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Tue, Apr 29 2008 100 Causes of Variability As far as I am able to judge, after long attending to the
subject, the conditions of life appear to act in two ways,-
directly on the whole organization or on certain parts alone,
and indirectly by affecting the reproductive system.

With respect to the direct action, we must bear in mind that
in every case, as Professor Weismann has lately insisted,
and as I have incidentally shown in my work on Variation
under Domestication, there are two factors: namely, the
nature of the organism, and the nature of the conditions.
August Weismann
August Weismann


The former seems to be much the more important; for nearly
similar variations sometimes arise under, as far as we can
judge, dissimilar conditions; and, on the other hand,
dissimilar variations arise under conditions which appear to
be nearly uniform.
The effects on the offspring are either definite or indefinite.
They may be considered as definite when all or nearly all the
offspring of individuals exposed to
certain conditions during several generations are modified in
the same manner.
It is extremely difficult to come to any
conclusion in regard to the extent of the changes which have
been thus definitely induced.

There can, however, be little doubt about many slight changes,-
such as size from the amount of food, colour from the nature
of the food, thickness of the skin and hair from climate, &c.

Each of the endless variations which we see in the plumage
of our fowls must have had some efficient cause; and if the
same cause were to act uniformly during a long series of
generations on. many individuals, all probably would be
modified in the same manner.
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Such facts as the complex and extraordinary out-growths
which variably follow from the insertion of a minute drop of
poison by a gall-producing insect, show us what singular
modifications might result in the case of plants from a
chemical change in the nature of the sap. Indefinite variability
is a much more common result of changed conditions than
definite variability, and has probably played a more important
part in the formation of our domestic races.
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We see indefinite variability in the endless slight peculiarities
which distinguish the individuals of the same species,
and which cannot be accounted for by inheritance from
either parent or from some more remote ancestor.

Even strongly marked differences occasionally appear in the
young of the same litter, and in seedlings from the same
seed-capsule.
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At long intervals of time, out of millions of individuals reared in
the same country and fed on nearly the same food, deviations
of structure so strongly pronounced as to deserve to be called
monstrosities arise;
but monstrosities cannot be separated by any distinct line from slighter variations.
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All such changes of structure, whether extremely slight or
strongly marked, which appear amongst many individuals
living together, may be considered as the indefinite effects of
the conditions of life on each individual organism, in nearly
the same manner as the chill affects different men in an
indefinite manner, according to their state of body or
constitution, causing coughs or colds, rheumatism,
or inflammation of various organs.
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With respect to what I have called the indirect action of
changed conditions, namely, through the reproductive system
of being affected, we may infer that variability is thus induced,
partly from the fact of this system being extremely sensitive
to any change in the conditions, and partly from the
similarity, as Kreuter and others have remarked, between the
variability which follows from the crossing of distinct species,
and that which may be observed with plants and animals
when reared under new or unnatural conditions.
Many facts clearly show how eminently susceptible the
reproductive system is to very slight changes in the surrounding conditions.
Nothing is more easy than to tame an animal,
and few things more difficult than to get it to breed freely
under confinement, even when the male and female unite.

How many animals there are which will not breed, though
kept in an almost free state in their native country!
This is generally, but erroneously, attributed to vitiated
instincts.

Many cultivated plants display the utmost vigour, and yet
rarely or never seed! In some few cases it has been
discovered that a very trifling change, such as a little more or
less water at some particular period of growth, will determine
whether or not a plant will produce seeds.
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I cannot here give the details which I have collected and
elsewhere published on this curious subject; but to show how
singular the laws are which determine the reproduction of
animals under confinement, I may mention that carnivorous
animals, even from the tropics, breed in this country pretty
freely under confinement, with the exception of the
plantigrades or bear family, which seldom produce young;
whereas carnivorous birds, with the rarest exceptions,
hardly ever lay fertile eggs.
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Many exotic plants have pollen utterly worthless,
in the same condition as in the most sterile hybrids.
When, on the one hand, we see domesticated animals and
plants, though often weak and sickly, breeding freely under
confinement; and when, on the other hand, we see
individuals, though taken young from a state of nature
perfectly tamed, long-lived and healthy
(of which I could give numerous instances),
yet having their reproductive system so seriously affected by
unperceived causes as to fail to act, we need not be
surprised at this system, when it does act under
confinement, acting irregularly, and producing offspring
somewhat unlike their parents.
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Causes of Variability
Tue, Apr 29 2008 200 Causes of Variability I may add, that as some organisms breed freely under the most unnatural conditions
(for instance, rabbits and ferrets kept in hutches),
showing that their reproductive organs are not easily affected;

rabbit
rabbit

ferret
ferret


so will some animals and plants withstand domestication or
cultivation, and vary very slightly- perhaps hardly more than in
a state of nature. Some naturalists have maintained that all
variations are connected with the act of sexual reproduction;

but this is certainly an error; for I have given in another work a
long list of "sporting plants," as they are called by gardeners;-
that is, of plants which have suddenly produced a single bud
with a new and sometimes widely different character from that
of the other buds on the same plant.
These bud variations, as they may be named, can be
propagated by grafts, offsets, &c., and sometimes by seed.

graft
graft


They occur rarely under nature, but are far from rare under culture.
As a single bud out of the many thousands, produced year
after year on the same tree under uniform conditions, has
been known suddenly to assume a new character; and as
buds on distinct trees, growing under different conditions,
have sometimes yielded nearly the same variety- for instance,
buds on peach-trees producing nectarines, and buds on
common roses producing moss-roses- we clearly see that
the nature of the conditions is of subordinate importance in
comparison with the nature of the organism in determining
each particular form of variation;-
bud
bud

tree
tree

peach
peach

nectarine
nectarine

rose
rose

Moss Rose
Moss Rose


perhaps of not more importance than the nature of the spark,
by which a mass of combustible matter is ignited, has in
determining the nature of the flames.
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Causes of Variability