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 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 Linnaeus 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. (22/1. 'Des Jacinthes' etc. Amsterdam
1768 page 43; Verlot 'Des Varietes' etc. page 86. On the reindeer see Linnaeus
'Tour in Lapland' translated by Sir J.E. Smith volume 1 page 314. The
statement in regard to German shepherds is given on the authority of Dr.
Weinland.)
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 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 (22/2. Muller 'Physiology' English
translation, volume 2 page 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 Medicale' tome 1 page 523, in which a
curious case is recorded:--"J'ai donne mes soins a deux freres jumeaux, tous
deux si extraordinairement ressemblants qu'il m'etait impossible de les
reconnaitre, a moin de les voir l'un a cote de l'autre. Cette ressemblance
physique s'etendait plus loin: ils avaient, permettez-moi l'expression, une
similitude pathologique plus remarquable encore. Ainsi l'un d'eux que je
voyais aux neothermes a 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 frere alors a Vienne, et qui lui ecrivait en effet--'J'ai
mon ophthalmie, tu dois avoir la tienne.' Quelque singulier que ceci puisse
paraitre, le fait n'en est pas moins exact: on ne me l'a pas raconte, je l'ai
vu, et j'en ai vu d'autres analogues dans ma pratique. Ces deux jumeaux
etaient aussi tous deux asthmatiques, et asthmatiques a un effroyable degre.
Originaires de Marseille, ils n'ont jamais pu demeurer dans cette ville, ou
leurs interets 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 etre gueris de leurs attaques de Marseille. Voyageant sans cesse et dans
tous pays pour leurs affaires, ils avaient remarque que certaines localites
leur etaient funestes, que dans d'autres ils etaient exempts de tout phenomene
d'oppression.") 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 (22/3. Isid. Geoffroy St.-Hilaire 'Hist. des Anomalies' tome 3 page
352; Moquin-Tandon 'Teratologie Vegetale' 1841 page 115.); 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 (22/4.
Metzger 'Die Getreidarten' 1841 s. 39.); 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. (22/5. On the date-palm see Vogel 'Annals and Mag.
of Nat. Hist.' 1854 page 460. On Indian varieties Dr. F. Hamilton 'Transact.
Linn. Soc.' volume 14 page 296. On the varieties cultivated in Tahiti see Dr.
Bennett in Loudon's 'Mag. of N. Hist.' volume 5 1832 page 484. Also Ellis
'Polynesian Researches' volume 1 pages 370, 375. On twenty varieties of the
Pandanus and other trees in the Marianne Island see 'Hooker's Miscellany'
volume 1 page 308. On the bamboo in China see Huc 'Chinese Empire' volume 2
page 307.) 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 (22/6. 'Treatise on the Culture of the Apple' etc. page 3.)
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. (22/7. Gallesio 'Teoria
della Riproduzione Veg.' page 125.) 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. (22/8. See Dr. Hooker's Memoir on
Arctic Plants in 'Linn. Transact.' volume 23 part 2. Mr. Woodward, and a
higher authority cannot be quoted, speaks of the Arctic mollusca in his
'Rudimentary Treatise' 1856 page 355 as remarkably subject to variation.)
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 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. (22/9. Bechstein in his 'Naturgeschichte der Stubenvogel' 1840 s. 238,
has some good remarks on this subject. He states that his canary-birds varied
in colour, though kept on uniform food.)
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. (22/10. 'The
Plant' by Schleiden translated by Henfrey 1848 page 169. See also Alex. Braun
in 'Bot. Memoirs' Ray Soc. 1853 page 313.) 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 (22/11. Messrs.
Hardy and Son of Maldon in 'Gardener's Chronicle' 1856 page 458. Carriere
'Production et Fixation des Varietes' 1865 page 31.): "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 Carriere, who
has had great experience with flower-garden seeds, "On remarque en general les
plantes de vigeur moyenne sont celles qui conservent le mieux leurs
caracteres."
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 (Crataegus 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 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 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 (22/12.
'Quadrupedes du Paraguay' 1801 tome 2 page 319.) 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. (22/13. M'Clelland on Indian Cyprinidae 'Asiatic Researches'
volume 19 part 2 1839 pages 266, 268, 313.)
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. (22/14. Quoted by
Sageret 'Pom. Phys.' 1830 page 43. This statement, however, is not believed by
Decaisne.) 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 (22/15. 'The
Fruits of America' 1845 page 5.), "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. (22/16. M. Cardan in 'Comptes Rendus' December 1848 quoted in
'Gardener's Chronicle' 1849 page 101.) 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." (22/17. M. Alexis Jordan mentions four excellent
pears found in woods in France, and alludes to others ('Mem. Acad. de Lyon'
tome 2 1852 page 159). Poiteau's remark is quoted in 'Gardener's Mag.' volume
4 1828 page 385. See 'Gardener's Chronicle' 1862 page 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' page 901. Mr. Rivers has given me
similar information.) 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 (22/18. Duval 'Hist. du Poirier' 1849 page 2.), 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 (22/19. I infer that this is the fact from
Van Mons' statement ('Arbres Fruitiers' 1835 tome 1 page 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.) 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. (22/20. Downing 'Fruit-trees of North America' page 422;
Foley in 'Transact. Hort. Soc.' volume 6 page 412.)
With respect to wheat, some writers have spoken (22/21. 'Gardener's Chronicle'
1847 page 244.) 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. (22/22. 'Gardener's Chronicle' 1841 page 383; 1850 page 700;
1854 page 650.)]
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 (22/23. 'Die Getreidearten' 1843 s.
66, 116, 117.), 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. (22/24.
Sabine in 'Hort. Transact.' volume 3 page 225; Bronn 'Geschichte der Natur' b.
2 s. 119.) 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."
(22/25. 'Journal of Horticulture' 1861 page 112; on Zinnia 'Gardener's
Chronicle' 1860 page 852.) 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 (22/26.
'The Chrysanthemum, its History, etc.' 1865 page 3.) 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 (22/27.
'Gardener's Chronicle' 1855 page 54; 'Journal of Horticulture' May 9, 1865
page 363.), 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 (22/28. Quoted by Verlot 'Des Varietes' etc. 1865 page 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 (22/29. 'Examination of the
Characteristics of Genera and Species' Charleston 1855 page 14.) 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 (22/30. Mr. Hewitt 'Journal of Hort.' 1863 page 39.), 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 (22/31. Devay 'Mariages Consanguins' pages 97, 125. In conversation I
have found two or three naturalists of the same opinion.) 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. (22/32. Muller has
conclusively argued against this belief, 'Elements of Phys.' English
translation volume 2 1842 page 1405.) 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 (22/33. 'Act. Acad.
St. Petersburg' 1780 part 2 page 84 etc.) 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.
[Gartner declares (22/34. 'Bastarderzeugung' s. 249, 255, 295.), 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
Kolreuter, 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. (22/35. 'Nova Acta, St. Petersburg' 1794 page 378; 1795
pages 307, 313, 316; 1787 page 407.)
Prof. Lecoq (22/36. 'De la Fecondation' 1862 page 311.) 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 (22/37.
'Amaryllidaceae' 1837 page 362.) 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. Carriere (22/38. Abstracted in
'Gardener's Chronicle' 1860 page 1081.) 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 (22/39. This was the opinion of the elder De Candolle, as quoted in
'Dic. Class. d'Hist. Nat.' tome 8 page 405. Puvis in his work 'De la
Degeneration' 1837 page 37, has discussed this same point.) 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 (22/40. 'Comptes Rendus' Novembre 21, 1864 page 838.)
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 the first generation
are generally uniform, but those subsequently produced display an almost
infinite diversity of character. He who wishes, says Kolreuter (22/41. 'Nova
Acta, St. Petersburg' 1794 page 391.), 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 Gartner (22/42.
'Bastarderzeugung' s. 507, 516, 572.), on whose authority the foregoing
statements are made, never succeeded in effecting a union; but Max Wichura
(22/43. 'Die Bastardbefruchtung' etc. 1865 s. 24.) 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 Gartner (22/44.
'Bastarderzeugung' s. 452, 507.) 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 (22/45. 'Die
Bastardbefruchtung' s. 56.) insists strongly on an analogous result with his
hybrid willows. Again Gartner (22/46. 'Bastarderzeugung' s. 423.) 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.
Kolreuter (22/47. 'Dritte Fortsetzung' etc. 1766 s. 85.) 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 (22/48. 'Die Bastardbefruchtung' etc. 1865 s. 92: see also the
Rev. M.J. Berkeley on the same subject in 'Journal of Royal Hort. Soc.' 1866
page 80.) 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.
(22/49. Dr. P. Lucas has given a history of opinion on this subject 'Hered.
Nat.' 1847 tome 1 page 175.) 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 (22/50. 'Hist. des Anomalies' tome 3 page 499.) 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. (22/51. Ibid tome 3
pages 392, 502. The several memoirs by M. Dareste hereafter referred to are of
special value on this whole subject.) 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 (22/52. See his interesting work 'Metamorphoses de l'Homme' etc.
1862 page 129.) 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 Gartner'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 Kolreuter and Gartner (22/53. 'Dritte
Fortsetzung' etc. s. 123; 'Bastarderzeugung' s. 249.) 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.