vegetables which were then cultivated, with some excellent drawings made a
hundred and fifty years previously, was struck with surprise at the great
improvement which had been effected; and remarks that these ancient flowers
and vegetables would now be rejected, not only by a florist but by a
village gardener. Since the time of Buffon the work of improvement has
steadily and rapidly gone on. Every florist who compares our present
flowers with those figured in books published not long since, is astonished
at the change. A well-known amateur,[524] in speaking of the varieties of
Pelargonium raised by Mr. Garth only twenty-two years before, remarks,
"what a rage they excited: surely we had attained perfection, it was said;
and now not one of the flowers of those days will be looked at. But none
the less is the debt of gratitude which we owe to those who saw what was to
be done, and did it." Mr. Paul, the well-known horticulturist, in writing
of the same flower,[525] says he remembers when young being delighted with
the portraits in Sweet's work; "but what are they in point of beauty
compared with the Pelargoniums of this day? Here again nature did not
advance by leaps; the improvement was gradual, and, if we had neglected
those very gradual advances, we must have foregone the present grand
results." How well this practical horticulturist appreciates and
illustrates the gradual and accumulative force of selection! The Dahlia has
advanced in beauty in a like manner; the line of improvement being guided
by fashion, and by the successive modifications which the flower slowly
underwent.[526] A steady and gradual change has been noticed in many other
flowers: thus an old florist,[527] after describing the leading varieties
of the Pink which were grown in 1813, adds, "the pinks of those days would
now be scarcely grown as border-flowers." The improvement of {217} so many
flowers and the number of the varieties which have been raised is all the
more striking when we hear that the earliest known flower-garden in Europe,
namely at Padua, dates only from the year 1545.[528]
* * * * *
_Effects of Selection, as shown by the parts most valued by man presenting
the greatest amount of Difference._--The power of long-continued selection,
whether methodical or unconscious, or both combined, is well shown in a
general way, namely, by the comparison of the differences between the
varieties of distinct species, which are valued for different parts, such
as for the leaves, or stems, or tubers, the seed, or fruit, or flowers.
Whatever part man values most, that part will be found to present the
greatest amount of difference. With trees cultivated for their fruit,
Sageret remarks that the fruit is larger than in the parent-species, whilst
with those cultivated for the seed, as with nuts, walnuts, almonds,
chesnuts, &c., it is the seed itself which is larger; and he accounts for
this fact by the fruit in the one case, and by the seed in the other,
having been carefully attended to and selected during many ages. Gallesio
has made the same observation. Godron insists on the diversity of the tuber
in the potato, of the bulb in the onion, and of the fruit in the melon; and
on the close similarity in these same plants of the other parts.[529]
In order to judge how far my own impression on this subject was correct, I
cultivated numerous varieties of the same species close to each other. The
comparison of the amount of difference between widely different organs is
necessarily vague; I will therefore give the results in only a few cases.
We have previously seen in the ninth chapter how greatly the varieties of
the cabbage differ in their foliage and stems, which are the selected
parts, and how closely they resembled each other in their flowers,
capsules, and seeds. In seven varieties of the radish, the roots differed
greatly in colour and shape, but no difference {218} whatever could be
detected in their foliage, flowers, or seeds. Now what a contrast is
presented, if we compare the flowers of the varieties of these two plants
with those of any species cultivated in our flower-gardens for ornament; or
if we compare their seeds with those of the varieties of maize, peas,
beans, &c., which are valued and cultivated for their seeds. In the ninth
chapter it was shown that the varieties of the pea differ but little except
in the tallness of the plant, moderately in the shape of the pod, and
greatly in the pea itself, and these are all selected points. The
varieties, however, of the _Pois sans parchemin_ differ much more in their
pods, and these are eaten and valued. I cultivated twelve varieties of the
common bean; one alone, the Dwarf Fan, differed considerably in general
appearance; two differed in the colour of their flowers, one being an
albino, and the other being wholly instead of partially purple; several
differed considerably in the shape and size of the pod, but far more in the
bean itself, and this is the valued and selected part. Toker's bean, for
instance, is twice-and-a-half as long and broad as the horse-bean, and is
much thinner and of a different shape.
The varieties of the gooseberry, as formerly described, differ much in
their fruit, but hardly perceptibly in their flowers or organs of
vegetation. With the plum, the differences likewise appear to be greater in
the fruit than in the flowers or leaves. On the other hand, the seed of the
strawberry, which corresponds with the fruit of the plum, differs hardly at
all; whilst every one knows how greatly the fruit--that is, the enlarged
receptacle--differs in the several varieties. In apples, pears, and peaches
the flowers and leaves differ considerably, but not, as far as I can judge,
in proportion with the fruit. The Chinese double-flowering peaches, on the
other hand, show that varieties of this tree have been formed, which differ
more in the flower than in fruit. If, as is highly probable, the peach is
the modified descendant of the almond, a surprising amount of change has
been effected in the same species, in the fleshy covering of the former and
in the kernels of the latter.
When parts stand in such close relation to each other as the fleshy
covering of the fruit (whatever its homological nature may be) and the
seed, when one part is modified, so generally is the other, but by no means
necessarily in the same degree. With {219} the plum-tree, for instance,
some varieties produce plums which are nearly alike, but include stones
extremely dissimilar in shape; whilst conversely other varieties produce
dissimilar fruit with barely distinguishable stones; and generally the
stones, though they have never been subjected to selection, differ greatly
in the several varieties of the plum. In other cases organs which are not
manifestly related, through some unknown bond vary together, and are
consequently liable, without any intention on man's part, to be
simultaneously acted on by selection. Thus the varieties of the stock
(Matthiola) have been selected solely for the beauty of their flowers, but
the seeds differ greatly in colour and somewhat in size. Varieties of the
lettuce have been selected solely on account of their leaves, yet produce
seeds which likewise differ in colour. Generally, through the law of
correlation, when a variety differs greatly from its fellow-varieties in
any one character, it differs to a certain extent in several other
characters. I observed this fact when I cultivated together many varieties
of the same species, for I used first to make a list of the varieties which
differed most from each other in their foliage and manner of growth,
afterwards of those that differed most in their flowers, then in their
seed-capsules, and lastly in their mature seed; and I found that the same
names generally occurred in two, three, or four of the successive lists.
Nevertheless the greatest amount of difference between the varieties was
always exhibited, as far as I could judge, by that part or organ for which
the plant was cultivated.
When we bear in mind that each plant was at first cultivated because useful
to man, and that its variation was a subsequent, often a long subsequent,
event, we cannot explain the greater amount of diversity in the valuable
parts by supposing that species endowed with an especial tendency to vary
in any particular manner, were originally chosen. We must attribute the
result to the variations in these parts having been successively preserved,
and thus continually augmented; whilst other variations, excepting such as
inevitably appeared through correlation, were neglected and lost. Hence we
may infer that most plants might be made, through long-continued selection,
to yield races as different from each other in any character {220} as they
now are in those parts for which they are valued and cultivated.
With animals we see something of the same kind; but they have not been
domesticated in sufficient number or yielded sufficient varieties for a
fair comparison. Sheep are valued for their wool, and the wool differs much
more in the several races than the hair in cattle. Neither sheep, goats,
European cattle, nor pigs are valued for their fleetness or strength; and
we do not possess breeds differing in these respects like the race-horse
and dray-horse. But fleetness and strength are valued in camels and dogs;
and we have with the former the swift dromedary and heavy camel; with the
latter the greyhound and mastiff. But dogs are valued even in a higher
degree for their mental qualities and senses; and every one knows how
greatly the races differ in these respects. On the other hand, where the
dog is valued solely to serve for food, as in the Polynesian islands and
China, it is described as an extremely stupid animal.[530] Blumenbach
remarks that "many dogs, such as the badger-dog, have a build so marked and
so appropriate for particular purposes, that I should find it very
difficult to persuade myself that this astonishing figure was an accidental
consequence of degeneration."[531] But had Blumenbach reflected on the
great principle of selection, he would not have used the term degeneration,
and he would not have been astonished that dogs and other animals should
become excellently adapted for the service of man.
On the whole we may conclude that whatever part or character is most
valued--whether the leaves, stems, tubers, bulbs, flowers, fruit, or seed
of plants, or the size, strength, fleetness, hairy covering, or intellect
of animals--that character will almost invariably be found to present the
greatest amount of difference both in kind and degree. And this result may
be safely attributed to man having preserved during a long course of
generations the variations which were useful to him, and neglected the
others.
I will conclude this chapter by some remarks on an important subject. With
animals such as the giraffe, of which {221} the whole structure is
admirably co-ordinated for certain purposes, it has been supposed that all
the parts must have been simultaneously modified; and it has been argued
that, on the principle of natural selection, this is scarcely possible. But
in thus arguing, it has been tacitly assumed that the variations must have
been abrupt and great. No doubt, if the neck of a ruminant were suddenly to
become greatly elongated, the fore limbs and back would have to be
simultaneously strengthened and modified; but it cannot be denied that an
animal might have its neck, or head, or tongue, or fore-limbs elongated a
very little without any corresponding modification in other parts of the
body; and animals thus slightly modified would, during a dearth, have a
slight advantage, and be enabled to browse on higher twigs, and thus
survive. A few mouthfuls more or less every day would make all the
difference between life and death. By the repetition of the same process,
and by the occasional intercrossing of the survivors, there would be some
progress, slow and fluctuating though it would be, towards the admirably
co-ordinated structure of the giraffe. If the short-faced tumbler-pigeon,
with its small conical beak, globular head, rounded body, short wings, and
small feet--characters which appear all in harmony--had been a natural
species, its whole structure would have been viewed as well fitted for its
life; but in this case we know that inexperienced breeders are urged to
attend to point after point, and not to attempt improving the whole
structure at the same time. Look at the greyhound, that perfect image of
grace, symmetry, and vigour; no natural species can boast of a more
admirably co-ordinated structure, with its tapering head, slim body, deep
chest, tucked-up abdomen, rat-like tail, and long muscular limbs, all
adapted for extreme fleetness, and for running down weak prey. Now, from
what we see of the variability of animals, and from what we know of the
method which different men follow in improving their stock--some chiefly
attending to one point, others to another point, others again correcting
defects by crosses, and so forth--we may feel assured that if we could see
the long line of ancestors of a first-rate greyhound, up to its wild
wolf-like progenitor, we should behold an infinite number of the finest
gradations, sometimes in one character and sometimes in another, but all
leading towards our {222} present perfect type. By small and doubtful steps
such as these, nature, as we may confidently believe, has progressed on her
grand march of improvement and development.
A similar line of reasoning is as applicable to separate organs as to the
whole organisation. A writer[532] has recently maintained that "it is
probably no exaggeration to suppose that, in order to improve such an organ
as the eye at all, it must be improved in ten different ways at once. And
the improbability of any complex organ being produced and brought to
perfection in any such way is an improbability of the same kind and degree
as that of producing a poem or a mathematical demonstration by throwing
letters at random on a table." If the eye were abruptly and greatly
modified, no doubt many parts would have to be simultaneously altered, in
order that the organ should remain serviceable.
But is this the case with smaller changes? There are persons who can see
distinctly only in a dull light, and this condition depends, I believe, on
the abnormal sensitiveness of the retina, and is known to be inherited.
Now, if a bird, for instance, received some great advantage from seeing
well in the twilight, all the individuals with the most sensitive retina
would succeed best and be the most likely to survive; and why should not
all those which happened to have the eye itself a little larger, or the
pupil capable of greater dilatation, be likewise preserved, whether or not
these modifications were strictly simultaneous? These individuals would
subsequently intercross and blend their respective advantages. By such
slight successive changes, the eye of a diurnal bird would be brought into
the condition of that of an owl, which has often been advanced as an
excellent instance of adaptation. Short-sight, which is often inherited,
permits a person to see distinctly a minute object at so near a distance
that it would be indistinct to ordinary eyes; and here we have a capacity
which might be serviceable under certain conditions, abruptly gained. The
Fuegians on board the {223} Beagle could certainly see distant objects more
distinctly than our sailors with all their long practice; I do not know
whether this depends on nervous sensitiveness or on the power of adjustment
in the focus; but this capacity for distant vision might, it is probable,
be slightly augmented by successive modifications of either kind.
Amphibious animals, which are enabled to see both in the water and in the
air, require and possess, as M. Plateau has shown,[533] eyes constructed on
the following plan: "the cornea is always flat, or at least much flattened
in front of the crystalline and over a space equal to the diameter of that
lens, whilst the lateral portions may be much curved." The crystalline is
very nearly a sphere, and the humours have nearly the same density as
water. Now, as a terrestrial animal slowly became more and more aquatic in
its habits, very slight changes, first in the curvature of the cornea or
crystalline, and then in the density of the humours, or conversely, might
successively occur, and would be advantageous to the animal whilst under
water, without serious detriment to its power of vision in the air. It is
of course impossible to conjecture by what steps the fundamental structure
of the eye in the Vertebrata was originally acquired, for we know
absolutely nothing about this organ in the first progenitors of the class.
With respect to the lowest animals in the scale, the transitional states
through which the eye at first probably passed, can by the aid of analogy
be indicated, as I have attempted to show in my 'Origin of Species.'[534]
* * * * *
{224}
CHAPTER XXI.
SELECTION, _continued_.
NATURAL SELECTION AS AFFECTING DOMESTIC PRODUCTIONS--CHARACTERS WHICH
APPEAR OF TRIFLING VALUE OFTEN OF REAL IMPORTANCE--CIRCUMSTANCES
FAVOURABLE TO SELECTION BY MAN--FACILITY IN PREVENTING CROSSES, AND THE
NATURE OF THE CONDITIONS--CLOSE ATTENTION AND PERSEVERANCE
INDISPENSABLE--THE PRODUCTION OF A LARGE NUMBER OF INDIVIDUALS
ESPECIALLY FAVOURABLE--WHEN NO SELECTION IS APPLIED, DISTINCT RACES ARE
NOT FORMED--HIGHLY-BRED ANIMALS LIABLE TO DEGENERATION--TENDENCY IN MAN
TO CARRY THE SELECTION OF EACH CHARACTER TO AN EXTREME POINT, LEADING
TO DIVERGENCE OF CHARACTER, RARELY TO CONVERGENCE--CHARACTERS
CONTINUING TO VARY IN THE SAME DIRECTION IN WHICH THEY HAVE ALREADY
VARIED--DIVERGENCE OF CHARACTER, WITH THE EXTINCTION OF INTERMEDIATE
VARIETIES, LEADS TO DISTINCTNESS IN OUR DOMESTIC RACES--LIMIT TO THE
POWER OF SELECTION--LAPSE OF TIME IMPORTANT--MANNER IN WHICH DOMESTIC
RACES HAVE ORIGINATED--SUMMARY.
_Natural Selection, or the Survival of the Fittest, as affecting domestic
productions._--We know little on this head. But as animals kept by savages
have to provide their own food, either entirely or to a large extent,
throughout the year, it can hardly be doubted that, in different countries,
varieties differing in constitution and in various characters would succeed
best, and so be naturally selected. Hence perhaps it is that the few
domesticated animals kept by savages partake, as has been remarked by more
than one writer, of the wild appearance of their masters, and likewise
resemble natural species. Even in long-civilised countries, at least in the
wilder parts, natural selection must act on our domestic races. It is
obvious that varieties, having very different habits, constitution, and
structure, would succeed best on mountains and on rich lowland pastures.
For example, the improved Leicester sheep were formerly taken to the
Lammermuir Hills; but an intelligent sheep-master reported that "our coarse
lean pastures were unequal to the task of supporting such heavy-bodied
sheep; and they gradually dwindled away into less and less bulk: {225} each
generation was inferior to the preceding one; and when the spring was
severe, seldom more than two-thirds of the lambs survived the ravages of
the storms."[535] So with the mountain cattle of North Wales and the
Hebrides, it has been found that they could not withstand being crossed
with the larger and more delicate lowland breeds. Two French naturalists,
in describing the horses of Circassia, remark that, subjected as they are
to extreme vicissitudes of climate, having to search for scanty pasture,
and exposed to constant danger from wolves, the strongest and most vigorous
alone survive.[536]
Every one must have been struck with the surpassing grace, strength, and
vigour of the Game-cock, with its bold and confident air, its long, yet
firm neck, compact body, powerful and closely pressed wings, muscular
thighs, strong beak massive at the base, dense and sharp spurs set low on
the legs for delivering the fatal blow, and its compact, glossy, and
mail-like plumage serving as a defence. Now the English game-cock has not
only been improved during many years by man's careful selection, but in
addition, as Mr. Tegetmeier has remarked,[537] by a kind of natural
selection, for the strongest, most active and courageous birds have
stricken down their antagonists in the cockpit, generation after
generation, and have subsequently served as the progenitors of their kind.
In Great Britain, in former times, almost every district had its own breed
of cattle and sheep; "they were indigenous to the soil, climate, and
pasturage of the locality on which they grazed: they seemed to have been
formed for it and by it."[538] But in this case we are quite unable to
disentangle the effects of the direct action of the conditions of life,--of
use or habit--of natural selection--and of that kind of selection which we
have seen is occasionally and unconsciously followed by man even during the
rudest periods of history.
Let us now look to the action of natural selection on special characters.
Although nature is difficult to resist, yet man often strives against her
power, and sometimes, as we shall see, with {226} success. From the facts
to be given, it will also be seen that natural selection would powerfully
affect many of our domestic productions if left unprotected. This is a
point of much interest, for we thus learn that differences apparently of
very slight importance would certainly determine the survival of a form
when forced to struggle for its own existence. It may have occurred to some
naturalists, as it formerly did to me, that, though selection acting under
natural conditions would determine the structure of all important organs,
yet that it could not affect characters which are esteemed by us of little
importance; but this is an error to which we are eminently liable, from our
ignorance of what characters are of real value to each living creature.
When man attempts to breed an animal with some serious defect in structure,
or in the mutual relation of parts, he will either partially or completely
fail, or encounter much difficulty; and this is in fact a form of natural
selection. We have seen that the attempt was once made in Yorkshire to
breed cattle with enormous buttocks, but the cows perished so often in
bringing forth their calves, that the attempt had to be given up. In
rearing short-faced tumblers, Mr. Eaton says,[539] "I am convinced that
better head and beak birds have perished in the shell than ever were
hatched; the reason being that the amazingly short-faced bird cannot reach
and break the shell with its beak, and so perishes." Here is a more curious
case, in which natural selection comes into play only at long intervals of
time: during ordinary seasons the Niata cattle can graze as well as others,
but occasionally, as from 1827 to 1830, the plains of La Plata suffer from
long-continued droughts and the pasture is burnt up; at such times common
cattle and horses perish by the thousand, but many survive by browsing on
twigs, reeds, &c.; this the Niata cattle cannot so well effect from their
upturned jaws and the shape of their lips; consequently, if not attended
to, they perish before the other cattle. In Colombia, according to Roulin,
there is a breed of nearly hairless cattle, called Pelones; these succeed
in their native hot district, but are found too tender for the Cordillera;
in this case, natural selection {227} determines only the range of the
variety. It is obvious that a host of artificial races could never survive
in a state of nature;--such as Italian greyhounds,--hairless and almost
toothless Turkish dogs,--fantail pigeons, which cannot fly well against a
strong wind,--barbs with their vision impeded by their eye-wattle,--Polish
fowls with their vision impeded by their great topknots,--hornless bulls
and rams which consequently cannot cope with other males, and thus have a
poor chance of leaving offspring,--seedless plants, and many other such
cases.
Colour is generally esteemed by the systematic naturalist as unimportant:
let us, therefore, see how far it indirectly affects our domestic
productions, and how far it would affect them if they were left exposed to
the full force of natural selection. In a future chapter I shall have to
show that constitutional peculiarities of the strangest kind, entailing
liability to the action of certain poisons, are correlated with the colour
of the skin. I will here give a single case, on the high authority of
Professor Wyman; he informs me that, being surprised at all the pigs in a
part of Virginia being black, he made inquiries, and ascertained that these
animals feed on the roots of the _Lachnanthes tinctoria_, which colours
their bones pink, and, excepting in the case of the black varieties, causes
the hoofs to drop off. Hence, as one of the squatters remarked, "we select
the black members of the litter for raising, as they alone have a good
chance of living." So that here we have artificial and natural selection
working hand in hand. I may add that in the Tarentino the inhabitants keep
black sheep alone, because the _Hypericum crispum_ abounds there; and this
plant does not injure black sheep, but kills the white ones in about a
fortnight's time.[540]
Complexion, and liability to certain diseases, are believed to run together
in man and the lower animals. Thus white terriers suffer more than terriers
of any other colour from the fatal Distemper.[541] In North America
plum-trees are liable to a disease which Downing[542] believes is not
caused by insects; the kinds bearing purple fruit are most affected, "and
we have never known the green or yellow fruited varieties infected {228}
until the other sorts had first become filled with the knots." On the other
hand, peaches in North America suffer much from a disease called the
_yellows_, which seems to be peculiar to that continent, and "more than
nine-tenths of the victims, when the disease first appeared, were the
yellow-fleshed peaches. The white-fleshed kinds are much more rarely
attacked; in some parts of the country never." In Mauritius, the white
sugar-canes have of late years been so severely attacked by a disease, that
many planters have been compelled to give up growing this variety (although
fresh plants were imported from China for trial), and cultivate only red
canes.[543] Now, if these plants had been forced to struggle with other
competing plants and enemies, there cannot be a doubt that the colour of
the flesh or skin of the fruit, unimportant as these characters are
considered, would have rigorously determined their existence.
Liability to the attacks of parasites is also connected with colour. It
appears that white chickens are certainly more subject than dark-coloured
chickens to the _gapes_, which is caused by a parasitic worm in the
trachea.[544] On the other hand, experience has shown that in France the
caterpillars which produce white cocoons resist the deadly fungus better
than those producing yellow cocoons.[545] Analogous facts have been
observed with plants: a new and beautiful white onion, imported from
France, though planted close to other kinds, was alone attacked by a
parasitic fungus.[546] White verbenas are especially liable to mildew.[547]
Near Malaga, during an early period of the vine-disease, the green sorts
suffered most; "and red and black grapes, even when interwoven with the
sick plants, suffered not at all." In France whole groups of varieties were
comparatively free, and others, such as the Chasselas, did not afford a
single fortunate exception; but I do not know whether any correlation
between colour and liability to disease was here observed.[548] In a former
chapter it was shown how curiously liable one variety of the strawberry is
to mildew.
{229}
It is certain that insects regulate in many cases the range and even the
existence of the higher animals, whilst living under their natural
conditions. Under domestication light-coloured animals suffer most: in
Thuringia[549] the inhabitants do not like grey, white, or pale cattle,
because they are much more troubled by various kinds of flies than the
brown, red, or black cattle. An Albino negro, it has been remarked,[550]
was peculiarly sensitive to the bites of insects. In the West Indies[551]
it is said that "the only horned cattle fit for work are those which have a
good deal of black in them. The white are terribly tormented by the
insects; and they are weak and sluggish in proportion to the white."
In Devonshire there is a prejudice against white pigs, because it is
believed that the sun blisters them when turned out;[552] and I knew a man
who would not keep white pigs in Kent, for the same reason. The scorching
of flowers by the sun seems likewise to depend much on colour; thus, dark
pelargoniums suffer most; and from various accounts it is clear that the
cloth-of-gold variety will not withstand a degree of exposure to sunshine
which other varieties enjoy. Another amateur asserts that not only all
dark-coloured verbenas, but likewise scarlets, suffer from the sun; "the
paler kinds stand better, and pale blue is perhaps the best of all." So
again with the heartsease (_Viola tricolor_); hot weather suits the
blotched sorts, whilst it destroys the beautiful markings of some other
kinds.[553] During one extremely cold season in Holland all red-flowered
hyacinths were observed to be very inferior in quality. It is believed by
many agriculturists that red wheat is hardier in northern climates than
white wheat.[554]
With animals, white varieties from being conspicuous are the most liable to
be attacked by beasts and birds of prey. In parts of France and Germany
where hawks abound, persons are advised not to keep white pigeons; for, as
Parmentier says, "it {230} is certain that in a flock the white always
first fall victims to the kite." In Belgium, where so many societies have
been established for the flight of carrier-pigeons, white is the one colour
which for the same reason is disliked.[555] On the other hand, it is said
that the sea-eagle (_Falco ossifragus_, Linn.) on the west coast of Ireland
picks out the black fowls, so that "the villagers avoid as much as possible
rearing birds of that colour." M. Daudin,[556] speaking of white rabbits
kept in warrens in Russia, remarks that their colour is a great
disadvantage, as they are thus more exposed to attack, and can be seen
during bright nights from a distance. A gentleman in Kent, who failed to
stock his woods with a nearly white and hardy kind of rabbit, accounted in
the same manner for their early disappearance. Any one who will watch a
white cat prowling after her prey will soon perceive under what a
disadvantage she lies.
The white Tartarian cherry, "owing either to its colour being so much like
that of the leaves, or to the fruit always appearing from a distance
unripe," is not so readily attacked by birds as other sorts. The
yellow-fruited raspberry, which generally comes nearly true by seed, "is
very little molested by birds, who evidently are not fond of it; so that
nets may be dispensed with in places where nothing else will protect the
red fruit."[557] This immunity, though a benefit to the gardener, would be
a disadvantage in a state of nature both to the cherry and raspberry, as
their dissemination depends on birds. I noticed during several winters that
some trees of the yellow-berried holly, which were raised from seed from a
wild tree found by my father, remained covered with fruit, whilst not a
scarlet berry could be seen on the adjoining trees of the common kind. A
friend informs me that a mountain-ash (_Pyrus aucuparia_) growing in his
garden bears berries which, though not differently coloured, are always
devoured by birds before those on the other trees. This variety of the
mountain-ash would thus be more freely disseminated, and the yellow-berried
variety of the holly less freely, than the common varieties of these two
trees.
{231}
Independently of colour, other trifling differences are sometimes found to
be of importance to plants under cultivation, and would be of paramount
importance if they had to fight their own battle and to struggle with many
competitors. The thin-shelled peas, called _pois sans parchemin_, are
attacked by birds[558] much more than common peas. On the other hand, the
purple-podded pea, which has a hard shell, escaped the attacks of tomtits
(_Parus major_) in my garden far better than any other kind. The
thin-shelled walnut likewise suffers greatly from the tomtit.[559] These
same birds have been observed to pass over and thus favour the filbert,
destroying only the other kinds of nuts which grew in the same
orchard.[560]
Certain varieties of the pear have soft bark, and these suffer severely
from boring wood-beetles; whilst other varieties are known to resist their
attacks much better.[561] In North America the smoothness, or absence of
down on the fruit, makes a great difference in the attacks of the weevil,
"which is the uncompromising foe of all smooth stone-fruits;" and the
cultivator "has the frequent mortification of seeing nearly all, or indeed
often the whole crop, fall from the trees when half or two-thirds grown."
Hence the nectarine suffers more than the peach. A particular variety of
the Morello cherry, raised in North America, is without any assignable
cause more liable to be injured by this same insect than other
cherry-trees.[562] From some unknown cause, the Winter Majetin apple enjoys
the great advantage of not being infested by the coccus. On the other hand,
a particular case has been recorded in which aphides confined themselves to
the Winter Nelis pear, and touched no other kind in an extensive
orchard.[563] The existence of minute glands on the leaves of peaches,
nectarines, and apricots, would not be esteemed by botanists as a character
of the least importance, for they are present or absent in closely related
sub-varieties, descended from the same parent-tree; yet there is good
evidence[564] that the {232} absence of glands leads to mildew, which is
highly injurious to these trees.
A difference either in flavour or in the amount of nutriment in certain
varieties causes them to be more eagerly attacked by various enemies than
other varieties of the same species. Bullfinches (_Pyrrhula vulgaris_)
injure our fruit-trees by devouring the flower-buds, and a pair of these
birds have been seen "to denude a large plum-tree in a couple of days of
almost every bud;" but certain varieties[565] of the apple and thorn
(_Crataegus oxyacantha_) are more especially liable to be attacked. A
striking instance of this was observed in Mr. Rivers's garden, in which two
rows of a particular variety of plum[566] had to be carefully protected, as
they were usually stripped of all their buds during the winter, whilst
other sorts growing near them escaped. The root (or enlarged stem) of
Laing's Swedish turnip is preferred by hares, and therefore suffers more
than other varieties. Hares and rabbits eat down common rye before St.
John's-day-rye, when both grow together.[567] In the South of France, when
an orchard of almond-trees is formed, the nuts of the bitter variety are
sown, "in order that they may not be devoured by field-mice;"[568] so we
see the use of the bitter principle in almonds.
Other slight differences, which would be thought quite unimportant, are no
doubt sometimes of great service both to plants and animals. The
Whitesmith's gooseberry, as formerly stated, produces its leaves later than
other varieties, and, as the flowers are thus left unprotected, the fruit
often fails. In one variety of the cherry, according to Mr. Rivers,[569]
the petals are much curled backwards, and in consequence of this the
stigmas were observed to be killed by a severe frost; whilst at the same
time, in another variety with incurved petals, the stigmas were not in the
least injured. The straw of the Fenton wheat is remarkably unequal in
height; and a competent observer believes that this variety is highly
productive, partly because the ears, from being distributed at various
heights above the ground, {233} are less crowded together. The same
observer maintains that in the upright varieties the divergent awns are
serviceable by breaking the shocks when the ears are dashed together by the
wind.[570] If several varieties of a plant are grown together, and the seed
is indiscriminately harvested, it is clear that the hardier and more
productive kinds will, by a sort of natural selection, gradually prevail
over the others; this takes place, as Colonel Le Couteur believes,[571] in
our wheat-fields, for, as formerly shown, no variety is quite uniform in
character. The same thing, as I am assured by nurserymen, would take place
in our flower-gardens, if the seed of the different varieties were not
separately saved. When the eggs of the wild and tame duck are hatched
together, the young wild ducks almost invariably perish, from being of
smaller size and not getting their fair share of food.[572]
Facts in sufficient number have now been given showing that natural
selection often checks, but occasionally favours, man's power of selection.
These facts teach us, in addition, a valuable lesson, namely, that we ought
to be extremely cautious in judging what characters are of importance in a
state of nature to animals and plants, which have to struggle from the hour
of their birth to that of their death for existence,--their existence
depending on conditions, about which we are profoundly ignorant.
_Circumstances favourable to Selection by Man._
The possibility of selection rests on variability, and this, as we shall
see in the following chapters, mainly depends on changed conditions of
life, but is governed by infinitely complex, and, to a great extent,
unknown laws. Domestication, even when long continued, occasionally causes
but a small amount of variability, as in the case of the goose and turkey.
The slight differences, however, which characterise each individual animal
and plant would in most, probably in all cases, suffice for the production
of distinct races through careful and prolonged selection. We see what
selection, though acting on mere individual differences, can effect when
families of cattle, sheep, {234} pigeons, &c., of the same race, have been
separately bred during a number of years by different men without any wish
on their part to modify the breed. We see the same fact in the difference
between hounds bred for hunting in different districts,[573] and in many
other such cases.
In order that selection should produce any result, it is manifest that the
crossing of distinct races must be prevented; hence facility in pairing, as
with the pigeon, is highly favourable for the work; and difficulty in
pairing, as with cats, prevents the formation of distinct breeds. On nearly
the same principle the cattle of the small island of Jersey have been
improved in their milking qualities "with a rapidity that could not have
been obtained in a widely extended country like France."[574] Although free
crossing is a danger on the one side which every one can see, too close
interbreeding is a hidden danger on the other side. Unfavourable conditions
of life overrule the power of selection. Our improved heavy breeds of
cattle and sheep could not have been formed on mountainous pastures; nor
could dray-horses have been raised on a barren and inhospitable land, such
as the Falkland islands, where even the light horses of La Plata rapidly
decrease in size. Nor could the wool of sheep have been much increased in
length within the Tropics; yet selection has kept Merino sheep nearly true
under diversified and unfavourable conditions of life. The power of
selection is so great, that breeds of the dog, sheep, and poultry, of the
largest and least size, long and short beaked pigeons, and other breeds
with opposite characters, have had their characteristic qualities
augmented, though treated in every way alike, being exposed to the same
climate and fed on the same food. Selection, however, is either checked or
favoured by the effects of use or habit. Our wonderfully-improved pigs
could never have been formed if they had been forced to search for their
own food; the English racehorse and greyhound could not have been improved
up to their present high standard of excellence without constant training.
As conspicuous deviations of structure occur rarely, the improvement of
each breed is generally the result, as already {235} remarked, of the
selection of slight individual differences. Hence the closest attention,
the sharpest powers of observation, and indomitable perseverance, are
indispensable. It is, also, highly important that many individuals of the
breed which is to be improved should be raised; for thus there will be a
better chance of the appearance of variations in the right direction, and
individuals varying in an unfavourable manner may be freely rejected or
destroyed. But that a large number of individuals should be raised, it is
necessary that the conditions of life should favour the propagation of the
species. Had the peacock been bred as easily as the fowl, we should
probably ere this have had many distinct races. We see the importance of a
large number of plants, from the fact of nursery gardeners almost always
beating amateurs in the exhibition of new varieties. In 1845 it was
estimated[575] that between 4000 and 5000 pelargoniums were annually raised
from seed in England, yet a decidedly improved variety is rarely obtained.
At Messrs. Carter's grounds, in Essex, where such flowers as the Lobelia,
Nemophila, Mignonette, &c., are grown by the acre for seed, "scarcely a
season passes without some new kinds being raised, or some improvement
affected on old kinds."[576] At Kew, as Mr. Beaton remarks, where many
seedlings of common plants are raised, "you see new forms of Laburnums,
Spiraeas, and other shrubs."[577] So with animals: Marshall,[578] in
speaking of the sheep in one part of Yorkshire, remarks, "as they belong to
poor people, and are mostly in small lots, they never can be improved."
Lord Rivers, when asked how he succeeded in always having first-rate
greyhounds, answered, "I breed many, and hang many." This, as another man
remarks, "was the secret of his success; and the same will be found in
exhibiting fowls,--successful competitors breed largely, and keep the
best."[579]
It follows from this that the capacity of breeding at an early age and at
short successive intervals, as with pigeons, rabbits, &c., facilitates
selection; for the result is thus soon made visible, and perseverance in
the work is encouraged. It can hardly be {236} accidental that the great
majority of the culinary and agricultural plants which have yielded
numerous races are annuals or biennials, which therefore are capable of
rapid propagation and thus of improvement. Sea-kale, asparagus, common and
Jerusalem artichokes, potatoes, and onions, alone are perennials. Onions
are propagated like annuals, and of the other plants just specified, none,
with the exception of the potato, have yielded more than one or two
varieties. No doubt fruit-trees, which cannot be propagated quickly by
seed, have yielded a host of varieties, though not permanent races; but
these, judging from pre-historic remains, were produced at a later and more
civilised epoch than the races of culinary and agricultural plants.
A species may be highly variable, but distinct races will not be formed, if
from any cause selection be not applied. The carp is highly variable, but
it would be extremely difficult to select slight variations in fishes
whilst living in their natural state, and distinct races have not been
formed;[580] on the other hand, a closely allied species, the gold-fish,
from being reared in glass or open vessels, and from having been carefully
attended to by the Chinese, has yielded many races. Neither the bee, which
has been semi-domesticated from an extremely remote period, nor the
cochineal insect, which was cultivated by the aboriginal Mexicans, has
yielded races; and it would be impossible to match the queen-bee with any
particular drone, and most difficult to match cochineal insects.
Silk-moths, on the other hand, have been subjected to rigorous selection,
and have produced a host of races. Cats, which from their nocturnal habits
cannot be selected for breeding, do not, as formerly remarked, yield
distinct races in the same country. The ass in England varies much in
colour and size; but it is an animal of little value, bred by poor people;
consequently there has been no selection, and distinct races have not been
formed. We must not attribute the inferiority of our asses to climate, for
in India they are of even smaller size than in Europe. But when selection
is brought to bear on the ass, all is changed. Near Cordova, as I am
informed (Feb. 1860) by Mr. W. E. Webb, C.E., they are carefully bred, as
much as 200l. having been paid for a stallion ass, {237} and they have been
immensely improved. In Kentucky, asses have been imported (for breeding
mules) from Spain, Malta, and France; these "seldom averaged more than
fourteen hands high; but the Kentuckians, by great care, have raised them
up to fifteen hands, and sometimes even to sixteen. The prices paid for
these splendid animals, for such they really are, will prove how much they
are in request. One male, of great celebrity, was sold for upwards of one
thousand pounds sterling." These choice asses are sent to cattle-shows, one
day being given to their exhibition.[581]
Analogous facts have been observed with plants: the nutmeg-tree in the
Malay archipelago is highly variable, but there has been no selection, and
there are no distinct races.[582] The common mignonette (_Reseda odorata_),
from bearing inconspicuous flowers, valued solely for their fragrance,
"remains in the same unimproved condition as when first introduced."[583]
Our common forest-trees are very variable, as may be seen in every
extensive nursery-ground; but as they are not valued like fruit-trees, and
as they seed late in life, no selection has been applied to them;
consequently, as Mr. Patrick Matthews remarks,[584] they have not yielded
distinct races, leafing at different periods, growing to different sizes,
and producing timber fit for different purposes. We have gained only some
fanciful and semi-monstrous varieties, which no doubt appeared suddenly as
we now see them.
Some botanists have argued that plants cannot have so strong a tendency to
vary as is generally supposed, because many species long grown in botanic
gardens, or unintentionally cultivated year after year mingled with our
corn crops, have not produced distinct races; but this is accounted for by
slight variations not having been selected and propagated. Let a plant
which is now grown in a botanic garden, or any common weed, be cultivated
on a large scale, and let a sharp-sighted gardener look out for each slight
variety and sow the seed, and then, if distinct races are not produced, the
argument will be valid.
{238}
The importance of selection is likewise shown by considering special
characters. For instance, with most breeds of fowls the form of the comb
and the colour of the plumage have been attended to, and are eminently
characteristic of each race; but in Dorkings, fashion has never demanded
uniformity of comb or colour; and the utmost diversity in these respects
prevails. Rose-combs, double-combs, cup-combs, &c., and colours of all
kinds, may be seen in purely-bred and closely related Dorking fowls, whilst
other points, such as the general form of body, and the presence of an
additional toe, have been attended to, and are invariably present. It has
also been ascertained that colour can be fixed in this breed, as well as in
any other.[585]
* * * * *
During the formation or improvement of a breed, its members will always be
found to vary much in those characters to which especial attention is
directed, and of which each slight improvement is eagerly sought and
selected. Thus with short-faced tumbler-pigeons, the shortness of the beak,
shape of head and plumage,--with carriers, the length of the beak and
wattle,--with fantails, the tail and carriage,--with Spanish fowls, the
white face and comb,--with long-eared rabbits, the length of ear, are all
points which are eminently variable. So it is in every case, and the large
price paid for first-rate animals proves the difficulty of breeding them up
to the highest standard of excellence. This subject has been discussed by
fanciers,[586] and the greater prizes given for highly improved breeds, in
comparison with those given for old breeds which are not now undergoing
rapid improvement, has been fully justified. Nathusius makes[587] a similar
remark when discussing the less uniform character of improved Shorthorn
cattle and of the English horse, in comparison, for example, with the
unennobled cattle of Hungary, or with the horses of the Asiatic steppes.
This want of uniformity in the parts which at the time are undergoing
selection, chiefly depends on the strength of the principle of reversion
but it likewise depends to a certain extent on the continued {239}
variability of the parts which have recently varied. That the same parts do
continue varying in the same manner we must admit, for, if it were not so,
there could be no improvement beyond an early standard of excellence, and
we know that such improvement is not only possible, but is of general
occurrence.
As a consequence of continued variability, and more especially of
reversion, all highly improved races, if neglected or not subjected to
incessant selection, soon degenerate. Youatt gives a curious instance of
this in some cattle formerly kept in Glamorganshire; but in this case the
cattle were not fed with sufficient care. Mr. Baker, in his memoir on the
Horse, sums up: "It must have been observed in the preceding pages that,
whenever there has been neglect, the breed has proportionally
deteriorated."[588] If a considerable number of improved cattle, sheep, or
other animals of the same race, were allowed to breed freely together, with
no selection, but with no change in their condition of life, there can be
no doubt that after a score or hundred generations they would be very far
from excellent of their kind; but, from what we see of the many common
races of dogs, cattle, fowls, pigeons, &c., which without any particular
care have long retained nearly the same character, we have no grounds for
believing that they would altogether depart from their type.
It is a general belief amongst breeders that characters of all kinds become
fixed by long-continued inheritance. But I have attempted to show in the
fourteenth chapter that this belief apparently resolves itself into the
following proposition, namely, that all characters whatever, whether
recently acquired or ancient, tend to be transmitted, but that those which
have already long withstood all counteracting influences, will, as a
general rule, continue to withstand them, and consequently be faithfully
transmitted.
_Tendency in Man to carry the practice of Selection to an extreme point._
It is an important principle that in the process of selection man almost
invariably wishes to go to an extreme point. Thus, in useful qualities,
there is no limit to his desire to breed certain {240} horses and dogs as
fleet as possible, and others as strong as possible; certain kinds of sheep
for extreme fineness, and others for extreme length of wool; and he wishes
to produce fruit, grain, tubers, and other useful parts of plants, as large
and excellent as possible. With animals bred for amusement, the same
principle is even more powerful; for fashion, as we see even in our dress,
always runs to extremes. This view has been expressly admitted by fanciers.
Instances were given in the chapters on the pigeon, but here is another:
Mr. Eaton, after describing a comparatively new variety, namely, the
Archangel, remarks, "What fanciers intend doing with this bird I am at a
loss to know, whether they intend to breed it down to the tumbler's head
and beak, or carry it out to the carrier's head and beak; leaving it as
they found it, is not progressing." Ferguson, speaking of fowls, says,
"their peculiarities, whatever they may be, must necessarily be fully
developed: a little peculiarity forms nought but ugliness, seeing it
violates the existing laws of symmetry." So Mr. Brent, in discussing the
merits of the sub-varieties of the Belgian canary-bird, remarks, "Fanciers
always go to extremes; they do not admire indefinite properties."[589]
This principle, which necessarily leads to divergence of character,
explains the present state of various domestic races. We can thus see how
it is that race-horses and dray-horses, greyhounds and mastiffs, which are
opposed to each other in every character,--how varieties so distinct as
Cochin-China fowls and bantams, or carrier-pigeons with very long beaks,
and tumblers with excessively short beaks, have been derived from the same
stock. As each breed is slowly improved, the inferior varieties are first
neglected and finally lost. In a few cases, by the aid of old records, or
from intermediate varieties still existing in countries where other
fashions have prevailed, we are enabled partially to trace the graduated
changes through which certain breeds have passed. Selection, whether
methodical or unconscious, always tending towards an extreme point,
together with the neglect and slow extinction of the intermediate and
less-valued forms, is the key which unlocks the mystery how man has
produced such wonderful results.
{241}
In a few instances selection, guided by utility for a single purpose, has
led to convergence of character. All the improved and different races of
the pig, as Nathusius has well shown,[590] closely approach each other in
character, in their shortened legs and muzzles, their almost hairless,
large, rounded bodies, and small tusks. We see some degree of convergence
in the similar outline of the body in well-bred cattle belonging to
distinct races.[591] I know of no other such cases.
Continued divergence of character depends on, and is indeed a clear proof,
as previously remarked, of the same parts continuing to vary in the same
direction. The tendency to mere general variability or plasticity of
organisation can certainly be inherited, even from one parent, as has been
shown by Gaertner and Koelreuter, in the production of varying hybrids from
two species, of which one alone was variable. It is in itself probable
that, when an organ has varied in any manner, it will again vary in the
same manner, if the conditions which first caused the being to vary remain,
as far as can be judged, the same. This is either tacitly or expressly
admitted by all horticulturists: if a gardener observes one or two
additional petals in a flower, he feels confident that in a few generations
he will be able to raise a double flower, crowded with petals. Some of the
seedlings from the weeping Moccas oak were so prostrate that they only
crawled along the ground. A seedling from the fastigate or upright Irish
yew is described as differing greatly from the parent-form "by the
exaggeration of the fastigate habit of its branches."[592] Mr. Sheriff, who
has been more successful than any other man in raising new kinds of wheat,
remarks, "A good variety may safely be regarded as the forerunner of a
better one."[593] A great rose-grower, Mr. Rivers, has made the same remark
with respect to roses. Sageret,[594] who had large experience, in speaking
of the future progress of fruit-trees, observes that the most important
principle is "that the more plants have departed from their original type,
the more they tend to depart from it." There is apparently much truth in
this {242} remark; for we can in no other way understand the surprising
amount of difference between varieties in the parts or qualities which are
valued, whilst other parts retain nearly their original character.
The foregoing discussion naturally leads to the question, what is the limit
to the possible amount of variation in any part or quality, and,
consequently, is there any limit to what selection can effect? Will a
race-horse ever be reared fleeter than Eclipse? Can our prize-cattle and
sheep be still further improved? Will a gooseberry ever weigh more than
that produced by "London" in 1852? Will the beet-root in France yield a
greater percentage of sugar? Will future varieties of wheat and other grain
produce heavier crops than our present varieties? These questions cannot be
positively answered; but it is certain that we ought to be cautious in
answering by a negative. In some lines of variation the limit has probably
been reached. Youatt believes that the reduction of bone in some of our
sheep has already been carried so far that it entails great delicacy of
constitution.[595] But seeing the great improvement within recent times in
our cattle and sheep, and especially in our pigs; seeing the wonderful
increase in weight in our poultry of all kinds during the last few years;
he would be a bold man who would assert that perfection has been reached.
Eclipse perhaps may never be beaten until all our race-horses have been
rendered swifter, through the selection of the best horses during many
generations; and then the old Eclipse may possibly be eclipsed; but, as Mr.
Wallace has remarked, there must be an ultimate limit to the fleetness of
every animal, whether under nature or domestication; and with the horse
this limit has perhaps been reached. Until our fields are better manured,
it may be impossible for a new variety of wheat to yield a heavier crop.
But in many cases those who are best qualified to judge do not believe that
the extreme point has as yet been reached even with respect to characters
which have already been carried to a high standard of perfection. For
instance, the short-faced tumbler-pigeon has been greatly modified;
nevertheless, according to Mr. Eaton,[596] "the field is still as open for
fresh competitors as it was one hundred years ago." Over and over again it
has been said that {243} perfection had been attained with our flowers, but
a higher standard has soon been reached. Hardly any fruit has been more
improved than the strawberry, yet a great authority remarks,[597] "it must
not be concealed that we are far from the extreme limits at which we may
arrive."
Time is an important element in the formation of our domestic races, as it
permits innumerable individuals to be born, and these when exposed to
diversified conditions are rendered variable. Methodical selection has been
occasionally practised from an ancient period to the present day, even by
semi-civilised people, and during former times will have produced some
effect. Unconscious selection will have been still more effective; for
during a lengthened period the more valuable individual animals will
occasionally have been saved, and the less valuable neglected. In the
course, also, of time, different varieties, especially in the less
civilised countries, will have been more or less modified through natural
selection. It is generally believed, though on this head we have little or
no evidence, that new characters in time become fixed; and after having
long remained fixed it seems possible that under new conditions they might
again be rendered variable.
How great the lapse of time has been since man first domesticated animals
and cultivated plants, we begin dimly to see. When the lake-buildings of
Switzerland were inhabited during the Neolithic period, several animals
were already domesticated and various plants cultivated. If we may judge
from what we now see of the habits of savages, it is probable that the men
of the earlier Stone period--when many great quadrupeds were living which
are now extinct, and when the face of the country was widely different from
what it now is--possessed at least some few domesticated animals, although
their remains have not as yet been discovered. If the science of language
can be trusted, the art of ploughing and sowing the land was followed, and
the chief animals had been already domesticated, at an epoch so immensely
remote, that the Sanskrit, Greek, Latin, Gothic, Celtic, and Sclavonic
languages had not as yet diverged from their common parent-tongue.[598]
{244}
It is scarcely possible to overrate the effects of selection occasionally
carried on in various ways and places during thousands of generations. All
that we know, and, in a still stronger degree, all that we do not
know,[599] of the history of the great majority of our breeds, even of our
more modern breeds, agrees with the view that their production, through the
action of unconscious and methodical selection, has been almost insensibly
slow. When a man attends rather more closely than is usual to the breeding
of his animals, he is almost sure to improve them to a slight extent. They
are in consequence valued in his immediate neighbourhood, and are bred by
others; and their characteristic features, whatever these may be, will then
slowly but steadily be increased, sometimes by methodical and almost always
by unconscious selection. At last a strain, deserving to be called a
sub-variety, becomes a little more widely known, receives a local name, and
spreads. The spreading will have been extremely slow during ancient and
less civilised times, but now is rapid. By the time that the new breed had
assumed a somewhat distinct character, its history, hardly noticed at the
time, will have been completely forgotten; for, as Low remarks,[600] "we
know how quickly the memory of such events is effaced."
As soon as a new breed is thus formed, it is liable through the same
process to break up into new strains and sub-varieties. For different
varieties are suited for, and are valued under, different circumstances.
Fashion changes, but, should a fashion last for even a moderate length of
time, so strong is the principle of inheritance, that some effect will
probably be impressed on the breed. Thus varieties go on increasing in
number, and history shows us how wonderfully they have increased since the
earliest records.[601] As each new variety is produced, the earlier,
intermediate, and less valuable forms will be neglected, and perish. When a
breed, from not being valued, is kept in small numbers, its extinction
almost inevitably follows sooner or later, either from accidental causes of
destruction or from close interbreeding; and this is an event which, in the
case of well-marked breeds, excites attention. The birth or production of a
new domestic race is so slow a process that it {245} escapes notice; its
death or destruction is comparatively sudden, is often recorded, and when
too late sometimes regretted.
Several authors have drawn a wide distinction between artificial and
natural races. The latter are more uniform in character, possessing in a
high degree the character of natural species, and are of ancient origin.
They are generally found in less civilised countries, and have probably
been largely modified by natural selection, and only to a small extent by
man's unconscious and methodical selection. They have, also, during a long
period, been directly acted on by the physical conditions of the countries
which they inhabit. The so-called artificial races, on the other hand, are
not so uniform in character; some have a semi-monstrous character, such as
"the wry-legged terriers so useful in rabbit-shooting,"[602] turnspit dogs,
ancon sheep, niata oxen, Polish fowls, fantail-pigeons, &c.; their
characteristic features have generally been acquired suddenly, though
subsequently increased in many cases by careful selection. Other races,
which certainly must be called artificial, for they have been largely
modified by methodical selection and by crossing, as the English
race-horse, terrier-dogs, the English game-cock, Antwerp carrier-pigeons,
&c., nevertheless cannot be said to have an unnatural appearance; and no
distinct line, as it seems to me, can be drawn between natural and
artificial races.
It is not surprising that domestic races should generally present a
different aspect from natural species. Man selects and propagates
modifications solely for his own use or fancy, and not for the creature's
own good. His attention is struck by strongly marked modifications, which
have appeared suddenly, due to some great disturbing cause in the
organisation. He attends almost exclusively to external characters; and
when he succeeds in modifying internal organs,--when for instance he
reduces the bones and offal, or loads the viscera with fat, or gives early
maturity, &c.,--the chances are strong that he will at the same time weaken
the constitution. On the other hand, when an animal has to struggle
throughout its life with many competitors and enemies, under circumstances
inconceivably complex and liable to change, modifications of the most
varied nature--in the internal organs as well as in external characters, in
the {246} functions and mutual relations of parts--will be rigorously
tested, preserved, or rejected. Natural selection often checks man's
comparatively feeble and capricious attempts at improvement; and if this
were not so, the result of his work, and of nature's work, would be even
still more different. Nevertheless, we must not overrate the amount of
difference between natural species and domestic races; the most experienced
naturalists have often disputed whether the latter are descended from one
or from several aboriginal stocks, and this clearly shows that there is no
palpable difference between species and races.
Domestic races propagate their kind far more truly, and endure for much
longer periods, than most naturalists are willing to admit. Breeders feel
no doubt on this head; ask a man who has long reared Shorthorn or Hereford
cattle, Leicester or Southdown sheep, Spanish or Game poultry, tumbler or
carrier-pigeons, whether these races may not have been derived from common
progenitors, and he will probably laugh you to scorn. The breeder admits
that he may hope to produce sheep with finer or longer wool and with better
carcases, or handsomer fowls, or carrier-pigeons with beaks just
perceptibly longer to the practised eye, and thus be successful at an
exhibition. Thus far he will go, but no farther. He does not reflect on
what follows from adding up during a long course of time many, slight,
successive modifications; nor does he reflect on the former existence of
numerous varieties, connecting the links in each divergent line of descent.
He concludes, as was shown in the earlier chapters, that all the chief
breeds to which he has long attended are aboriginal productions. The
systematic naturalist, on the other hand, who generally knows nothing of
the art of breeding, who does not pretend to know how and when the several
domestic races were formed, who cannot have seen the intermediate
gradations, for they do not now exist, nevertheless feels no doubt that
these races are sprung from a single source. But ask him whether the
closely allied natural species which he has studied may not have descended
from a common progenitor, and he in his turn will perhaps reject the notion
with scorn. Thus the naturalist and breeder may mutually learn a useful
lesson from each other.
* * * * *
_Summary on Selection by Man._--There can be no doubt that {247} methodical
selection has effected and will effect wonderful results. It was
occasionally practised in ancient times, and is still practised by
semi-civilised people. Characters of the highest importance, and others of
trifling value, have been attended to, and modified. I need not here repeat
what has been so often said on the part which unconscious selection has
played: we see its power in the difference between flocks which have been
separately bred, and in the slow changes, as circumstances have slowly
changed, which many animals have undergone in the same country, or when
transported into a foreign land. We see the combined effects of methodical
and unconscious selection in the great amount of difference between
varieties in those parts or qualities which are valued by man, in
comparison with those which are not valued, and consequently have not been
attended to. Natural selection often determines man's power of selection.
We sometimes err in imagining that characters, which are considered as
unimportant by the systematic naturalist, could not be affected by the
struggle for existence, and therefore be acted on by natural selection; but
striking cases have been given, showing how great an error this is.
The possibility of selection coming into action rests on variability; and
this is mainly caused, as we shall hereafter see, by changes in the
conditions of life. Selection is sometimes rendered difficult, or even
impossible, by the conditions being opposed to the desired character or
quality. It is sometimes checked by the lessened fertility and weakened
constitution which follow from long-continued close interbreeding. That
methodical selection may be successful, the closest attention and
discernment, combined with unwearied patience, are absolutely necessary;
and these same qualities, though not indispensable, are highly serviceable
in the case of unconscious selection. It is almost necessary that a large
number of individuals should be reared; for thus there will be a fair
chance of variations of the desired nature arising, and every individual
with the slightest blemish or in any degree inferior may be freely
rejected. Hence length of time is an important element of success. Thus,
also, propagation at an early age and at short intervals favours the work.
Facility in pairing animals, or their inhabiting a confined area, is
advantageous as a check to free crossing. Whenever and {248} wherever
selection is not practised, distinct races are not formed. When any one
part of the body or quality is not attended to, it remains either unchanged
or varies in a fluctuating manner, whilst at the same time other parts and
other qualities may become permanently and greatly modified. But from the
tendency to reversion and to continued variability, those parts or organs
which are now undergoing rapid improvement through selection, are likewise
found to vary much. Consequently highly-bred animals, when neglected, soon
degenerate; but we have no reason to believe that the effects of
long-continued selection would, if the conditions of life remained the
same, be soon and completely lost.
Man always tends to go to an extreme point in the selection, whether
methodical or unconscious, of all useful and pleasing qualities. This is an
important principle, as it leads to continued divergence, and in some rare
cases to convergence of character. The possibility of continued divergence
rests on the tendency in each part or organ to go on varying in the same
manner in which it has already varied; and that this occurs, is proved by
the steady and gradual improvement of many animals and plants during
lengthened periods. The principle of divergence of character, combined with
the neglect and final extinction of all previous, less-valued, and
intermediate varieties, explains the amount of difference and the
distinctness of our several races. Although we may have reached the utmost
limit to which certain characters can be modified, yet we are far from
having reached, as we have good reason to believe, the limit in the
majority of cases. Finally, from the difference between selection as
carried on by man and by nature, we can understand how it is that domestic
races often, though by no means always, differ in general aspect from
closely allied natural species.
Throughout this chapter and elsewhere I have spoken of selection as the
paramount power, yet its action absolutely depends on what we in our
ignorance call spontaneous or accidental variability. Let an architect be
compelled to build an edifice with uncut stones, fallen from a precipice.
The shape of each fragment may be called accidental; yet the shape of each
has been determined by the force of gravity, the nature {249} of the rock,
and the slope of the precipice,--events and circumstances, all of which
depend on natural laws; but there is no relation between these laws and the
purpose for which each fragment is used by the builder. In the same manner
the variations of each creature are determined by fixed and immutable laws;
but these bear no relation to the living structure which is slowly built up
through the power of selection, whether this be natural or artificial
selection.
If our architect succeeded in rearing a noble edifice, using the rough
wedge-shaped fragments for the arches, the longer stones for the lintels,
and so forth, we should admire his skill even in a higher degree than if he
had used stones shaped for the purpose. So it is with selection, whether
applied by man or by nature; for though variability is indispensably
necessary, yet, when we look at some highly complex and excellently adapted
organism, variability sinks to a quite subordinate position in importance
in comparison with selection, in the same manner as the shape of each
fragment used by our supposed architect is unimportant in comparison with
his skill.
* * * * *
{250}
CHAPTER XXII.
CAUSES OF VARIABILITY.
VARIABILITY DOES NOT NECESSARILY ACCOMPANY REPRODUCTION--CAUSES
ASSIGNED BY VARIOUS AUTHORS--INDIVIDUAL DIFFERENCES--VARIABILITY OF
EVERY KIND DUE TO CHANGED CONDITIONS OF LIFE--ON THE NATURE OF SUCH
CHANGES--CLIMATE, FOOD, EXCESS OF NUTRIMENT--SLIGHT CHANGES
SUFFICIENT--EFFECTS OF GRAFTING ON THE VARIABILITY OF
SEEDLING-TREES--DOMESTIC PRODUCTIONS BECOME HABITUATED TO CHANGED
CONDITIONS--ON THE ACCUMULATIVE ACTION OF CHANGED CONDITIONS--CLOSE
INTERBREEDING AND THE IMAGINATION OF THE MOTHER SUPPOSED TO CAUSE
VARIABILITY--CROSSING AS A CAUSE OF THE APPEARANCE OF NEW
CHARACTERS--VARIABILITY FROM THE COMMINGLING OF CHARACTERS AND FROM
REVERSION--ON THE MANNER AND PERIOD OF ACTION OF THE CAUSES WHICH
EITHER DIRECTLY, OR INDIRECTLY THROUGH THE REPRODUCTIVE SYSTEM, INDUCE
VARIABILITY.
We will now consider, as far as we can, the causes of the almost universal
variability of our domesticated productions. The subject is an obscure one;
but it may be useful to probe our ignorance. Some authors, for instance Dr.
Prosper Lucas, look at variability as a necessary contingent on
reproduction, and as much an aboriginal law, as growth or inheritance.
Others have of late encouraged, perhaps unintentionally, this view by
speaking of inheritance and variability as equal and antagonistic
principles. Pallas maintained, and he has had some followers, that
variability depends exclusively on the crossing of primordially distinct
forms. Other authors attribute the tendency to variability to an excess of
food, and with animals to an excess relatively to the amount of exercise
taken, or again to the effects of a more genial climate. That these causes
are all effective is highly probable. But we must, I think, take a broader
view, and conclude that organic beings, when subjected during several
generations to any change whatever in their conditions, tend to vary; the
kind of variation which ensues depending in a far higher degree on the
nature or constitution of the being, than on the nature of the changed
conditions. {251}
Those authors who believe that it is a law of nature that each individual
should differ in some slight degree from every other, may maintain,
apparently with truth, that this is the fact, not only with all
domesticated animals and cultivated plants, but likewise with all organic
beings in a state of nature. The Laplander by long practice knows and gives
a name to each reindeer, though, as Linnaeus remarks, "to distinguish one
from another among such multitudes was beyond my comprehension, for they
were like ants on an ant-hill." In Germany shepherds have won wagers by
recognising each sheep in a flock of a hundred, which they had never seen
until the previous fortnight. This power of discrimination, however, is as
nothing compared to that which some florists have acquired. Verlot mentions
a gardener who could distinguish 150 kinds of camellia, when not in flower;
and it has been positively asserted that the famous old Dutch florist
Voorhelm, who kept above 1200 varieties of the hyacinth, was hardly ever
deceived in knowing each variety by the bulb alone. Hence we must conclude
that the bulbs of the hyacinth and the branches and leaves of the camellia,
though appearing to an unpractised eye absolutely undistinguishable, yet
really differ.[603]
As Linnaeus has compared the reindeer in number to ants, I may add that
each ant knows its fellow of the same community. Several times I carried
ants of the same species (_Formica rufa_) from one ant-hill to another,
inhabited apparently by tens of thousands of ants; but the strangers were
instantly detected and killed. I then put some ants taken from a very large
nest into a bottle strongly perfumed with assafoetida, and after an
interval of twenty-four hours returned them to their home; they were at
first threatened by their fellows, but were soon recognised and allowed to
pass. Hence each ant certainly recognises, independently of odour, its
fellow; and if all the ants of the same community have not some countersign
or watchword, they must present to each other's senses some distinguishable
character.
{252}
The dissimilarity of brothers or sisters of the same family, and of
seedlings from the same capsule, may be in part accounted for by the
unequal blending of the characters of the two parents, and by the more or
less complete recovery through reversion of ancestral characters on either
side; but we thus only push the difficulty further back in time, for what
made the parents or their progenitors different? Hence the belief[604] that
an innate tendency to vary exists, independently of external conditions,
seems at first sight probable. But even the seeds nurtured in the same
capsule are not subjected to absolutely uniform conditions, as they draw
their nourishment from different points; and we shall see in a future
chapter that this difference sometimes suffices greatly to affect the
character of the future plant. The less close similarity of the successive
children of the same family in comparison with human twins, which often
resemble each other in external appearance, mental disposition, and
constitution, in so extraordinary a manner, apparently proves that the
state of the parents at the exact period of conception, or the nature of
the subsequent embryonic development, has a direct and powerful influence
on the character of the offspring. Nevertheless, when we reflect on the
{253} individual differences between organic beings in a state of nature,
as shown by every wild animal knowing its mate; and when we reflect on the
infinite diversity of the many varieties of our domesticated productions,
we may well be inclined to exclaim, though falsely as I believe, that
Variability must be looked at as an ultimate fact, necessarily contingent
on reproduction.
Those authors who adopt this latter view would probably deny that each
separate variation has its own proper exciting cause. Although we can
seldom trace the precise relation between cause and effect, yet the
considerations presently to be given lead to the conclusion that each
modification must have its own distinct cause. When we hear of an infant
born, for instance, with a crooked finger, a misplaced tooth, or other
slight deviation of structure, it is difficult to bring the conviction home
to the mind that such abnormal cases are the result of fixed laws, and not
of what we blindly call accident. Under this point of view the following
case, which has been carefully examined and communicated to me by Dr.
William Ogle, is highly instructive. Two girls, born as twins, and in all
respects extremely alike, had their little fingers on both hands crooked;
and in both children the second bicuspid tooth in the upper jaw, of the
second dentition, was misplaced; for these teeth, instead of standing in a
line with the others, grew from the roof of the mouth behind the first
bicuspids. Neither the parents nor any other member of the family had
exhibited any similar peculiarity. Now, as both these children were
affected in exactly the same manner by both deviations of structure, the
idea of accident is at once excluded; and we are compelled to admit that
there must have existed some precise and sufficient cause which, if it had
occurred a hundred times, would have affected a hundred children.
We will now consider the general arguments, which appear to me to have
great weight, in favour of the view that variations of all kinds and
degrees are directly or indirectly caused by the conditions of life to
which each being, and more especially its ancestors, have been exposed.
No one doubts that domesticated productions are more variable than organic
beings which have never been removed from their {254} natural conditions.
Monstrosities graduate so insensibly into mere variations that it is
impossible to separate them; and all those who have studied monstrosities
believe that they are far commoner with domesticated than with wild animals
and plants;[605] and in the case of plants, monstrosities would be equally
noticeable in the natural as in the cultivated state. Under nature, the
individuals of the same species are exposed to nearly uniform conditions,
for they are rigorously kept to their proper places by a host of competing
animals and plants; they have, also, long been habituated to their
conditions of life; but it cannot be said that they are subject to quite
uniform conditions, and they are liable to a certain amount of variation.
The circumstances under which our domestic productions are reared are
widely different: they are protected from competition; they have not only
been removed from their natural conditions and often from their native
land, but they are frequently carried from district to district, where they
are treated differently, so that they never remain during a considerable
length of time exposed to closely similar conditions. In conformity with
this, all our domesticated productions, with the rarest exceptions, vary
far more than natural species. The hive-bee, which feeds itself and follows
in most respects its natural habits of life, is the least variable of all
domesticated animals, and probably the goose is the next least variable;
but even the goose varies more than almost any wild bird, so that it cannot
be affiliated with perfect certainty to any natural species. Hardly a
single plant can be named, which has long been cultivated and propagated by
seed, that is not highly variable; common rye (_Secale cereale_) has
afforded fewer and less marked varieties than almost any other cultivated
plant;[606] but it may be doubted whether the variations of this, the least
valuable of all our cereals, have been closely observed.
Bud-variation, which was fully discussed in a former chapter, shows us that
variability may be quite independent of seminal reproduction, and likewise
of reversion to long-lost ancestral characters. No one will maintain that
the sudden appearance {255} of a moss-rose on a Provence-rose is a return
to a former state, for mossiness of the calyx has been observed in no
natural species; the same argument is applicable to variegated and
laciniated leaves; nor can the appearance of nectarines on peach-trees be
accounted for with any probability on the principle of reversion. But
bud-variations more immediately concern us, as they occur far more
frequently on plants which have been highly cultivated during a length of
time, than on other and less highly cultivated plants; and very few
well-marked instances have been observed with plants growing under strictly
natural conditions. I have given one instance of an ash-tree growing in a
gentleman's pleasure-grounds; and occasionally there may be seen, on beech
and other trees, twigs leafing at a different period from the other
branches. But our forest trees in England can hardly be considered as
living under strictly natural conditions; the seedlings are raised and
protected in nursery-grounds, and must often be transplanted into places
where wild trees of the kind would not naturally grow. It would be esteemed
a prodigy if a dog-rose growing in a hedge produced by bud-variation a
moss-rose, or a wild bullace or wild cherry-tree yielded a branch bearing
fruit of a different shape and colour from the ordinary fruit. The prodigy
would be enhanced if these varying branches were found capable of
propagation, not only by grafts, but sometimes by seed; yet analogous cases
have occurred with many of our highly cultivated trees and herbs.
These several considerations alone render it probable that variability of
every kind is directly or indirectly caused by changed conditions of life.
Or, to put the case under another point of view, if it were possible to
expose all the individuals of a species during many generations to
absolutely uniform conditions of life, there would be no variability.
_On the Nature of the Changes in the Conditions of Life which induce
Variability._
From a remote period to the present day, under climates and circumstances
as different as it is possible to conceive, organic beings of all kinds,
when domesticated or cultivated, have {256} varied. We see this with the
many domestic races of quadrupeds and birds belonging to different orders,
with gold-fish and silkworms, with plants of many kinds, raised in various
quarters of the world. In the deserts of northern Africa the date-palm has
yielded thirty-eight varieties; in the fertile plains of India it is
notorious how many varieties of rice and of a host of other plants exist;
in a single Polynesian island, twenty-four varieties of the bread-fruit,
the same number of the banana, and twenty-two varieties of the arum, are
cultivated by the natives; the mulberry-tree in India and Europe has
yielded many varieties serving as food for the silkworm; and in China
sixty-three varieties of the bamboo are used for various domestic
purposes.[607] These facts alone, and innumerable others could be added,
indicate that a change of almost any kind in the conditions of life
suffices to cause variability--different changes acting on different
organisms.
Andrew Knight[608] attributed the variation of both animals and plants to a
more abundant supply of nourishment, or to a more favourable climate, than
that natural to the species. A more genial climate, however, is far from
necessary; the kidney-bean, which is often injured by our spring frosts,
and peaches, which require the protection of a wall, have varied much in
England, as has the orange-tree in northern Italy, where it is barely able
to exist.[609] Nor can we overlook the fact, though not immediately
connected with our present subject, that the plants and shells of the
arctic regions are eminently variable.[610] Moreover, it does not appear
that a change of climate, whether more or less genial, is one of the most
potent causes of variability; for in regard to plants Alph. De Candolle, in
his 'Geographie {257} Botanique,' repeatedly shows that the native country
of a plant, where in most cases it has been longest cultivated, is that
where it has yielded the greatest number of varieties.
It is doubtful whether a change in the nature of the food is a potent cause
of variability. Scarcely any domesticated animal has varied more than the
pigeon or the fowl, but their food, especially that of highly-bred pigeons,
is generally the same. Nor can our cattle and sheep have been subjected to
any great change in this respect. But in all these cases the food probably
is much less varied in kind than that which was consumed by the species in
its natural state.[611]
Of all the causes which induce variability, excess of food, whether or not
changed in nature, is probably the most powerful. This view was held with
regard to plants by Andrew Knight, and is now held by Schleiden, more
especially in reference to the inorganic elements of the food.[612] In
order to give a plant more food it suffices in most cases to grow it
separately, and thus prevent other plants robbing its roots. It is
surprising, as I have often seen, how vigorously our common wild plants
flourish when planted by themselves, though not in highly manured land.
Growing plants separately is, in fact, the first step in cultivation. We
see the converse of the belief that excess of food induces variability in
the following statement by a great raiser of seeds of all kinds.[613] "It
is a rule invariably with us, when we desire to keep a true stock of any
one kind of seed, to grow it on poor land without dung; but when we grow
for quantity, we act contrary, and sometimes have dearly to repent of it."
In the case of animals the want of a proper amount of exercise, as
Bechstein has remarked, has perhaps played, independently of the direct
effects of the disuse of any particular organ, an important part in causing
variability. We can see in a vague manner that, when the organised and
nutrient fluids of the body are not used during growth, or by the wear and
tear of the tissues, {258} they will be in excess; and as growth,
nutrition, and reproduction are intimately allied processes, this
superfluity might disturb the due and proper action of the reproductive
organs, and consequently affect the character of the future offspring. But
it may be argued that neither an excess of food nor a superfluity in the
organised fluids of the body necessarily induces variability. The goose and
the turkey have been well fed for many generations, yet have varied very
little. Our fruit-trees and culinary plants, which are so variable, have
been cultivated from an ancient period, and, though they probably still
receive more nutriment than in their natural state, yet they must have
received during many generations nearly the same amount; and it might be
thought that they would have become habituated to the excess. Nevertheless,
on the whole, Knight's view, that excess of food is one of the most potent
causes of variability, appears, as far as I can judge, probable.
Whether or not our various cultivated plants have received nutriment in
excess, all have been exposed to changes of various kinds. Fruit-trees are
grafted on different stocks, and grown in various soils. The seeds of
culinary and agricultural plants are carried from place to place; and
during the last century the rotation of our crops and the manures used have
been greatly changed.
Slight changes of treatment often suffice to induce variability. The simple
fact of almost all our cultivated plants and domesticated animals having
varied in all places and at all times, leads to this conclusion. Seeds
taken from common English forest-trees, grown under their native climate,
not highly manured or otherwise artificially treated, yield seedlings which
vary much, as may be seen in every extensive seed-bed. I have shown in a
former chapter what a number of well marked and singular varieties the
thorn (_Crataegus oxyacantha_) has produced; yet this tree has been
subjected to hardly any cultivation. In Staffordshire I carefully examined
a large number of two British plants, namely, _Geranium phaeum_ and
_Pyrenaicum_, which have never been highly cultivated. These plants had
spread spontaneously by seed from a common garden into an open plantation;
and the seedlings varied in almost every single character, both in their
flowers and foliage, to a degree which {259} I have never seen exceeded;
yet they could not have been exposed to any great change in their
conditions.
With respect to animals, Azara has remarked with much surprise,[614] that,
whilst the feral horses on the Pampas are always of one of three colours,
and the cattle always of a uniform colour, yet these animals, when bred on
the unenclosed estancias, though kept in a state which can hardly be called
domesticated, and apparently exposed to almost identically the same
conditions as when they are feral, nevertheless display a great diversity
of colour. So again in India several species of fresh-water fish are only
so far treated artificially, that they are reared in great tanks; but this
small change is sufficient to induce much variability.[615]
Some facts on the effects of grafting, in regard to the variability of
trees, deserve attention. Cabanis asserts that when certain pears are
grafted on the quince, their seeds yield more varieties than do the seeds
of the same variety of pear when grafted on the wild pear.[616] But as the
pear and quince are distinct species, though so closely related that the
one can be readily grafted and succeeds admirably on the other, the fact of
variability being thus caused is not surprising; we are, however, here
enabled to see the cause, namely, the different nature of the stock with
its roots and the rest of the tree. Several North American varieties of the
plum and peach are well known to reproduce themselves truly by seed; but
Downing asserts,[617] "that when a graft is taken from one of these trees
and placed upon another stock, this grafted tree is found to lose its
singular property of producing the same variety by seed, and becomes like
all other worked trees;"--that is, its seedlings become highly variable.
Another case is worth giving: the Lalande variety of the walnut-tree leafs
between April 20th and May 15th, and its seedlings invariably inherit the
same habit; whilst several other varieties of the walnut leaf in June. Now,
if seedlings are raised from the May-leafing Lalande variety, grafted on
another May-leafing variety, though both stock and graft have the same
early habit of leafing, yet the seedlings leaf at various times, {260} even
as late as the 5th of June.[618] Such facts as these are well fitted to
show, on what obscure and slight causes variability rests.
I may here just allude to the appearance of new and valuable varieties
of fruit-trees and of wheat in woods and waste places, which at first
sight seems a most anomalous circumstance. In France a considerable
number of the best pears have been discovered in woods; and this has
occurred so frequently, that Poiteau asserts that "improved varieties
of our cultivated fruits rarely originate with nurserymen."[619] In
England, on the other hand, no instance of a good pear having been
found wild has been recorded; and Mr. Rivers informs me that he knows
of only one instance with apples, namely, the Bess Poole, which was
discovered in a wood in Nottinghamshire. This difference between the
two countries may be in part accounted for by the more favourable
climate of France, but chiefly from the great number of seedlings which
spring up there in the woods. I infer that this is the case from a
remark made by a French gardener,[620] who regards it as a national
calamity that such a number of pear-trees are periodically cut down for
firewood, before they have borne fruit. The new varieties which thus
spring up in the woods, though they cannot have received any excess of
nutriment, will have been exposed to abruptly changed conditions, but
whether this is the cause of their production is very doubtful. These
varieties, however, are probably all descended[621] from old cultivated
kinds growing in adjoining orchards,--a circumstance which will account
for their variability; and out of a vast number of varying trees there
will always be a good chance of the appearance of a valuable kind. In
North America, where fruit-trees frequently spring up in waste places,
the Washington pear was found in a hedge, and the Emperor peach in a
wood.[622]
With respect to wheat, some writers have spoken[623] as if it were an
ordinary event for new varieties to be found in waste places; the
Fenton wheat was certainly discovered growing on a pile of basaltic
detritus in a quarry, but in such a situation the plant would probably
receive a sufficient amount {261} of nutriment. The Chidham wheat was
raised from an ear found _on_ a hedge; and Hunter's wheat was
discovered _by_ the roadside in Scotland, but it is not said that this
latter variety grew where it was found.[624]
Whether our domestic productions would ever become so completely habituated
to the conditions under which they now live, as to cease varying, we have
no sufficient means for judging. But, in fact, our domestic productions are
never exposed for a great length of time to uniform conditions, and it is
certain that our most anciently cultivated plants, as well as animals,
still go on varying, for all have recently undergone marked improvement. In
some few cases, however, plants have become habituated to new conditions.
Thus Metzger, who cultivated in Germany during many years numerous
varieties of wheat, brought from different countries,[625] states that some
kinds were at first extremely variable, but gradually, in one instance
after an interval of twenty-five years, became constant; and it does not
appear that this resulted from the selection of the more constant forms.
* * * * *
_On the Accumulative Action of changed Conditions of Life._--We have good
grounds for believing that the influence of changed conditions accumulates,
so that no effect is produced on a species until it has been exposed during
several generations to continued cultivation or domestication. Universal
experience shows us that when new flowers are first introduced into our
gardens they do not vary; but ultimately all, with the rarest exceptions,
vary to a greater or less extent. In a few cases the requisite number of
generations, as well as the successive steps in the progress of variation,
have been recorded, as in the often-quoted instance of the Dahlia.[626]
After several years' culture the Zinnia has only lately (1860) begun to
vary in any great degree. "In the first seven or eight years of high
cultivation the Swan River daisy (_Brachycome iberidifolia_) kept to its
original colour; it then varied into lilac and purple and other minor
shades."[627] Analogous facts have been recorded with the Scotch rose. In
discussing the variability of plants several experienced horticulturists
have spoken to the {262} same general effect. Mr. Salter[628] remarks,
"Every one knows that the chief difficulty is in breaking through the
original form and colour of the species, and every one will be on the
look-out for any natural sport, either from seed or branch; that being once
obtained, however trifling the change may be, the result depends upon
himself." M. de Jonghe, who has had so much success in raising new
varieties of pears and strawberries,[629] remarks with respect to the
former, "There is another principle, namely, that the more a type has
entered into a state of variation, the greater is its tendency to continue
doing so; and the more it has varied from the original type, the more it is
disposed to vary still farther." We have, indeed, already discussed this
latter point when treating of the power which man possesses, through
selection, of continually augmenting in the same direction each
modification; for this power depends on continued variability of the same
general kind. The most celebrated horticulturist in France, namely,
Vilmorin,[630] even maintains that, when any particular variation is
desired, the first step is to get the plant to vary in any manner whatever,
and to go on selecting the most variable individuals, even though they vary
in the wrong direction; for the fixed character of the species being once
broken, the desired variation will sooner or later appear.
As nearly all our animals were domesticated at an extremely remote epoch,
we cannot, of course, say whether they varied quickly or slowly when first
subjected to new conditions. But Dr. Bachman[631] states that he has seen
turkeys raised from the eggs of the wild species lose their metallic tints
and become spotted with white in the third generation. Mr. Yarrell many
years ago informed me that the wild ducks bred on the ponds in St. James's
Park, which had never been crossed, as it is believed, with domestic ducks,
lost their true plumage after a few generations. An excellent
observer,[632] who has often reared birds from the eggs of the wild duck,
and who took precautions {263} that there should be no crossing with
domestic breeds, has given, as previously stated, full details on the
changes which they gradually undergo. He found that he could not breed
these wild ducks true for more than five or six generations, "as they then
proved so much less beautiful. The white collar round the neck of the
mallard became much broader and more irregular, and white feathers appeared
in the ducklings' wings." They increased also in size of body; their legs
became less fine, and they lost their elegant carriage. Fresh eggs were
then procured from wild birds; but again the same result followed. In these
cases of the duck and turkey we see that animals, like plants, do not
depart from their primitive type until they have been subjected during
several generations to domestication. On the other hand, Mr. Yarrell
informed me that the Australian dingos, bred in the Zoological Gardens,
almost invariably produced in the first generation puppies marked with
white and other colours; but these introduced dingos had probably been
procured from the natives, who keep them in a semi-domesticated state. It
is certainly a remarkable fact that changed conditions should at first
produce, as far as we can see, absolutely no effect; but that they should
subsequently cause the character of the species to change. In the chapter
on pangenesis I shall attempt to throw a little light on this fact.
* * * * *
Returning now to the causes which are supposed to induce variability. Some
authors[633] believe that close interbreeding gives this tendency, and
leads to the production of monstrosities. In the seventeenth chapter some
few facts were advanced, showing that monstrosities are, as it appears,
occasionally thus caused; and there can be no doubt that close
interbreeding induces lessened fertility and a weakened constitution; hence
it may lead to variability: but I have not sufficient evidence on this
head. On the other hand, close interbreeding, if not carried to an
injurious extreme, far from causing variability, tends to fix the character
of each breed.
It was formerly a common belief, still held by some persons, that the
imagination of the mother affects the child in {264} the womb.[634] This
view is evidently not applicable to the lower animals, which lay
unimpregnated eggs, or to plants. Dr. William Hunter, in the last century,
told my father that during many years every woman in a large London
Lying-in Hospital was asked before her confinement whether anything had
specially affected her mind, and the answer was written down; and it so
happened that in no one instance could a coincidence be detected between
the woman's answer and any abnormal structure; but when she knew the nature
of the structure, she frequently suggested some fresh cause. The belief in
the power of the mother's imagination may perhaps have arisen from the
children of a second marriage resembling the previous father, as certainly
sometimes occurs, in accordance with the facts given in the eleventh
chapter.
* * * * *
_Crossing as a Cause of Variability._--In an early part of this chapter it
was stated that Pallas[635] and a few other naturalists maintain that
variability is wholly due to crossing. If this means that new characters
never spontaneously appear in our domestic races, but that they are all
directly derived from certain aboriginal species, the doctrine is little
less than absurd; for it implies that animals like Italian greyhounds,
pug-dogs, bull-dogs, pouter and fantail pigeons, &c., were able to exist in
a state of nature. But the doctrine may mean something widely different,
namely, that the crossing of distinct species is the sole cause of the
first appearance of new characters, and that without this aid man could not
have formed his various breeds. As, however, new characters have appeared
in certain cases by bud-variation, we may conclude with certainty that
crossing is not necessary for variability. It is, moreover, almost certain
that the breeds of various animals, such as of the rabbit, pigeon, duck,
&c., and the varieties of several plants, are the modified descendants of a
single wild species. Nevertheless, it is probable that the crossing of two
forms, when one or both have long been domesticated or cultivated, adds to
the variability of the offspring, independently of the commingling of the
characters derived from the two parent-forms; and this implies {265} that
new characters actually arise. But we must not forget the facts advanced in
the thirteenth chapter, which clearly prove that the act of crossing often
leads to the reappearance or reversion of long-lost characters; and in most
cases it would be impossible to distinguish between the reappearance of
ancient characters and the first appearance of new characters. Practically,
whether new or old, they would be new to the breed in which they
reappeared.
Gaertner declares,[636] and his experience is of the highest value on
such a point, that, when he crossed native plants which had not been
cultivated, he never once saw in the offspring any new character; but
that from the odd manner in which the characters derived from the
parents were combined, they sometimes appeared as if new. When, on the
other hand, he crossed cultivated plants, he admits that new characters
occasionally appeared, but he is strongly inclined to attribute their
appearance to ordinary variability, not in any way to the cross. An
opposite conclusion, however, appears to me the more probable.
According to Koelreuter, hybrids in the genus Mirabilis vary almost
infinitely, and he describes new and singular characters in the form of
the seeds, in the colour of the anthers, in the cotyledons being of
immense size, in new and highly peculiar odours, in the flowers
expanding early in the season, and in their closing at night. With
respect to one lot of these hybrids, he remarks that they presented
characters exactly the reverse of what might have been expected from
their parentage.[637]
Prof. Lecoq[638] speaks strongly to the same effect in regard to this
same genus, and asserts that many of the hybrids from _Mirabilis
jalapa_ and _multiflora_ might easily be mistaken for distinct species,
and adds that they differed in a greater degree, than the other species
of the genus, from _M. jalapa_. Herbert, also, has described[639] the
offspring from a hybrid Rhododendron as being "as _unlike all others_
in foliage, as if they had been a separate species." The common
experience of floriculturists proves that the crossing and recrossing
of distinct but allied plants, such as the species of Petunia,
Calceolaria, Fuchsia, Verbena, &c., induces excessive variability;
hence the appearance of quite new characters is probable. M.
Carriere[640] has lately discussed this subject: he states that
_Erythrina cristagalli_ had been multiplied by seed for many years, but
had not yielded any varieties: it was then crossed with the allied _E.
herbacea_, and "the resistance was now overcome, and varieties were
produced with flowers of extremely different size, form, and colour."
From the general and apparently well-founded belief that the crossing
{266} of distinct species, besides commingling their characters, adds
greatly to their variability, it has probably arisen that some
botanists have gone so far as to maintain[641] that, when a genus
includes only a single species, this when cultivated never varies. The
proposition made so broadly cannot be admitted; but it is probably true
that the variability of cultivated monotypic genera is much less than
that of genera including numerous species, and this quite independently
of the effects of crossing. I have stated in my 'Origin of Species,'
and in a future work shall more fully show, that the species belonging
to small genera generally yield a less number of varieties in a state
of nature than those belonging to large genera. Hence the species of
small genera would, it is probable, produce fewer varieties under
cultivation than the already variable species of larger genera.
Although we have not at present sufficient evidence that the crossing
of species, which have never been cultivated, leads to the appearance
of new characters, this apparently does occur with species which have
been already rendered in some degree variable through cultivation.
Hence crossing, like any other change in the conditions of life, seems
to be an element, probably a potent one, in causing variability. But we
seldom have the means of distinguishing, as previously remarked,
between the appearance of really new characters and the reappearance of
long-lost characters, evoked through the act of crossing. I will give
an instance of the difficulty in distinguishing such cases. The species
of Datura may be divided into two sections, those having white flowers
with green stems, and those having purple flowers with brown stems: now
Naudin[642] crossed _Datura laevis_ and _ferox_, both of which belong
to the white section, and raised from them 205 hybrids. Of these
hybrids, every one had brown stems and bore purple flowers; so that
they resembled the species of the other section of the genus, and not
their own two parents. Naudin was so much astonished at this fact, that
he was led carefully to observe both parent-species, and he discovered
that the pure seedlings of _D. ferox_, immediately after germination,
had dark purple stems, extending from the young roots up to the
cotyledons, and that this tint remained ever afterwards as a ring round
the base of the stem of the plant when old. Now I have shown in the
thirteenth chapter that the retention or exaggeration of an early
character is so intimately related to reversion, that it evidently
comes under the same principle. Hence probably we ought to look at the
purple flowers and brown stems of these hybrids, not as new characters
due to variability, but as a return to the former state of some ancient
progenitor.
Independently of the appearance of new characters from crossing, a few
words may be added to what has been said in former chapters on the
unequal combination and transmission of the characters proper to the
two parent-forms. When two species or races are crossed, the offspring
of {267} the first generation are generally uniform, but subsequently
they display an almost infinite diversity of character. He who wishes,
says Koelreuter,[643] to obtain an endless number of varieties from
hybrids should cross and recross them. There is also much variability
when hybrids or mongrels are reduced or absorbed by repeated crosses
with either pure parent-form; and a still higher degree of variability
when three distinct species, and most of all when four species, are
blended together by successive crosses. Beyond this point
Gaertner,[644] on whose authority the foregoing statements are made,
never succeeded in effecting a union; but Max Wichura[645] united six
distinct species of willows into a single hybrid. The sex of the
parent-species affects in an inexplicable manner the degree of
variability of hybrids; for Gaertner[646] repeatedly found that when a
hybrid was used as the father, and either one of the pure
parent-species, or a third species, was used as the mother, the
offspring were more variable than when the same hybrid was used as the
mother, and either pure parent or the same third species as the father:
thus seedlings from _Dianthus barbatus_ crossed by the hybrid _D.
chinensi-barbatus_ were more variable than those raised from this
latter hybrid fertilised by the pure _D. barbatus_. Max Wichura[647]
insists strongly on an analogous result with his hybrid willows. Again
Gaertner[648] asserts that the degree of variability sometimes differs
in hybrids raised from reciprocal crosses between the same two species;
and here the sole difference is, that the one species is first used as
the father and then as the mother. On the whole we see that,
independently of the appearance of new characters, the variability of
successive crossed generations is extremely complex, partly from the
offspring partaking unequally of the characters of the two
parent-forms, and more especially from their unequal tendency to revert
to these same characters or to those of more ancient progenitors.
* * * * *
_On the Manner and on the Period of Action of the Causes which induce
Variability._--This is an extremely obscure subject, and we need here only
briefly consider, firstly, whether inherited variations are caused by the
organisation being directly acted on, or indirectly through the
reproductive system; and secondly, at what period of life or growth they
are primarily caused. We shall see in the two following chapters that
various agencies, such as an abundant supply of food, exposure to a
different climate, increased use or disuse of parts, &c., prolonged during
several generations, certainly modify either the whole organisation or
certain organs. This direct action of changed conditions perhaps comes into
play much more frequently than can be proved, and it is at least clear that
in all cases of {268} bud-variation the action cannot have been through the
reproductive system.
With respect to the part which the reproductive system takes in causing
variability, we have seen in the eighteenth chapter that even slight
changes in the conditions of life have a remarkable power in causing a
greater or less degree of sterility. Hence it seems not improbable that
being generated though a system so easily affected should themselves be
affected, or should fail to inherit, or inherit in excess, characters
proper to their parents. We know that certain groups of organic beings,
but with exceptions in each group, have their reproductive systems much
more easily affected by changed conditions than other groups; for
instance, carnivorous birds more readily than carnivorous mammals, and
parrots more readily than pigeons; and this fact harmonizes with the
apparently capricious manner and degree in which various groups of
animals and plants vary under domestication.
Koelreuter[649] was struck with the parallelism between the excessive
variability of hybrids when crossed and recrossed in various
ways,--these hybrids having their reproductive powers more or less
affected,--and the variability of anciently cultivated plants. Max
Wichura[650] has gone one step farther, and shows that with many of our
highly cultivated plants, such as the hyacinth, tulip, auricula,
snapdragon, potato, cabbage, &c., which there is no reason to believe
have been hybridized, the anthers contain many irregular pollen-grains,
in the same state as in hybrids. He finds also in certain wild forms,
the same coincidence between the state of the pollen and a high degree
of variability, as in many species of Rubus; but in _R. caesius_ and
_idaeus_, which are not highly variable species, the pollen is sound.
It is also notorious that many cultivated plants, such as the banana,
pine-apple, breadfruit, and others previously mentioned, have their
reproductive organs so seriously affected as to be generally quite
sterile; and when they do yield seed, the seedlings, judging from the
large number of cultivated races which exist, must be variable in an
extreme degree. These facts indicate that there is some relation
between the state of the reproductive organs and a tendency to
variability; but we must not conclude that the relation is strict.
Although many of our highly cultivated plants may have their pollen in
a deteriorated condition, yet, as we have previously seen, they yield
more seed, and our anciently domesticated animals are more prolific,
than the corresponding species in a state of nature. The peacock is
almost the only bird which is believed to be less fertile under
domestication than in its native state, and it has varied in a
remarkably small degree. From these considerations it would seem that
changes in the conditions of life lead either to sterility or to
variability, or to both; and not that sterility induces variability. On
the whole it is probable that any cause affecting the organs of
reproduction would likewise affect their product,--that is, the
offspring thus generated.
{269}
The period of life at which the causes that induce variability act, is
another obscure subject, which has been discussed by various
authors.[651] In some of the cases, to be given in the following
chapter, of modifications from the direct action of changed conditions,
which are inherited, there can be no doubt that the causes have acted
on the mature or nearly mature animal. On the other hand,
monstrosities, which cannot be distinctly separated from lesser
variations, are often caused by the embryo being injured whilst in the
mother's womb or in the egg. Thus I. Geoffroy St. Hilaire[652] asserts
that poor women who work hard during their pregnancy, and the mothers
of illegitimate children troubled in their minds and forced to conceal
their state, are far more liable to give birth to monsters than women
in easy circumstances. The eggs of the fowl when placed upright or
otherwise treated unnaturally frequently produce monstrous chickens. It
would, however, appear that complex monstrosities are induced more
frequently during a rather late than during a very early period of
embryonic life; but this may partly result from some one part, which
has been injured during an early period, affecting by its abnormal
growth other parts subsequently developed; and this would be less
likely to occur with parts injured at a later period.[653] When any
part or organ becomes monstrous through abortion, a rudiment is
generally left, and this likewise indicates that its development had
already commenced.
Insects sometimes have their antennae or legs in a monstrous condition,
and yet the larvae from which they are metamorphosed do not possess
either antennae or legs; and in those cases, as Quatrefages[654]
believes, we are enabled to see the precise period at which the normal
progress of development has been troubled. But the nature of the food
given to a caterpillar sometimes affects the colours of the moth,
without the caterpillar itself being affected; therefore it seems
possible that other characters in the mature insect might be indirectly
modified through the larvae. There is no reason to suppose that organs
which have been rendered monstrous have always been acted on during
their development; the cause may have acted on the organisation at a
much earlier stage. It is even probable that either the male or female
sexual elements, or both, before their union, may be affected in such a
manner as to lead to modifications in organs developed at a late period
of life; in nearly the same manner as a child may inherit from his
father a disease which does not appear until old age.
In accordance with the facts above given, which prove that in many
cases a close relation exists between variability and the sterility
following from changed conditions, we may conclude that the exciting
cause often acts at the earliest possible period, namely, on the sexual
elements, before impregnation has taken place. That an affection of the
female sexual element may induce variability we may likewise infer as
probable from the occurrence of bud-variations; for a bud seems to be
the analogue of an ovule. But the male element is apparently much
oftener affected by changed {270} conditions, at least in a visible
manner, than the female element or ovule; and we know from Gaertner's
and Wichura's statements that a hybrid used as the father and crossed
with a pure species gives a greater degree of variability to the
offspring, than does the same hybrid when used as the mother. Lastly,
it is certain that variability may be transmitted through either sexual
element, whether or not originally excited in them, for Koelreuter and
Gaertner[655] found that when two species were crossed, if either one
was variable, the offspring were rendered variable.
* * * * *
_Summary._--From the facts given in this chapter, we may conclude that the
variability of organic beings under domestication, although so general, is
not an inevitable contingent on growth and reproduction, but results from
the conditions to which the parents have been exposed. Changes of any kind
in the conditions of life, even extremely slight changes, often suffice to
cause variability. Excess of nutriment is perhaps the most efficient single
exciting cause. Animals and plants continue to be variable for an immense
period after their first domestication; but the conditions to which they
are exposed never long remain quite constant. In the course of time they
can be habituated to certain changes, so as to become less variable; and it
is possible that when first domesticated they may have been even more
variable than at present. There is good evidence that the power of changed
conditions accumulates; so that two, three, or more generations must be
exposed to new conditions before any effect is visible. The crossing of
distinct forms, which have already become variable, increases in the
offspring the tendency to further variability, by the unequal commingling
of the characters of the two parents, by the reappearance of long-lost
characters, and by the appearance of absolutely new characters. Some
variations are induced by the direct action of the surrounding conditions
on the whole organisation, or on certain parts alone, and other variations
are induced indirectly through the reproductive system being affected in
the same manner as is so common with organic beings when removed from their
natural conditions. The causes which induce variability act on the mature
organism, on the embryo, and, as we have good reason to believe, on both
sexual elements before impregnation has been effected.
* * * * *
{271}
CHAPTER XXIII.
DIRECT AND DEFINITE ACTION OF THE EXTERNAL CONDITIONS OF LIFE.
SLIGHT MODIFICATIONS IN PLANTS FROM THE DEFINITE ACTION OF CHANGED
CONDITIONS IN SIZE, COLOUR, CHEMICAL PROPERTIES, AND IN THE STATE OF
THE TISSUES--LOCAL DISEASES--CONSPICUOUS MODIFICATIONS FROM CHANGED
CLIMATE OR FOOD, ETC.--PLUMAGE OF BIRDS AFFECTED BY PECULIAR NUTRIMENT,
AND BY THE INOCULATION OF POISON--LAND-SHELLS--MODIFICATIONS OF ORGANIC
BEINGS IN A STATE OF NATURE THROUGH THE DEFINITE ACTION OF EXTERNAL
CONDITIONS--COMPARISON OF AMERICAN AND EUROPEAN TREES--GALLS--EFFECTS
OF PARASITIC FUNGI--CONSIDERATIONS OPPOSED TO THE BELIEF IN THE POTENT
INFLUENCE OF CHANGED EXTERNAL CONDITIONS--PARALLEL SERIES OF
VARIETIES--AMOUNT OF VARIATION DOES NOT CORRESPOND WITH THE DEGREE OF
CHANGE IN THE CONDITIONS--BUD-VARIATION--MONSTROSITIES PRODUCED BY
UNNATURAL TREATMENT--SUMMARY.
If we ask ourselves why this or that character has been modified under
domestication, we are, in most cases lost in utter darkness. Many
naturalists, especially of the French school, attribute every modification
to the "monde ambiant," that is, to changed climate, with all its
diversities of heat and cold, dampness and dryness, light and electricity,
to the nature of the soil, and to varied kinds and amount of food. By the
term definite action, as used in this chapter, I mean an action of such a
nature that, when many individuals of the same variety are exposed during
several generations to any change in their physical conditions of life,
all, or nearly all the individuals, are modified in the same manner. A new
sub-variety would thus be produced without the aid of selection.
I do not include under the term of definite action the effects of habit or
of the increased use and disuse of various organs. Modifications of this
nature, no doubt, are definitely caused by the conditions to which the
beings are subjected; but they depend much less on the nature of the
conditions than on the laws of growth; hence they are included under a
distinct head in the {272} following chapter. We know, however, far too
little of the causes and laws of variation to make a sound classification.
The direct action of the conditions of life, whether leading to definite or
indefinite results, is a totally distinct consideration from the effects of
natural selection; for natural selection depends on the survival under
various and complex circumstances of the best-fitted individuals, but has
no relation whatever to the primary cause of any modification of structure.
I will first give in detail all the facts which I have been able to
collect, rendering it probable that climate, food, &c., have acted so
definitely and powerfully on the organisation of our domesticated
productions, that they have sufficed to form new sub-varieties or races,
without the aid of selection by man or of natural selection. I will then
give the facts and considerations opposed to this conclusion, and finally
we will weigh, as fairly as we can, the evidence on both sides.
When we reflect that distinct races of almost all our domesticated animals
exist in each kingdom of Europe, and formerly even in each district of
England, we are at first strongly inclined to attribute their origin to the
definite action of the physical conditions of each country; and this has
been the conclusion of many authors. But we should bear in mind that man
annually has to choose which animals shall be preserved for breeding, and
which shall be slaughtered. We have also seen that both methodical and
unconscious selection were formerly practised, and are now occasionally
practised by the most barbarous races, to a much greater extent than might
have been anticipated. Hence it is very difficult to judge how far the
difference in conditions between, for instance, the several districts in
England, could have sufficed without the aid of selection to modify the
breeds which have been reared in each. It may be argued that, as numerous
wild animals and plants have ranged during many ages throughout Great
Britain, and still retain the same character, the difference in conditions
between the several districts could not have modified in so marked a manner
the various native races of cattle, sheep, pigs, and horses. The same
difficulty of distinguishing between selection and the definite effects of
the conditions of life, is encountered in a still higher degree when we
compare closely allied natural {273} forms, inhabiting two countries, such
as North America and Europe, which do not differ greatly in climate, nature
of soil, &c., for in this case natural selection will inevitably and
rigorously have acted during a long succession of ages.
From the importance of the difficulty just alluded to, it will be
advisable to give as large a body of facts as possible, showing that
extremely slight differences in treatment, either in different parts of
the same country, or during different seasons, certainly cause an
appreciable effect, at least on varieties which are already in an
unstable condition. Ornamental flowers are good for this purpose, as
they are highly variable, and are carefully observed. All
floriculturists are unanimous that certain varieties are affected by
very slight differences in the nature of the artificial compost in
which they are grown, and by the natural soil of the district, and by
the season. Thus, a skilful judge, in writing on Carnations and
Picotees,[656] asks "where can Admiral Curzon be seen possessing the
colour, size, and strength which it has in Derbyshire? Where can
Flora's Garland be found equal to those at Slough? Where do
high-coloured flowers revel better than at Woolwich and Birmingham? Yet
in no two of these districts do the same varieties attain an equal
degree of excellence, although each may be receiving the attention of
the most skilful cultivators." The same writer then recommends every
cultivator to keep five different kinds of soil and manure, "and to
endeavour to suit the respective appetites of the plants you are
dealing with, for without such attention all hope of general success
will be vain." So it is with the Dahlia:[657] the Lady Cooper rarely
succeeds near London, but does admirably in other districts; the
reverse holds good with other varieties; and again, there are others
which succeed equally well in various situations. A skilful
gardener[658] states that he procured cuttings of an old and well-known
variety (pulchella) of Verbena, which from having been propagated in a
different situation presented a slightly different shade of colour; the
two varieties were afterwards multiplied by cuttings, being carefully
kept distinct; but in the second year they could hardly be
distinguished, and in the third year no one could distinguish them.