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 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 most cases in a far higher degree on the nature or constitution of
the being, than on the nature of the changed conditions.
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 Linnæus remarks,
“to distinguish one from another among such multitudes was beyond my
comprehension, for they were like ants on an anthill.” 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.[1]
As Linnæus 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
assafœtida, 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
recognised, 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.
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[2] that an innate tendency to vary exists, independently of
external differences, 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 to affect the character of the future plant. The greater
dissimilarity of the successive children of the same family in
comparison with 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 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, and is not the result of
what we blindly call accident. The following striking case has been
communicated to me by Dr. William Ogle. 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 of the second
dentition on the right side in the upper jaw was misplaced; for,
instead of standing in a line with the others, it grew from the roof of
the mouth behind the first bicuspid. Neither the parents nor any other
members of the family were known to have exhibited any similar
peculiarity; but a son of one of these girls had the same tooth
similarly misplaced. Now, as both the girls were affected in exactly
the same manner, 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
given crooked fingers and misplaced bicuspid teeth to a hundred
children. It is of course possible that this case may have been due to
reversion to some long-forgotten progenitor, and this would much weaken
the value of the argument. I have been led to think of the probability
of reversion, from having been told by Mr. Galton of another case of
twin girls born with their little fingers slightly crooked, which they
inherited from their maternal grandmother.
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 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;[3] 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 rarely remain during any 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;[4] 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 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 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 varied. We
see this with the many domestic races of quadrupeds and birds belonging
to different orders, with goldfish 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.[5]
These facts, and innumerable others which 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[6] 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.[7] 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.[8]
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 ‘Géographie 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.[9]
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.[10] 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 species flourish when planted by themselves, though not in
highly manured land; separate growth 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:[11] “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.” According also to Carrière, who
has had great experience with flower-garden seeds, “On remarque en
général les plantes de vigeur moyenne sont celles qui conservent le
mieux leurs caractères.”
In the case of animals the want of a proper amount of exercise, as
Bechstein 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, 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 (_Cratægus oxycantha_) 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 phæum_ 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 flower and foliage, to a
degree which 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[12]
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.[13]
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 a greater number of varieties
than do the seeds of the same variety of pear when grafted on the wild
pear.[14] 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; as we are here enabled to see the cause, namely, the
very different nature of the stock and graft. Several North American
varieties of the plum and peach are well known to reproduce themselves
truly by seed; but Downing asserts,[15] “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, even as late
as the 5th of June.[16] Such facts as these are well fitted to show on
what obscure and slight causes variability depends.
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.”[17] 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,[18] 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[19] 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.[20]
With respect to wheat, some writers have spoken[21] 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 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.[22]
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,[23] 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.[24] 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.”[25] Analogous
facts have been recorded with the Scotch rose. In discussing the
variability of plants several experienced horticulturists have spoken
to the same general effect. Mr. Salter[26] 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,[27] 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,[28] 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[29] 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,[30] who has often reared ducks from
the eggs of the wild bird, and who took precautions 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[31] 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 induced; and there can be no doubt
that close interbreeding causes 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 the womb.[32] 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[33] 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,
etc., 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, certain that the breeds of
various animals, such as of the rabbit, pigeon, duck, etc., 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
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 absolutely new characters. Practically, whether new or old, they
would be new to the breed in which they reappeared.
Gärtner declares,[34] 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 Kölreuter, 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.[35]
Prof. Lecoq[36] 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[37]
certain hybrid Rhododendrons 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, etc., induces excessive variability; hence the
appearance of quite new characters is probable. M. Carrière[38] 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
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[39] 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 monotypic genera when cultivated is generally less than
that of genera including numerous species, and this quite independently
of the effects of crossing. I have shown in my ‘Origin of Species’ 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[40] crossed _Datura lævis_ 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 the first generation are generally uniform, but those subsequently
produced display an almost infinite diversity of character. He who
wishes, says Kölreuter,[41] 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 Gärtner,[42] on whose authority the foregoing statements are
made, never succeeded in effecting a union; but Max Wichura[43] 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 Gärtner[44] repeatedly found that when a
hybrid was used as a 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[45] insists strongly on an
analogous result with his hybrid willows. Again Gärtner[46] 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 such 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 consider, whether inherited variations are due to certain parts
being acted on after they have been formed, or through the reproductive
system being affected before their formation; and in the former case at
what period of growth or development the effect is produced. 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, etc., prolonged during several generations,
certainly modify either the whole organisation or certain organs; and
it is clear at least in the case of bud-variation that 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
beings generated through 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 harmonises with the apparently capricious manner and degree in
which various groups of animals and plants vary under domestication.
Kölreuter[47] 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[48] has gone one step farther, and shows that with many of our
highly cultivated plants, such as the hyacinth, tulip, auricula,
snapdragon, potato, cabbage, etc., which there is no reason to believe
have been hybridised, 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,
pineapple, bread-fruit, 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 seeds, 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.
The period of life at which the causes that induce variability act, is
likewise an obscure subject, which has been discussed by various
authors.[49] 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 Saint-Hilaire[50] 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.[51] 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,
the larvae of which do not possess either antennae or legs; and in
these cases, as Quatrefages[52] believes, we are enabled to see the
precise period at which the normal progress of development was
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 conditions, at least in a visible manner,
than the female element or ovule and we know from Gärtner’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 Kölreuter and
Gärtner[53] 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 life, 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; other variations appear to be
induced indirectly through the reproductive system being affected, as
we know is often the case with various beings, which when removed from
their natural conditions become sterile. The causes which induce
variability act on the mature organism, on the embryo, and, probably,
on the sexual elements before impregnation has been effected.
REFERENCES
[1] ‘Des Jacinthes,’ etc., Amsterdam, 1768, p. 43; Verlot, ‘Des
Variétés,’ etc., p. 86. On the reindeer _see_ Linnæus, ‘Tour in
Lapland,’ translated by Sir J. E. Smith, vol. i. p. 314. The statement
in regard to German shepherds is given on the authority of Dr.
Weinland.
[2] Müller’s ‘Physiology,’ Eng. translation, vol. ii. p. 1662. With
respect to the similarity of twins in constitution, Dr. William Ogle
has given me the following extract from Professor Trousseau’s Lectures
(‘Clinique Médicale,’ tom. i.1 p. 523), in which a curious case is
recorded:—“J’ai donné mes soins à deux frères jumeaux, tous deux si
extraordinairement ressemblants qu’il m’était impossible de les
reconnaitre, à moin de les voir l’un à côté de l’autre. Cette
ressemblance physique s’étendait plus loin: ils avaient, permettez-moi
l’expression, une similitude pathologique plus remarquable encore.
Ainsi l’un d’eux que je voyais aux néothermes à Paris malade d’une
ophthalmie rhumatismale me disait, ‘En ce moment mon frere doit avoir
une ophthalmie comme la mienne;’ et comme je m’etais recrie, il me
montrait quelques jours apres une lettre qu’il venait de recevoir de
ce frère alors à Vienne, et qui lui écrivait en effet—‘J’ai mon
ophthalmie, tu dois avoir la tienne.’ Quelque singulier que ceci
puisse paraître, le fait n’en est pas moins exact: on ne me l’a pas
raconté, je l’ai vu, et j’en ai vu d’autres analogues dans ma
pratique. Ces deux jumeaux étaient aussi tous deux asthmatiques, et
asthmatiques a un effroyable degré. Originaires de Marseille, ils
n’ont jamais pu demeurer dans cette ville, ou leurs intérêts les
appelaient souvent, sans etre pris de leurs acces; jamais ils n’en
eprouvaient a Paris. Bien mieux, il leur suffisait de gagner Toulon
pour être guéris de leurs attaques de Marseille. Voyageant sans cesse
et dans tous pays pour leurs affaires, ils avaient remarque que
certaines localités leur étaient funestes, que dans d’autres ils
etaient exempts de tout phénomène d’oppression.”
[3] Isid. Geoffroy St.-Hilaire, ‘Hist. des Anomalies,’ tom. iii. p.
352; Moquin-Tandon, ‘Tératologie Végétale,’ 1841, p. 115.
[4] Metzger, ‘Die Getreidarten,’ 1841, s. 39.
[5] On the date-palm _see_ Vogel, ‘Annals and Mag. of Nat. Hist.,’
1854, p. 460. On Indian varieties, Dr. F. Hamilton, ‘Transact. Linn.
Soc.,’ vol. xiv. p. 296. On the varieties cultivated in Tahiti, _see_
Dr. Bennett, in Loudon’s ‘Mag. of N. Hist.,’ vol. v. 1832, p. 484.
Also Ellis, ‘Polynesian Researches,’ vol. i. pp. 370, 375. On twenty
varieties of the Pandanus and other trees in the Marianne Island,
_see_ ‘Hooker’s Miscellany,’ vol. i. p. 308. On the bamboo in China,
_see_ Huc’s ‘Chinese Empire,’ vol. ii. p. 307.
[6] ‘Treatise on the Culture of the Apple,’ etc., p. 3.
[7] Gallesio, ‘Teoria della Riproduzione Veg.,’ p. 125.
[8] _See_ Dr. Hooker’s Memoir on Arctic Plants in ‘Linn. Transact.,’
vol. xxiii. part ii. Mr. Woodward, and a higher authority cannot be
quoted, speaks of the Arctic mollusca (in his ‘Rudimentary Treatise,’
1856, p. 355) as remarkably subject to variation.
[9] Bechstein, in his ‘Naturgeschichte der Stubenvögel,’ 1840, s. 238,
has some good remarks on this subject. He states that his canary-birds
varied in colour, though kept on uniform food.
[10] ‘The Plant,’ by Schleiden, translated by Henfrey, 1848, p. 169.
_See also_ Alex. Braun, in ‘Bot. Memoirs,’ Ray Soc., 1853, p. 313.
[11] Messrs. Hardy and Son, of Maldon, in ‘Gardener’s Chronicle,’
1856, p. 458. Carrière, ‘Production et Fixation des Variétés,’ 1865,
p. 31.
[12] ‘Quadrupedes du Paraguay,’ 1801, tom. ii. p. 319.
[13] M’Clelland on Indian Cyprinidæ, ‘Asiatic Researches,’ vol. xix.
part ii., 1839, pp. 266, 268, 313.
[14] Quoted by Sageret, ‘Pom. Phys.,’ 1830, p. 43. This statement,
however, is not believed by Decaisne.
[15] ‘The Fruits of America,’ 1845, p. 5.
[16] M. Cardan, in ‘Comptes Rendus,’ Dec. 1848, quoted in ‘Gardener’s
Chronicle,’ 1849, p. 101.
[17] M. Alexis Jordan mentions four excellent pears found in woods in
France, and alludes to others (‘Mém. Acad. de Lyon,’ tom. ii. 1852, p.
159). Poiteau’s remark is quoted in ‘Gardener’s Mag.,’ vol. iv., 1828,
p. 385. _See_ ‘Gardener’s Chronicle,’ 1862, p. 335, for another case
of a new variety of the pear found in a hedge in France. Also for
another case, _see_ Loudon’s ‘Encyclop. of Gardening,’ p. 901. Mr.
Rivers has given me similar information.
[18] Duval, ‘Hist. du Poirier,’ 1849, p. 2.
[19] I infer that this is the fact from Van Mons’ statement (‘Arbres
Fruitiers,’ 1835, tom. i. p. 446) that he finds in the woods seedlings
resembling all the chief cultivated races of both the pear and apple.
Van Mons, however, looked at these wild varieties as aboriginal
species.
[20] Downing, ‘Fruit-trees of North America,’ p. 422; Foley, in
‘Transact. Hort. Soc.,’ vol. vi. p. 412.
[21] ‘Gardener’s Chronicle,’ 1847, p. 244.
[22] ‘Gardener’s Chronicle,’ 1841, p. 383; 1850, p. 700; 1854, p. 650.
[23] ‘Die Getreidearten,’ 1843, s. 66, 116, 117.
[24] Sabine, in ‘Hort. Transact.,’ vol. iii. p. 225; Bronn,
‘Geschichte der Natur,’ b. ii. s. 119.
[25] ‘Journal of Horticulture,’ 1861, p. 112; on Zinnia, ‘Gardener’s
Chronicle,’ 1860, p. 852.
[26] ‘The Chrysanthemum, its History, etc.,’ 1865, p. 3.
[27] ‘Gardener’s Chronicle,’ 1855, p. 54; ‘Journal of Horticulture,’
May 9, 1865, p. 363.
[28] Quoted by Verlot, ‘Des Variétés,’ etc., 1865, p. 28.
[29] ‘Examination of the Characteristics of Genera and Species,’
Charleston, 1855, p. 14.
[30] Mr. Hewitt, ‘Journal of Hort.,’ 1863, p. 39.
[31] Devay, ‘Mariages Consanguins,’ pp. 97, 125. In conversation I
have found two or three naturalists of the same opinion.
[32] Müller has conclusively argued against this belief, ‘Elements of
Phys.,’ Eng. translat., vol. ii. 1842, p. 1405.
[33] ‘Act. Acad. St. Petersburg,’ 1780, part ii. p. 84, etc.
[34] ‘Bastarderzeugung,’ s. 249, 255, 295.
[35] ‘Nova Acta, St. Petersburg,’ 1794, p. 378; 1795, pp. 307, 313,
316; 1787, p. 407.
[36] ‘De la Fécondation,’ 1862, p. 311.
[37] ‘Amaryllidaceæ,’ 1837, p. 362.
[38] Abstracted in ‘Gardener’s Chronicle,’ 1860, p. 1081.
[39] This was the opinion of the elder De Candolle, as quoted in ‘Dic.
Class. d’Hist. Nat.,’ tom. viii. p. 405. Puvis, in his work, ‘De la
Dégénération,’ 1837, p. 37, has discussed this same point.
[40] ‘Comptes Rendus,’ Novembre 21, 1864, p. 838.
[41] ‘Nova Acta, St. Petersburg,’ 1794, p. 391.
[42] ‘Bastarderzeugung,’ s. 507, 516, 572.
[43] ‘Die Bastardbefruchtung,’ etc., 1865, s. 24.
[44] ‘Bastarderzeugung,’ s. 452, 507.
[45] ‘Die Bastardbefruchtung,’ s. 56.
[46] ‘Bastarderzeugung,’ s. 423.
[47] ‘Dritte Fortsetzung,’ etc., 1766, s. 85.
[48] ‘Die Bastardbefruchtung,’ etc., 1865, s. 92: _see also_ the Rev.
M. J. Berkeley on the same subject, in ‘Journal of Royal Hort. Soc.,’
1866, p. 80.
[49] Dr. P. Lucas has given a history of opinion on this subject:
‘Héréd. Nat.,’ 1847, tom. i. p. 175.
[50] ‘Hist. des Anomalies,’ tom. iii. p. 499.
[51] Ibid., tom. iii. pp. 392, 502. The several memoirs by M. Dareste
hereafter referred to are of special value on this whole subject.
[52] _See_ his interesting work, ‘Métamorphoses de l’Homme,’ etc.,
1862, p. 129.
[53] ‘Dritte Fortsetzung,’ etc., s. 123; ‘Bastarderzeugung’ s. 249.