The Book that Upset Tennessee (Nov. 24)

The Book that Upset Tennessee
The signal for the beginning of a great controversy, still raging,
was the publication of Darwin’s "Origin of Species." This was
the first complete statement of the evolution theory, which had
been privately advanced but never publicly taught. A new epoch
in science dates from this great work.
("Origin of Species" published Nov. 24, 1859.)
Read from Darwin’s ORIGIN OF SPECIES Vol. I I , pp. 23-30

 

ORIGIN OF SPECIES
CHAPTER I
VARIATION UNDER DOMESTICATION
Causes of variability—Effects of habit and the use or disuse of parts—
Correlated variation—Inheritance—Character of domestic varieties
—Difficulty of distinguishing between varieties and species—Origin
of domestic varieties from one or more species—Domestic pigeons,
their differences and origin—Principles of selection, anciently followed,
their effects—Methodical and unconscious selection—Unknown
origin of our domestic productions—Circumstances favourable
to man’s power of selection.

CAUSES OF VARIABILITY

WHEN we compare the individuals of the same variety or
sub-variety of our older cultivated plants and animals, one
of the first points which strikes us is, that they generally differ
more from each other than do the individuals of any one species
or variety in a state of nature. And if we reflect on the vast diversity
of the plants and animals which have been cultivated, and which have
varied during all ages under the most different climates and treatment,
we are driven to conclude that this great variability is due to
our domestic productions having been raised under conditions of life
not so uniform as, and somewhat different from, those to which the
parent species had been exposed under nature. There is, also, some
probability in the view propounded by Andrew Knight, that this
variability may be partly connected with excess of food. It seems
clear that organic beings must be exposed during several generations
to new conditions to cause any great amount of variation; and that,
when the organisation has once begun to vary, it generally continues
varying for many generations. No case is on record of a variable organism
ceasing to vary under cultivation. Our oldest cultivated
plants, such as wheat, still yield new varieties: our oldest domesticated
animals are still capable of rapid improvement or modification.
23

24 ORIGIN OF SPECIES
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 organisation 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. 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, etc. 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. Such facts as the complex and extraordinary
outgrowths 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. 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.
At long intervals of time, out of millions of individuals reared in the

VARIATION UNDER DOMESTICATION 25
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. 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.
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 Kolreuter 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. 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. Many exotic

26 ORIGIN OF SPECIES
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. 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; 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,
etc., and sometimes by seed. 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 condition is of subordinate importance in comparison with
the nature of the organism in determining each particular form of
variation; 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.

VARIATION UNDER DOMESTICATION 2 7
EFFECTS OF HABIT AND OF THE USE OR DISUSE OF PARTS;
CORRELATED VARIATION; INHERITANCE
Changed habits produce an inherited effect, as in the period of the
flowering of plants when transported from one climate to another.
With animals the increased use or disuse of parts has had a more
marked influence; thus I find in the domestic duck that the bones of
the wing weigh less and the bones of the leg more, in proportion to
the whole skeleton, than do the same bones in the wild duck; and this
change may be safely attributed to the domestic duck flying much
less, and walking more, than its wild parents. The great and inherited
development of the udders in cows and goats in countries
where they are habitually milked, in comparison with these organs
in other countries, is probably another instance of the effects of use.
Not one of our domestic animals can be named which has not in
some country drooping ears; and the view which has been suggested
that the drooping is due to disuse of the muscles of the ear, from the
animals being seldom much alarmed, seems probable.
Many laws regulate variation, some few of which can be dimly
seen, and will hereafter be briefly discussed. I will here only allude
to what may be called correlated variation. Important changes in the
embryo or larva will probably entail changes in the mature animal.
In monstrosities, the correlations between quite distinct parts are
very curious; and many instances are given in Isidore Geoffroy St.
Hilaire’s great work on this subject. Breeders believe that long limbs
are almost always accompanied by an elongated head. Some instances
of correlation are quite whimsical: thus cats which are entirely white
and have blue eyes are generally deaf; but it has been lately stated
by Mr. Tait that this is confined to the males. Colour and constitutional
peculiarities go together, of which many remarkable cases
could be given amongst animals and plants. From facts collected by
Heusinger, it appears that white sheep and pigs are injured by certain
plants, whilst dark-coloured individuals escape: Professor
Wyman has recently communicated to me a good illustration of this
fact; on asking some farmers in Virginia how it was that all their
pigs were black, they informed him that the pigs ate the paintroot
(Lachnanthes), which colored their bones pink, and which caused the
hoofs of all but the black varieties to drop off; and one of the

28 ORIGIN OF SPECIES
"crackers" (ix., Virginia squatters) added, "We select the black
members of a litter for raising, as they alone have a good chance of
living." Hairless dogs have imperfect teeth; long-haired and coarsehaired
animals are apt to have, as is asserted, long or many horns;
pigeons with feathered feet have skin between their outer toes;
pigeons with short beaks have small feet, and those with long beaks
large feet. Hence if man goes on selecting, and thus augmenting, any
peculiarity, he will almost certainly modify unintentionally other
parts of the structure, owing to the mysterious laws of correlation.
The results of the various, unknown, or but dimly understood laws
of variation are infinitely complex and diversified. It is well worth
while carefully to study the several treatises on some of our old cultivated
plants, as on the hyacinth, potato, even the dahlia, etc.; and it is
really surprising to note the endless points of structure and constitution
in which the varieties and sub-varieties differ slightly from
each other. The whole organisation seems to have become plastic,
and departs in a slight degree from that of the parental type.
Any variation which is not inherited is unimportant for us. But
the number and diversity of inheritable deviations of structure, both
those of slight and those of considerable physiological importance,
are endless. Dr. Prosper Lucas’s treatise, in two large volumes, is
the fullest and the best on this subject. No breeder doubts how strong
is the tendency to inheritance; that like produces like, is his fundamental
belief: doubts have been thrown on this principle only by
theoretical writers. When any deviation of structure often appears,
and we see it in the father and child, we cannot tell whether it may
not be due to the same cause having acted on both; but when amongst
individuals, apparently exposed to the same conditions, any very rare
deviation, due to some extraordinary combination of circumstances,
appears in the parent—say, once amongst several million individuals
—and it reappears in the child, the mere doctrine of chances almost
compels us to attribute its reappearance to inheritance. Every one
must have heard of cases of albinism, prickly skin, hairy bodies, etc.,
appearing in several members of the same family. If strange and
rare deviations of structure are really inherited, less strange and commoner
deviations may be freely admitted to be inheritable. Perhaps
the correct way of viewing the whole subject would be, to look

VARIATION UNDER DOMESTICATION 29
at the inheritance of every character whatever as the rule, and noninheritance
as the anomaly.
The laws governing inheritance are for the most part unknown.
No one can say why the same peculiarity in different individuals of
the same species, or in different species, is sometimes inherited and
sometimes not so; why the child often reverts in certain characters to
its grandfather or grandmother or more remote ancestor; why a
peculiarity is often transmitted from one sex to both sexes, or to one
sex alone, more commonly but not exclusively to the like sex. It is
a fact of some importance to us, that peculiarities appearing in the
males of our domestic breeds are often transmitted, either exclusively
or in a much greater degree, to the males alone. A much more important
rule, which I think may be trusted, is that, at whatever period
of life a peculiarity first appears, it tends to reappear in the offspring
at a corresponding age, though sometimes earlier. In many
cases this could not be otherwise; thus the inherited peculiarities in
the horns of cattle could appear only in the offspring when nearly
mature; peculiarities in the silkworm are known to appear at the corresponding
caterpillar or cocoon stage. But hereditary diseases and
some other facts make me believe that the rule has a wider extension,
and that, when there is no apparent reason why a peculiarity should
appear at any particular age, yet that it does tend to appear in the
offspring at the same period at which it first appeared in the parent.
I believe this rule to be of the highest importance in explaining the
laws of embryology. These remarks are of course confined to the first
appearance of the peculiarity, and not to the primary cause which
may have acted on the ovules or on the male element; in nearly the
same manner as the increased length of the horns in the offspring
from a short-horned cow by a long-horned bull, though appearing late
in life, is clearly due to the male element.
Having alluded to the subject of reversion, I may here refer to a
statement often made by naturalists—namely, that our domestic varieties,
when run wild, gradually but invariably revert in character to
their aboriginal stocks. Hence it has been argued that no deductions
can be drawn from domestic races to species in a state of nature. I
have in vain endeavoured to discover on what decisive facts the above
statement has so often and so boldly been made. There would be

30 ORIGIN OF SPECIES
great difficulty in proving its truth: we may safely conclude that very
many of the most strongly marked domestic varieties could not possibly
live in a wild state. In many cases we do not know what the
aboriginal stock was, and so could not tell whether or not nearly perfect
reversion had ensued. It would be necessary, in order to prevent
the effects of intercrossing, that only a single variety should have
been turned loose in its new home. Nevertheless, as our varieties
certainly do occasionally revert in some of their characters to ancestral
forms, it seems to me not improbable that if we could succeed in
naturalising, or were to cultivate, during many generations, the several
races, for instance, of the cabbage, in very poor soil (in which
case, however, some effect would have to be attributed to the definite
action of the poor soil), that they would, to a large extent, or even
wholly, revert to the wild aboriginal stock. Whether or not the experiment
would succeed, is not of great importance for our line of
argument; for by the experiment itself the conditions of life are
changed. If it could be shown that our domestic varieties manifested
a strong tendency to reversion,—that is, to lose their acquired
characters, whilst kept under the same conditions, and whilst kept in
a considerable body, so that free intercrossing might check, by blending
together, any slight deviations in their structure, in such case, I
grant that we could deduce nothing from domestic varieties in regard
to species. But there is not a shadow of evidence in favour of this
view: to assert that we could not breed our cart and race horses, long
and short horned cattle, and poultry of various breeds, and esculent
vegetables, for an unlimited number of generations, would be opposed
to all experience.

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