LAWS OF VARIATION, continued.--SUMMARY.
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'affinite
de soi pour soi, which has been discussed and illustrated by his son, Isidore,
with respect to monsters in the animal kingdom (26/1. 'Hist. des Anomalies'
1832 tome 1 pages 22, 537-556; tome 3 page 462.), 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 (26/2. 'Comptes
Rendus' 1855 pages 855, 1039.), 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 (26/3. 'Catalogue
of the Teratological Series in the Museum of the R. Coll. of Surgeons' 1872
page 16.): "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 (26/4. 'Archives de Zoolog. Exper.' January 1874
page 78.) 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 a un meme oeuf; qu'ils
s'unissent en meme temps qu'ils se forment, et que la soudure ne se produit
que pendant la premiere periode de la vie embryonnaire, celle ou les organes
ne sont encore constitues que par des blastemes homogenes."
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 (26/5. 'Teratologie Veg.'
1841 livre 3.) 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 (26/6. 'Hist. des Anomalies' tome 3 pages 4, 5, 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 (26/7. 'Teratologie Veg.'
page 156. See also my book on 'The Movements and Habits of Climbing Plants'
2nd edition 1875 page 202.), 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 (26/8. 'Memoires du Museum' etc. tome 8 page 178.)
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 (26/9. Prichard 'Phys. Hist. of
Mankind' 1851 volume 1 page 324.) 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 (26/10. 'Annales des Sc. Nat.' 1st series
tome 19 page 327.) 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 (26/11. 'Comptes Rendus'
December 1864 page 1039.) 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. Muller
(26/12. "Ueber fotale Rachites" 'Wurzburger Medicin. Zeitschrift' 1860 b. 1 s.
265.) 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 (26/13. 'Teratologie Veg.' page 192.);
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 (26/14. 'Journal of Horticulture' July 2, 1861 page 253.) 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. (26/15. 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.)
Morren has described (26/16. Quoted in 'Journal of Horticulture' February 24,
1863 page 152.) 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 (26/17. 'Gardener's Chronicle' 1866 page 612. For the
Phalaenopsis see ibid 1867 page 211.) three similar peloric flowers, which all
occupied a central position on the flower-branches. In the Orchideous genus,
Phalaenopsis, 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 (26/18. 'Memoires...des
Vegetaux' 1837 tome 2 page 170.) 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 (26/19. 'Journal of Horticulture' July 23, 1861 page 311.),
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 (26/20. 'Nouvelles Archives du Museum' tome 1 page 137.), 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." (26/21. Hugo von Mohl 'The Vegetable
Cell' English translation 1852 page 76.) 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 (26/22. The Rev. H.H. Dombrain in 'Journal of
Horticulture' 1861 June 4 page 174; and June 25 page 234; 1862 April 29 page
83.) 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 (26/23. 'Transact. Linn. Soc.' volume 23 1861 page 360.)
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 (26/24. 'Die Getreidearten'
1845 s. 208, 209.) 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 (26/25. 'Gardener's Chronicle' 1850 page 198.) 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. Chate (26/26. Quoted in 'Gardener's Chronicle' 1866 page 74.) 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 (26/27.
'Ueber den Begriff der Pflanzenart' 1834 s. 14.), 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 (26/28. 'Domesticated Animals'
1845 page 351.) 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. (26/29.
Bechstein 'Naturgeschichte Deutschlands' b. 4 1795 s. 31.)
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
(26/30. 'Proc. Entomolog. Soc. of Philadelphia' October 1863 page 213.) 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.