in
cats. Mr. Lawson Tait states ('Nature' 1873 page 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.)
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 (25/25. 'Annales des Sc. Nat.' Zoolog.
3rd series 1847 tome 8 page 239.) 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. (25/26. 'Revue des Cours Scientifiques'
June 5, 1869 page 430.) 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 (25/27. 'Gardener's Chronicle' 1864 page
1202.), 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. (25/28.
Verlot gives several other instances 'Des Varietes' 1865 page 72.) 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
(25/29. 'Arbres Fruitiers' 1836 tome 2 pages 204, 226.) 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 (25/30. 'Annales du Museum' tome 20 page 188.), 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 (25/31. 'Gardener's Chronicle' 1843
page 877.), "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
(25/32. Ibid 1845 page 102.) 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 (25/33. 'Hist. des Anomalies' tome 3 page 402. See also M. Camille
Dareste 'Recherches sur les Conditions' etc. 1863 pages 16, 48.), "que
certaines anomalies coexistent rarement entr'elles, d'autres frequemment,
d'autres enfin presque constamment, malgre la difference tres-grande de leur
nature, et quoiqu'elles puissent paraitre COMPLETEMENT INDEPENDANTES 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 (25/34. Rev. E.S. Dixon 'Ornamental Poultry' 1848 page 111;
Isidore Geoffroy 'Hist. Anomalies' tome 1 page 211.), 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 (25/35. 'On the
Breeding of Domestic Animals' 1829 page 6.) 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." (25/36. Youatt on 'Cattle'
1834 page 283.)
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 (25/37. Mr. Herbert Spencer 'Principles
of Biology' 1864 volume 1 pages 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.) 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. (25/38. Dr. Prosper Lucas apparently disbelieves in any
such connection; 'L'Hered. Nat.' tome 2 pages 88-94.) Thus Dr. Beddoe by his
tables shows (25/39. 'British Medical Journal' 1862 page 433.) that a relation
exists between liability to consumption and the colour of the hair, eyes, and
skin. It has been affirmed (25/40. Boudin 'Geograph. Medicale' tome 1 page
406.) 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 (25/41. 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 page 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.), 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 (Aethusa 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."
(25/42. Mr. Mogford in the 'Veterinarian' quoted in 'The Field' January 22,
1861 page 545.) 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.
(25/43. 'Edinburgh Veterinary Journal' October 1860 page 347.)
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.