THE FUSION OF HOMOLOGOUS PARTS—THE VARIABILITY OF MULTIPLE AND
HOMOLOGOUS PARTS—COMPENSATION OF GROWTH—MECHANICAL PRESSURE—RELATIVE
POSITION OF FLOWERS WITH RESPECT TO THE AXIS, AND OF SEEDS IN THE
OVARY, AS INDUCING VARIATION—ANALOGOUS OR PARALLEL VARIETIES—SUMMARY OF
THE THREE LAST CHAPTERS.
_The Fusion of Homologous Parts._—Geoffroy Saint-Hilaire formerly
propounded what he called _la loi de l’affinité de soi pour soi,_ which
has been discussed and illustrated by his son, Isidore, with respect to
monsters in the animal kingdom,[1] and by Moquin-Tandon, with respect
to monstrous plants. This law seems to imply that homologous parts
actually attract one another and then unite. No doubt there are many
wonderful cases, in which such parts become intimately fused together.
This is perhaps best seen in monsters with two heads, which are united,
summit to summit, or face to face, or Janus-like, back to back, or
obliquely side to side. In one instance of two heads united almost face
to face, but a little obliquely, four ears were developed, and on one
side a perfect face, which was manifestly formed by the fusion of two
half-faces. Whenever two bodies or two heads are united, each bone,
muscle, vessel, and nerve on the line of junction appears as if it had
sought out its fellow, and had become completely fused with it.
Lereboullet,[2] who carefully studied the development of double
monsters in fishes, observed in fifteen instances the steps by which
two heads gradually became united into one. In all such cases it is now
thought by the greater number of capable judges that the homologous
parts do not attract each other, but that in the words of Mr. Lowne:[3]
“As union takes place before the differentiation of distinct organs
occurs, these are formed in continuity with each other.” He adds that
organs already differentiated probably in no case become united to
homologous ones. M. Dareste does not speak[4] quite decisively against
the law of _soi pour soi,_ but concludes by saying, “On se rend
parfaitement compte de la formation des monstres, si l’on admet que les
embryons qui se soudent appartiennent à un même œuf; qu’ils s’unissent
en même temps qu’ils se forment, et que la soudure ne se produit que
pendant la première période de la vie embryonnaire, celle ou les
organes ne sont encore constitués que par des blastèmes homogènes.”
By whatever means the abnormal fusion of homologous parts is effected,
such cases throw light on the frequent presence of organs which are
double during an embryonic period (and throughout life in other and
lower members of the same class) but which afterwards unite by a normal
process into a single medial organ. In the vegetable kingdom
Moquin-Tandon[5] gives a long list of cases, showing how frequently
homologous parts, such as leaves, petals, stamens, and pistils,
flowers, and aggregates of homologous parts, such as buds, as well as
fruit, become blended, both normally and abnormally, with perfect
symmetry into one another.
_The Variability of Multiple and Homologous parts._—Isidore Geoffroy[6]
insists that, when any part or organ is repeated many times in the same
animal, it is particularly liable to vary both in number and structure.
With respect to number, the proposition may, I think, be considered as
fully established; but the evidence is chiefly derived from organic
beings living under their natural conditions, with which we are not
here concerned. Whenever such parts as the vertebrae or teeth, the rays
in the fins of fishes, or the feathers in the tails of birds, or
petals, stamens, pistils, or seeds, are very numerous, the number is
generally variable. With respect to the structure of multiple parts,
the evidence of variability is not so decisive; but the fact, as far as
it may be trusted, probably depends on multiple parts being of less
physiological importance than single parts; consequently their
structure has been less rigorously guarded by natural selection.
_Compensation of Growth, or Balancement._—This law, as applied to
natural species, was propounded by Goethe and Geoffroy Saint-Hilaire at
nearly the same time. It implies that, when much organised matter is
used in building up some one part, other parts are starved and become
reduced. Several authors, especially botanists, believe in this law;
others reject it. As far as I can judge, it occasionally holds good;
but its importance has probably been exaggerated. It is scarcely
possible to distinguish between the supposed effects of such
compensation, and the effects of long-continued selection which may
lead to the augmentation of one part, and simultaneously to the
diminution of another. Anyhow, there can be no doubt that an organ may
be greatly increased without any corresponding diminution of an
adjoining part. To recur to our former illustration of the Irish elk,
it may be asked what part has suffered in consequence of the immense
development of the horns?
It has already been observed that the struggle for existence does not
bear hard on our domesticated productions, and consequently the
principle of economy of growth will seldom come into play, so that we
ought not to expect to find with them frequent evidence of
compensation. We have, however, some such cases. Moquin-Tandon
describes a monstrous bean,[7] in which the stipules were enormously
developed, and the leaflets apparently in consequence completely
aborted; this case is interesting, as it represents the natural
condition of _Lathyrus aphaca,_ with its stipules of great size, and
its leaves reduced to mere threads, which act as tendrils. De
Candolle[8] has remarked that the varieties of _Raphanus sativus_ which
have small roots yield numerous seed containing much oil, whilst those
with large roots are not productive in oil; and so it is with _Brassica
asperifolia._ The varieties of _Cucurbita pepo_ which bear large fruit
yield a small crop, according to Naudin; whilst those producing small
fruit yield a vast number. Lastly, I have endeavoured to show in the
eighteenth chapter that with many cultivated plants unnatural treatment
checks the full and proper action of the reproductive organs, and they
are thus rendered more or less sterile; consequently, in the way of
compensation, the fruit becomes greatly enlarged, and, in double
flowers, the petals are greatly increased in number.
With animals, it has been found difficult to produce cows which yield
much milk, and are afterwards capable of fattening well. With fowls
which have large top-knots and beards the comb and wattles are
generally much reduced in size; though there are exceptions to this
rule. Perhaps the entire absence of the oil-gland in fantail pigeons
may be connected with the great development of their tails.
_Mechanical Pressure as a Cause of Modifications._—In some few cases
there is reason to believe that mere mechanical pressure has affected
certain structures. Vrolik and Weber[9] maintain that the shape of the
human head is influenced by the shape of the mother’s pelvis. The
kidneys in different birds differ much in form, and St. Ange[10]
believes that this is determined by the form of the pelvis, which
again, no doubt, stands in close relation with their power of
locomotion. In snakes, the viscera are curiously displaced, in
comparison with their position in other vertebrates; and this has been
attributed by some authors to the elongation of their bodies; but here,
as in so many previous cases, it is impossible to disentangle a direct
result of this kind from that consequent on natural selection. Godron
has argued[11] that the abortion of the spur on the inner side of the
flowers in Corydalis, is caused by the buds at a very early period of
growth whilst underground being closely pressed against one another and
against the stem. Some botanists believe that the singular difference
in the shape both of the seed and corolla, in the interior and exterior
florets in certain Compositous and Umbelliferous plants, is due to the
pressure to which the inner florets are subjected; but this conclusion
is doubtful.
The facts just given do not relate to domesticated productions, and
therefore do not strictly concern us. But here is a more appropriate
case: H. Müller[12] has shown that in shortfaced races of the dog some
of the molar teeth are placed in a slightly different position to that
which they occupy in other dogs, especially in those having elongated
muzzles; and as he remarks, any inherited change in the arrangement of
the teeth deserves notice, considering their classificatory importance.
This difference in position is due to the shortening of certain facial
bones and the consequent want of space; and the shortening results from
a peculiar and abnormal state of the embryonal cartilages of the bones.
_Relative Position of Flowers with respect to the Axis, and of
Seeds in the Ovary, as inducing Variation._
In the thirteenth chapter various peloric flowers were described, and
their production was shown to be due either to arrested development, or
to reversion to a primordial condition. Moquin-Tandon has remarked that
the flowers which stand on the summit of the main stem or of a lateral
branch are more liable to become peloric than those on the sides;[13]
and he adduces, amongst other instances, that of _Teucrium
campanulatum._ In another Labiate plant grown by me, viz., the
_Galeobdolon luteum,_ the peloric flowers were always produced on the
summit of the stem, where flowers are not usually borne. In
Pelargonium, a _single_ flower in the truss is frequently peloric, and
when this occurs I have during several years invariably observed it to
be the central flower. This is of such frequent occurrence that one
observer[14] gives the names of ten varieties flowering at the same
time, in every one of which the central flower was peloric.
Occasionally more than one flower in the truss is peloric, and then of
course the additional ones must be lateral. These flowers are
interesting as showing how the whole structure is correlated. In the
common Pelargonium the upper sepal is produced into a nectary which
coheres with the flower-peduncle; the two upper petals differ a little
in shape from the three lower ones, and are marked with dark shades of
colour; the stamens are graduated in length and upturned. In the
peloric flowers, the nectary aborts; all the petals become alike both
in shape and colour; the stamens are generally reduced in number and
become straight, so that the whole flower resembles that of the allied
genus Erodium. The correlation between these changes is well shown when
one of the two upper petals alone loses its dark mark, for in this case
the nectary does not entirely abort, but is usually much reduced in
length.[15]
Morren has described[16] a marvellous flask-shaped flower of the
Calceolaria, nearly four inches in length, which was almost completely
peloric; it grew on the summit of the plant, with a normal flower on
each side; Prof. Westwood also has described[17] three similar peloric
flowers, which all occupied a central position on the flower-branches.
In the Orchideous genus, Phalænopsis, the terminal flower has been seen
to become peloric.
In a Laburnum-tree I observed that about a fourth part of the racemes
produced terminal flowers which had lost their papilionaceous
structure. These were produced after almost all the other flowers on
the same racemes had withered. The most perfectly pelorised examples
had six petals, each marked with black striae like those on the
standard-petal. The keel seemed to resist the change more than the
other petals. Dutrochet has described[18] an exactly similar case in
France, and I believe these are the only two instances of pelorism in
the laburnum which have been recorded. Dutrochet remarks that the
racemes on this tree do not properly produce a terminal flower, so that
(as in the case of the Galeobdolon) their position as well as structure
are both anomalies, which no doubt are in some manner related. Dr.
Masters has briefly described another leguminous plant,[19] namely, a
species of clover, in which the uppermost and central flowers were
regular or had lost their papilionaceous structure. In some of these
plants the flower-heads were also proliferous.
Lastly, Linaria produces two kinds of peloric flowers, one having
simple petals, and the other having them all spurred. The two forms, as
Naudin remarks,[20] not rarely occur on the same plant, but in this
case the spurred form almost invariably stands on the summit of the
spike.
The tendency in the terminal or central flower to become peloric more
frequently than the other flowers, probably results from “the bud which
stands on the end of a shoot receiving the most sap; it grows out into
a stronger shoot than those situated lower down.”[21] I have discussed
the connection between pelorism and a central position, partly because
some few plants are known normally to produce a terminal flower
different in structure from the lateral ones; but chiefly on account of
the following case, in which we see a tendency to variability or to
reversion connected with the same position. A great judge of
Auriculas[22] states that when one throws up a side bloom it is pretty
sure to keep its character; but that if it grows from the centre or
heart of the plant, whatever the colour of the edging ought to be, “it
is just as likely to come in any other class as in the one to which it
properly belongs.” This is so notorious a fact, that some florists
regularly pinch off the central trusses of flowers. Whether in the
highly improved varieties the departure of the central trusses from
their proper type is due to reversion, I do not know. Mr. Dombrain
insists that, whatever may be the commonest kind of imperfection in
each variety, this is generally exaggerated in the central truss. Thus
one variety “sometimes has the fault of producing a little green floret
in the centre of the flower,” and in central blooms these become
excessive in size. In some central blooms, sent to me by Mr. Dombrain,
all the organs of the flower were rudimentary in structure, of minute
size, and of a green colour, so that by a little further change all
would have been converted into small leaves. In this case we clearly
see a tendency to prolification—a term which I may explain, for those
who have never attended to botany, to mean the production of a branch
or flower, or head of flowers, out of another flower. Now Dr.
Masters[23] states that the central or uppermost flower on a plant is
generally the most liable to prolification. Thus, in the varieties of
the Auricula, the loss of their proper character and a tendency to
prolification, also a tendency to prolification with pelorism, are all
connected together, and are due either to arrested development, or to
reversion to a former condition.
The following is a more interesting case; Metzger[24] cultivated in
Germany several kinds of maize brought from the hotter parts of
America, and he found, as previously described, that in two or three
generations the grains became greatly changed in form, size, and
colour; and with respect to two races he expressly states that in the
first generation, whilst the lower grains on each head retained their
proper character, the uppermost grains already began to assume that
character which in the third generation all the grains acquired. As we
do not know the aboriginal parent of the maize, we cannot tell whether
these changes are in any way connected with reversion.
In the two following cases, reversion comes into play and is determined
by the position of the seed in the capsule. The Blue Imperial pea is
the offspring of the Blue Prussian, and has larger seed and broader
pods than its parent. Now Mr. Masters, of Canterbury, a careful
observer and a raiser of new varieties of the pea, states[25] that the
Blue Imperial always has a strong tendency to revert to its
parent-stock, and the reversion “occurs in this manner: the last (or
uppermost) pea in the pod is frequently much smaller than the rest; and
if these small peas are carefully collected and sown separately, very
many more, in proportion, will revert to their origin, than those taken
from the other parts of the pod.” Again, M. Chaté[26] says that in
raising seedling stocks he succeeds in getting eighty per cent to bear
double flowers, by leaving only a few of the secondary branches to
seed; but in addition to this, “at the time of extracting the seeds,
the upper portion of the pod is separated and placed aside, because it
has been ascertained that the plants coming from the seeds situated in
this portion of the pod, give eighty per cent of single flowers.” Now
the production of single-flowering plants from the seed of
double-flowering plants is clearly a case of reversion. These latter
facts, as well as the connection between a central position and
pelorism and prolification, show in an interesting manner how small a
difference—namely, a little greater or less freedom in the flow of sap
towards one part of the plant—determines important changes of
structure.
_Analogous or Parallel Variation._—By this term I mean that similar
characters occasionally make their appearance in the several varieties
or races descended from the same species, and more rarely in the
offspring of widely distinct species. We are here concerned, not as
hitherto with the causes of variation, but with the results; but this
discussion could not have been more conveniently introduced elsewhere.
The cases of analogous variation, as far as their origin is concerned,
may be grouped, disregarding minor subdivisions, under two main heads;
firstly, those due to unknown causes acting on similarly constituted
organisms, and which consequently have varied in a similar manner; and
secondly, those due to the reappearance of characters which were
possessed by a more or less remote progenitor. But these two main
divisions can often be separated only conjecturally, and graduate, as
we shall presently see, into each other.
Under the first head of analogous variations, not due to reversion, we
have the many cases of trees belonging to quite different orders which
have produced pendulous and fastigiate varieties. The beech, hazel, and
barberry have given rise to purple-leaved varieties; and, as Bernhardi
remarks,[27] a multitude of plants, as distinct as possible, have
yielded varieties with deeply-cut or laciniated leaves. Varieties
descended from three distinct species of Brassica have their stems, or
so-called roots, enlarged into globular masses. The nectarine is the
offspring of the peach; and the varieties of peaches and nectarines
offer a remarkable parallelism in the fruit being white, red, or yellow
fleshed—in being clingstones or freestones—in the flowers being large
or small—in the leaves being serrated or crenated, furnished with
globose or reniform glands, or quite destitute of glands. It should be
remarked that each variety of the nectarine has not derived its
character from a corresponding variety of the peach. The several
varieties also of a closely allied genus, namely the apricot, differ
from one another in nearly the same parallel manner. There is no reason
to believe that any of these varieties have merely reacquired long-lost
characters; and in most of them this certainly is not the case.
Three species of Cucurbita have yielded a multitude of races which
correspond so closely in character that, as Naudin insists, they may be
arranged in almost strictly parallel series. Several varieties of the
melon are interesting from resembling, in important characters, other
species, either of the same genus or of allied genera; thus, one
variety has fruit so like, both externally and internally, the fruit of
a perfectly distinct species, namely, the cucumber, as hardly to be
distinguished from it; another has long cylindrical fruit twisting
about like a serpent; in another the seeds adhere to portions of the
pulp; in another the fruit, when ripe, suddenly cracks and falls into
pieces; and all these highly remarkable peculiarities are
characteristic of species belonging to allied genera. We can hardly
account for the appearance of so many unusual characters by reversion
to a single ancient form; but we must believe that all the members of
the family have inherited a nearly similar constitution from an early
progenitor. Our cereal and many other plants offer similar cases.
With animals we have fewer cases of analogous variation, independently
of direct reversion. We see something of the kind in the resemblance
between the short-muzzled races of the dog, such as the pug and
bull-dog; in feather-footed races of the fowl, pigeon, and canary-bird;
in horses of the most different races presenting the same range of
colour; in all black-and-tan dogs having tan-coloured eye-spots and
feet, but in this latter case reversion may possibly have played a
part. Low has remarked[28] that several breeds of cattle are
“sheeted,”—that is, have a broad band of white passing round their
bodies like a sheet; this character is strongly inherited, and
sometimes originates from a cross; it may be the first step in
reversion to an early type, for, as was shown in the third chapter,
white cattle with dark ears, dark feet and tip of tail, formerly
existed, and now exist in feral or semi-feral condition in several
quarters of the world.
Under our second main division, namely, of analogous variations due to
reversion, the best cases are afforded by pigeons. In all the most
distinct breeds, sub-varieties occasionally appear coloured exactly
like the parent rock-pigeon, with black wing-bars, white loins, banded
tail, etc.; and no one can doubt that these characters are due to
reversion. So with minor details; turbits properly have white tails,
but occasionally a bird is born with a dark-coloured and banded tail;
pouters properly have their primary wing-feathers white, but not rarely
a “sword-flighted” bird appears, that is, one with the few first
primaries dark-coloured; and in these cases we have characters proper
to the rock-pigeon, but new to the breed, evidently appearing from
reversion. In some domestic varieties the wing-bars, instead of being
simply black, as in the rock-pigeon, are beautifully edged with
different zones of colour, and they then present a striking analogy
with the wing-bars in certain natural species of the same family, such
as _Phaps chalcoptera_; and this may probably be accounted for by all
the species of the family being descended from the same remote
progenitor and having a tendency to vary in the same manner. Thus,
also, we can perhaps understand the fact of some Laugher-pigeons cooing
almost like turtle-doves, and for several races having peculiarities in
their flight, since certain natural species (viz., _C. torquatrix_ and
_palumbus_), display singular vagaries in this respect. In other cases
a race, instead of imitating a distinct species, resembles some other
race; thus, certain runts tremble and slightly elevate their tails,
like fantails; and turbits inflate the upper part of their oesophagus,
like pouter-pigeons.
It is a common circumstance to find certain coloured marks persistently
characterising all the species of a genus, but differing much in tint;
and the same thing occurs with the varieties of the pigeon: thus,
instead of the general plumage being blue, with the wing-bars black,
there are snow-white varieties with red bars, and black varieties with
white bars; in other varieties the wing-bars, as we have seen, are
elegantly zoned with different tints. The Spot pigeon is characterised
by the whole plumage being white, excepting a spot on the forehead and
the tail; but these parts may be red, yellow, or black. In the
rock-pigeon and in many varieties the tail is blue, with the outer
edges of the outer feathers white; but in the sub-variety of the
monk-pigeon we have a reversed style of coloration, for the tail is
white, except the outer edges of the outer feathers, which are
black.[29]
With some species of birds, for instance with gulls, certain coloured
parts appear as if almost washed out, and I have observed exactly the
same appearance in the terminal dark tail-bar in certain pigeons, and
in the whole plumage of certain varieties of the duck. Analogous facts
in the vegetable kingdom could be given.
Many sub-varieties of the pigeon have reversed and somewhat lengthened
feathers on the back part of their heads, and this is certainly not due
to reversion to the parent-species, which shows no trace of such
structure: but when we remember that sub-varieties of the fowl, turkey,
canary-bird, duck, and goose, all have either topknots or reversed
feathers on their heads; and when we remember that scarcely a single
large natural group of birds can be named, in which some members have
not a tuft of feathers on their heads, we may suspect that reversion to
some extremely remote form has come into action.
Several breeds of the fowl have either spangled or pencilled feathers;
and these cannot be derived from the parent-species, the _Gallus
bankiva_; though of course it is possible that one early progenitor of
this species may have been spangled, and another pencilled. But, as
many gallinaceous birds are either spangled or pencilled, it is a more
probable view that the several domestic breeds of the fowl have
acquired this kind of plumage from all the members of the family
inheriting a tendency to vary in a like manner. The same principle may
account for the ewes in certain breeds of sheep being hornless, like
the females of some other hollow-horned ruminants; it may account for
certain domestic cats having slightly-tufted ears, like those of the
lynx; and for the skulls of domestic rabbits often differing from one
another in the same characters by which the skulls of the various
species of the genus Lepus differ.
I will only allude to one other case, already discussed. Now that we
know that the wild parent of the ass commonly has striped legs, we may
feel confident that the occasional appearance of stripes on the legs of
the domestic ass is due to reversion; but this will not account for the
lower end of the shoulder-stripe being sometimes angularly bent or
slightly forked. So, again, when we see dun and other coloured horses
with stripes on the spine, shoulders, and legs, we are led, from
reasons formerly given, to believe that they reappear through reversion
to the wild parent-horse. But when horses have two or three
shoulder-stripes, with one of them occasionally forked at the lower
end, or when they have stripes on their faces, or are faintly striped
as foals over nearly their whole bodies, with the stripes angularly
bent one under the other on the forehead, or irregularly branched in
other parts, it would be rash to attribute such diversified characters
to the reappearance of those proper to the aboriginal wild horse. As
three African species of the genus are much striped, and as we have
seen that the crossing of the unstriped species often leads to the
hybrid offspring being conspicuously striped—bearing also in mind that
the act of crossing certainly causes the reappearance of long-lost
characters—it is a more probable view that the above-specified stripes
are due to reversion, not to the immediate wild parent-horse, but to
the striped progenitor of the whole genus.
I have discussed this subject of analogous variation at considerable
length, because it is well known that the varieties of one species
frequently resemble distinct species—a fact in perfect harmony with the
foregoing cases, and explicable on the theory of descent. Secondly,
because these facts are important from showing, as remarked in a former
chapter, that each trifling variation is governed by law, and is
determined in a much higher degree by the nature of the organisation,
than by the nature of the conditions to which the varying being has
been exposed. Thirdly, because these facts are to a certain extent
related to a more general law, namely, that which Mr. B. D. Walsh[30]
has called the “Law of _Equable Variability,_” or, as he explains it,
“if any given character is very variable in one species of a group, it
will tend to be variable in allied species; and if any given character
is perfectly constant in one species of a group, it will tend to be
constant in allied species.”
This leads me to recall a discussion in the chapter on Selection, in
which it was shown that with domestic races, which are now undergoing
rapid improvement, those parts or characters vary the most, which are
the most valued. This naturally follows from recently selected
characters continually tending to revert to their former less improved
standard, and from their being still acted on by the same agencies,
whatever these may be, which first caused the characters in question to
vary. The same principle is applicable to natural species, for, as
stated in my ‘Origin of Species’ generic characters are less variable
than specific characters; and the latter are those which have been
modified by variation and natural selection, since the period when all
the species belonging to the genus branched off from a common
progenitor, whilst generic characters are those which have remained
unaltered from a much more remote epoch, and accordingly are now less
variable. This statement makes a near approach to Mr. Walsh’s law of
Equable Variability. Secondary sexual characters, it may be added,
rarely serve to characterise distinct genera, for they usually differ
much in the species of the same genus, and they are highly variable in
the individuals of the same species; we have also seen in the earlier
chapters of this work how variable secondary sexual characters become
under domestication.
_Summary of the three previous Chapters on the Laws of
Variation._
In the twenty-third chapter we saw that changed conditions
occasionally, or even often, act in a definite manner on the
organisation, so that all, or nearly all, the individuals thus exposed
become modified in the same manner. But a far more frequent result of
changed conditions, whether acting directly on the organisation or
indirectly through the reproductive system, is indefinite and
fluctuating variability. In the three last chapters, some of the laws
by which such variability is regulated have been discussed.
Increased use adds to the size of muscles, together with the
blood-vessels, nerves, ligaments, the crests of bone and the whole
bones, to which they are attached. Increased functional activity
increases the size of various glands, and strengthens the sense-organs.
Increased and intermittent pressure thickens the epidermis. A change in
the nature of the food sometimes modifies the coats of the stomach, and
augments or decreases the length of the intestines. Continued disuse,
on the other hand, weakens and diminishes all parts of the
organisation. Animals which during many generations have taken but
little exercise, have their lungs reduced in size, and as a consequence
the bony fabric of the chest and the whole form of the body become
modified. With our anciently domesticated birds, the wings have been
little used, and they are slightly reduced; with their decrease, the
crest of the sternum, the scapulae, coracoids, and furculum, have all
been reduced.
With domesticated animals, the reduction of a part from disuse is never
carried so far that a mere rudiment is left; whereas we have reason to
believe that this has often occurred under nature; the effects of
disuse in this latter case being aided by economy of growth, together
with the intercrossing of many varying individuals. The cause of this
difference between organisms in a state of nature, and under
domestication, probably is that in the latter case there has not been
time sufficient for any very great change, and that the principle of
economy of growth does not come into action. On the contrary,
structures which are rudimentary in the parent-species, sometimes
become partially redeveloped in our domesticated productions. Such
rudiments as occasionally make their appearance under domestication,
seem always to be the result of a sudden arrest of development;
nevertheless they are of interest, as showing that rudiments are the
relics of organs once perfectly developed.
Corporeal, periodical, and mental habits, though the latter have been
almost passed over in this work, become changed under domestication,
and the changes are often inherited. Such changed habits in an organic
being, especially when living a free life, would often lead to the
augmented or diminished use of various organs, and consequently to
their modification. From long-continued habit, and more especially from
the occasional birth of individuals with a slightly different
constitution, domestic animals and cultivated plants become to a
certain extent acclimatised or adapted to a climate different from that
proper to the parent-species.
Through the principle of correlated variability, taken in its widest
sense, when one part varies other parts vary, either simultaneously, or
one after the other. Thus, an organ modified during an early embryonic
period affects other parts subsequently developed. When an organ, such
as the beak, increases or decreases in length, adjoining or correlated
parts, as the tongue and the orifice of the nostrils, tend to vary in
the same manner. When the whole body increases or decreases in size,
various parts become modified; thus, with pigeons the ribs increase or
decrease in number and breadth. Homologous parts which are identical
during their early development and are exposed to similar conditions,
tend to vary in the same or in some connected manner,—as in the case of
the right and left sides of the body, and of the front and hind limbs.
So it is with the organs of sight and hearing; for instance, white cats
with blue eyes are almost always deaf. There is a manifest relation
throughout the body between the skin and various dermal appendages,
such as hair, feathers, hoofs, horns, and teeth. In Paraguay, horses
with curly hair have hoofs like those of a mule; the wool and the horns
of sheep often vary together; hairless dogs are deficient in their
teeth; men with redundant hair have abnormal teeth, either by
deficiency or excess. Birds with long wing-feathers usually have long
tail-feathers. When long feathers grow from the outside of the legs and
toes of pigeons, the two outer toes are connected by membrane; for the
whole leg tends to assume the structure of the wing. There is a
manifest relation between a crest of feathers on the head and a
marvellous amount of change in the skull of various fowls; and in a
lesser degree, between the greatly elongated, lopping ears of rabbits
and the structure of their skulls. With plants, the leaves, various
parts of the flower, and the fruit, often vary together to a correlated
manner.
In some cases we find correlation without being able even to conjecture
what is the nature of the connection, as with various monstrosities and
diseases. This is likewise the case with the colour of the adult
pigeon, in connection with the presence of down on the young bird.
Numerous curious instances have been given of peculiarities of
constitution, in correlation with colour, as shown by the immunity of
individuals of one colour from certain diseases, from the attacks of
parasites and from the action of certain vegetable poisons.
Correlation is an important subject; for with species, and in a lesser
degree with domestic races, we continually find that certain parts have
been greatly modified to serve some useful purpose; but we almost
invariably find that other parts have likewise been more or less
modified, without our being able to discover any advantage in the
change. No doubt great caution is necessary with respect to this latter
point, for it is difficult to overrate our ignorance on the use of
various parts of the organisation; but from what we have seen, we may
believe that many modifications are of no direct service, having arisen
in correlation with other and useful changes.
Homologous parts during their early development often become fused
together. Multiple and homologous organs are especially liable to vary
in number and probably in form. As the supply of organised matter is
not unlimited, the principle of compensation sometimes comes into
action; so that, when one part is greatly developed, adjoining parts
are apt to be reduced; but this principle is probably of much less
importance than the more general one of the economy of growth. Through
mere mechanical pressure hard parts occasionally affect adjoining
parts. With plants the position of the flowers on the axis, and of the
seeds in the ovary, sometimes leads, through a more or less free flow
of sap, to changes of structure; but such changes are often due to
reversion. Modifications, in whatever manner caused, will be to a
certain extent regulated by that co-ordinating power, or so-called
_nisus formativus,_ which is in fact a remnant of that simple form of
reproduction, displayed by many lowly organised beings in their power
of fissiparous generation and budding. Finally, the effects of the laws
which directly or indirectly govern variability, may be largely
regulated by man’s selection, and will so far be determined by natural
selection that changes advantageous to any race will be favoured, and
disadvantageous changes will be checked.
Domestic races descended from the same species, or from two or more
allied species, are liable to revert to characters derived from their
common progenitor; and, as they inherit a somewhat similar
constitution, they are liable to vary in the same manner. From these
two causes analogous varieties often arise. When we reflect on the
several foregoing laws, imperfectly as we understand them, and when we
bear in mind how much remains to be discovered, we need not be
surprised at the intricate and to us unintelligible manner in which our
domestic productions have varied, and still go on varying.
REFERENCES
[1] ‘Hist. des Anomalies,’ 1832, tom. i. pp. 22, 537-556; tom. iii. p.
462.
[2] ‘Comptes Rendus,’ 1855, pp. 855, 1039.
[3] ‘Catalogue of the Teratological Series in the Museum of the R.
Coll. of Surgeons,’ 1872, p. 16.
[4] ‘Archives de Zoolog. Exper.,’ Jan. 1874, p. 78.
[5] ‘Tératologie Vég.,’ 1841, livre iii.
[6] ‘Hist. des Anomalies,’ tom. iii. pp. 4, 5, 6.
[7] ‘Tératologie Vég.,’ p. 156. _See also_ my book on ‘The Movements
and Habits of Climbing Plants,’ 2nd edit., 1875, p. 202.
[8] ‘Mémoires du Muséum,’ etc., tom. viii. p. 178.
[9] Prichard, ‘Phys. Hist. of Mankind,’ 1851, vol. i. p. 324.
[10] ‘Annales des Sc. Nat.,’ 1st series, tom. xix. p. 327.
[11] ‘Comptes Rendus,’ Dec. 1864, p. 1039.
[12] “Ueber fötale Rachites,” ‘Würzburger Medicin. Zeitschrift,’ 1860,
B. i. s. 265.
[13] ‘Tératologie Vég.,’ p. 192.
[14] ‘Journal of Horticulture,’ July 2nd, 1861, p. 253.
[15] It would be worth trial to fertilise with the same pollen the
central and lateral flowers of the pelargonium, or of other highly
cultivated plants, protecting them of course from insects: then to sow
the seed separately, and observe whether the one or the other lot of
seedlings varied the most.
[16] Quoted in ‘Journal of Horticulture,’ Feb. 24th, 1863, p. 152.
[17] ‘Gardener’s Chronicle,’ 1866, p. 612. For the Phalænopsis, _see_
ibid., 1867, p. 211.
[18] ‘Mémoires . . . des Végétaux,’ 1837, tom. ii. p. 170.
[19] ‘Journal of Horticulture,’ July 23rd, 1861, p. 311.
[20] ‘Nouvelles Archives du Muséum,’ tom. i. p. 137.
[21] Hugo von Mohl, ‘The Vegetable Cell,’ Eng. translat., 1852, p. 76.
[22] The Rev. H. H. Dombrain, in ‘Journal of Horticulture,’ 1861, June
4th, p. 174; and June 25th, p. 234; 1862, April 29th, p. 83.
[23] ‘Transact. Linn. Soc.,’ vol. xxiii. 1861, p. 360.
[24] ‘Die Getreidearten,’ 1845, s. 208, 209.
[25] ‘Gardener’s Chronicle,’ 1850, p. 198.
[26] Quoted in ‘Gardener’s Chronicle,’ 1866, p. 74.
[27] ‘Ueber den Begriff der Pflanzenart,’ 1834, s. 14.
[28] ‘Domesticated Animals,’ 1845, p. 351.
[29] Bechstein, ‘Naturgeschichte Deutschlands,’ B. iv. 1795, s. 31.
[30] ‘Proc. Entomolog. Soc. of Philadelphia,’ Oct. 1863, p. 213.