simple variation, striped on the legs like a zebra, or striped on
the shoulders like an ass. In the horse we see this tendency strong
whenever a dun tint appears--a tint that approaches to that of the
general coloring of the other species of the genus. The appearance of
the stripes is not accompanied by any change of form or by any other
new character. We see this tendency to become striped most strongly
displayed in hybrids from between several of the most distinct
species. Now observe the case of the several breeds of pigeons: they
are descended from a pigeon (including two or three sub-species or
geographical races) of a bluish color, with certain bars and other
marks; and, when any breed assumes by simple variation a bluish tint,
these bars and other marks invariably reappear; but without any other
change of form or character. When the oldest and truest breeds of
various colors are crossed, we see a strong tendency for the blue tint
and bars and marks to reappear in the mongrels. I have stated that
the most probable hypothesis to account for the reappearance of very
ancient characters is--that there is a _tendency_ in the young of each
successive generation to produce the long-lost character, and that this
tendency, from unknown causes, sometimes prevails. And we have just
seen that in several species of the horse-genus the stripes are either
plainer or appear more commonly in the young than in the old. Call the
breeds of pigeons, some of which have bred true for centuries, species;
and how exactly parallel is the case with that of the species of the
horse-genus! For myself, I venture confidently to look back thousands
on thousands of generations, and I see an animal striped like a zebra,
but perhaps otherwise very differently constructed, the common parent
of our domestic horse (whether or not it be descended from one or more
wild stocks), of the ass, the hemionus, quagga, and zebra.
He who believes that each equine species was independently created,
will, I presume, assert that each species has been created with a
tendency to vary, both under nature and under domestication, in this
particular manner, so as often to become striped like the other species
of the genus; and that each has been created with a strong tendency,
when crossed with species inhabiting distant quarters of the world, to
produce hybrids resembling in their stripes, not their own parents,
but other species of the genus. To admit this view is, as it seems to
me, to reject a real for an unreal, or at least for an unknown, cause.
It makes the works of God a mere mockery and deception; I would almost
as soon believe with the old and ignorant cosmogonists, that fossil
shells had never lived, but had been created in stone so as to mock the
shells living on the sea-shore.
VARIABILITY OF CULTIVATED PLANTS.
[Animals and
Plants, vol.
i, page 322.]
I shall not enter into so much detail on the variability of cultivated
plants as in the case of domesticated animals. The subject is
involved in much difficulty. Botanists have generally neglected
cultivated varieties, as beneath their notice. In several cases the
wild prototype is unknown or doubtfully known; and in other cases
it is hardly possible to distinguish between escaped seedlings and
truly wild plants, so that there is no safe standard of comparison by
which to judge of any supposed amount of change. Not a few botanists
believe that several of our anciently cultivated plants have become
so profoundly modified that it is not possible now to recognize their
aboriginal parent-forms. Equally perplexing are the doubts whether some
of them are descended from one species, or from several inextricably
commingled by crossing and variation. Variations often pass into, and
can not be distinguished from, monstrosities; and monstrosities are
of little significance for our purpose. Many varieties are propagated
solely by grafts, buds, layers, bulbs, etc., and frequently it is
not known how far their peculiarities can be transmitted by seminal
generation.
* * * * *
[Page 325.]
From innumerable experiments made through dire necessity by the
savages of every land, with the results handed down by tradition,
the nutritious, stimulating, and medicinal properties of the most
unpromising plants were probably first discovered. It appears, for
instance, at first an inexplicable fact that untutored man, in three
distant quarters of the world, should have discovered, among a host
of native plants, that the leaves of the tea-plant and mattee, and
the berries of the coffee, all included a stimulating and nutritious
essence, now known to be chemically the same. We can also see that
savages suffering from severe constipation would naturally observe
whether any of the roots which they devoured acted as aperients. We
probably owe our knowledge of the uses of almost all plants to man
having originally existed in a barbarous state, and having been often
compelled by severe want to try as food almost everything which he
could chew and swallow.
SAVAGE WISDOM IN THE CULTIVATION OF PLANTS.
[Page 326.]
The savage inhabitants of each land, having found out by many and
hard trials what plants were useful, or could be rendered useful by
various cooking processes, would after a time take the first step in
cultivation by planting them near their usual abodes. Livingstone
states that the savage Batokas sometimes left wild fruit-trees standing
in their gardens, and occasionally even planted them, “a practice
seen nowhere else among the natives.” But Du Chaillu saw a palm and
some other wild fruit-trees which had been planted; and these trees
were considered private property. The next step in cultivation, and
this would require but little forethought, would be to sow the seeds
of useful plants; and, as the soil near the hovels of the natives
would often be in some degree manured, improved varieties would sooner
or later arise. Or a wild and unusually good variety of a native
plant might attract the attention of some wise old savage; and he
would transplant it, or sow its seed. That superior varieties of wild
fruit-trees occasionally are found is certain, as in the case of the
American species of hawthorns, plums, cherries, grapes, and hickories,
specified by Professor Asa Gray.
* * * * *
[Page 336.]
We now know that man was sufficiently civilized to cultivate the ground
at an immensely remote period; so that wheat might have been improved
long ago up to that standard of excellence which was possible under the
then existing state of agriculture. One small class of facts supports
this view of the slow and gradual improvement of our cereals. In the
most ancient lake-habitations of Switzerland, when men employed only
flint-tools, the most extensively cultivated wheat was a peculiar kind,
with remarkably small ears and grains. “While the grains of the modern
forms are in section from seven to eight millimetres in length, the
larger grains from the lake-habitations are six, seldom seven, and
the smaller ones only four. The ear is thus much narrower, and the
spikelets stand out more horizontally, than in our present forms.” So
again with barley, the most ancient and most extensively cultivated
kind had small ears, and the grains were “smaller, shorter, and nearer
to each other, than in that now grown; without the husk they were two
and one half lines long, and scarcely one and one half broad, while
those now grown have a length of three lines, and almost the same
in breadth.” These small-grained varieties of wheat and barley are
believed by Heer to be the parent-forms of certain existing allied
varieties, which have supplanted their early progenitors.
UNKNOWN LAWS OF INHERITANCE.
[Origin of
Species,
page 10.]
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 silk-worm 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.
* * * * *
[Variation of
Animals and
Plants, vol.
i, page 445.]
If animals and plants had never been domesticated, and wild ones alone
had been observed, we should probably never have heard the saying that
“like begets like.” The proposition would have been as self-evident
as that all the buds on the same tree are alike, though neither
proposition is strictly true. For, as has often been remarked, probably
no two individuals are identically the same. All wild animals recognize
each other, which shows that there is some difference between them;
and, when the eye is well practiced, the shepherd knows each sheep, and
man can distinguish a fellow-man out of millions on millions of other
men.
* * * * *
[Page 446.]
The subject of inheritance is wonderful. When a new character arises,
whatever its nature may be, it generally tends to be inherited, at
least in a temporary and sometimes in a most persistent manner.
What can be more wonderful than that some trifling peculiarity, not
primordially attached to the species, should be transmitted through the
male or female sexual cells, which are so minute as not to be visible
to the naked eye, and afterward through the incessant changes of a long
course of development, undergone either in the womb or in the egg, and
ultimately appear in the offspring when mature, or even when quite
old, as in the case of certain diseases? Or, again, what can be more
wonderful than the well-ascertained fact that the minute ovule of a
good milking-cow will produce a male, from whom a cell, in union with
an ovule, will produce a female, and she, when mature, will have large
mammary glands, yielding an abundant supply of milk, and even milk of
a particular quality? Nevertheless, the real subject of surprise is,
as Sir H. Holland has well remarked, not that a character should be
inherited, but that any should ever fail to be inherited.
LAWS OF INHERITANCE THAT ARE FAIRLY WELL ESTABLISHED.
[Animals and
Plants, vol.
ii, page 61.]
Though much remains obscure with respect to inheritance, we may look
at the following laws as fairly well established: Firstly, a tendency
in every character, new and old, to be transmitted by seminal and bud
generation, though often counteracted by various known and unknown
causes. Secondly, reversion or atavism, which depends on transmission
and development being distinct powers: it acts in various degrees and
manners through both seminal and bud generation. Thirdly, prepotency of
transmission, which may be confined to one sex, or be common to both
sexes. Fourthly, transmission, as limited by sex, generally to the same
sex in which the inherited character first appeared; and this in many,
probably most cases, depends on the new character having first appeared
at a rather late period of life. Fifthly, inheritance at corresponding
periods of life, with some tendency to the earlier development of the
inherited character. In these laws of inheritance, as displayed under
domestication, we see an ample provision for the production, through
variability and natural selection, of new specific forms.
INHERITED PECULIARITIES IN MAN.
[Animals and
Plants, vol.
i, page 450.]
Gait, gestures, voice, and general bearing, are all inherited, as
the illustrious Hunter and Sir A. Carlisle have insisted. My father
communicated to me some striking instances, in one of which a man died
during the early infancy of his son, and my father, who did not see
this son until grown up and out of health, declared that it seemed to
him as if his old friend had risen from the grave, with all his highly
peculiar habits and manners. Peculiar manners pass into tricks, and
several instances could be given of their inheritance; as in the case,
often quoted, of the father who generally slept on his back, with his
right leg crossed over the left, and whose daughter, while an infant in
the cradle, followed exactly the same habit, though an attempt was made
to cure her. I will give one instance which has fallen under my own
observation, and which is curious from being a trick associated with
a peculiar state of mind, namely, pleasurable emotion. A boy had the
singular habit, when pleased, of rapidly moving his fingers parallel
to each other, and, when much excited, of raising both hands, with the
fingers still moving, to the sides of his face on a level with the
eyes: when this boy was almost an old man, he could still hardly resist
this trick when much pleased, but from its absurdity concealed it. He
had eight children. Of these, a girl, when pleased, at the age of four
and a half years, moved her fingers in exactly the same way, and, what
is still odder, when much excited, she raised both her hands, with her
fingers still moving, to the sides of her face, in exactly the same
manner as her father had done, and sometimes even still continued to do
so when alone. I never heard of any one, excepting this one man and his
little daughter, who had this strange habit; and certainly imitation
was in this instance out of the question.
INHERITED DISEASES.
[Animals and
Plants, vol.
ii, page 54.]
Large classes of diseases usually appear at certain ages, such as St.
Vitus’s dance in youth, consumption in early mid-life, gout later,
and apoplexy still later; and these are naturally inherited at the
same period. But, even in diseases of this class, instances have been
recorded, as with St. Vitus’s dance, showing that an unusually early
or late tendency to the disease is inheritable. In most cases the
appearance of any inherited disease is largely determined by certain
critical periods in each person’s life, as well as by unfavorable
conditions. There are many other diseases, which are not attached to
any particular period, but which certainly tend to appear in the child
at about the same age at which the parent was first attacked. An array
of high authorities, ancient and modern, could be given in support of
this proposition. The illustrious Hunter believed in it; and Piorry
cautions the physician to look closely to the child at the period when
any grave inheritable disease attacked the parent. Dr. Prosper Lucas,
after collecting facts from every source, asserts that affections of
all kinds, though not related to any particular period of life, tend
to reappear in the offspring at whatever period of life they first
appeared in the progenitor.
* * * * *
[Page 55.]
Esquirol gives several striking instances of insanity coming on at
the same age as that of a grandfather, father, and son, who all
committed suicide near their fiftieth year. Many other cases could
be given, as of a whole family who became insane at the age of
forty. Other cerebral affections sometimes follow the same rule--for
instance, epilepsy and apoplexy. A woman died of the latter disease
when sixty-three years old; one of her daughters at forty-three, and
the other at sixty-seven: the latter had twelve children, who all
died from tubercular meningitis. I mention this latter case because
it illustrates a frequent occurrence, namely, a change in the precise
nature of an inherited disease, though still affecting the same organ.
* * * * *
Two brothers, their father, their paternal uncles, seven cousins,
and their paternal grandfather, were all similarly affected by a
skin-disease, called pityriasis versicolor; “the disease, strictly
limited to the males of the family (though transmitted through the
females), usually appeared at puberty, and disappeared at about the
age of forty or forty-five years.” The second case is that of four
brothers, who, when about twelve years old, suffered almost every week
from severe headaches, which were relieved only by a recumbent position
in a dark room. Their father, paternal uncles, paternal grandfather,
and grand-uncles all suffered in the same way from headaches, which
ceased at the age of fifty-four or fifty-five in all those who lived so
long. None of the females of the family were affected.
CAUSES OF NON-INHERITANCE.
[Animals and
Plants, vol.
i, page 470.]
A large number of cases of non-inheritance are intelligible on the
principle that a strong tendency to inheritance does exist, but that it
is overborne by hostile or unfavorable conditions of life. No one would
expect that our improved pigs, if forced during several generations
to travel about and root in the ground for their own subsistence,
would transmit, as truly as they now do, their short muzzles and legs,
and their tendency to fatten. Dray-horses assuredly would not long
transmit their great size and massive limbs, if compelled to live
in a cold, damp, mountainous region; we have, indeed, evidence of
such deterioration in the horses which have run wild on the Falkland
Islands. European dogs in India often fail to transmit their true
character. Our sheep in tropical countries lose their wool in a few
generations. There seems also to be a close relation between certain
peculiar pastures and the inheritance of an enlarged tail in fat-tailed
sheep, which form one of the most ancient breeds in the world. With
plants, we have seen that tropical varieties of maize lose their proper
character in the course of two or three generations, when cultivated
in Europe; and conversely so it is with European varieties cultivated
in Brazil. Our cabbages, which here come so true by seed, can not form
heads in hot countries. According to Carrière, the purple-leafed beech
and barberry transmit their character by seed far less truly in certain
districts than in others. Under changed circumstances, periodical
habits of life soon fail to be transmitted, as the period of maturity
in summer and winter wheat, barley, and vetches. So it is with animals:
for instance, a person, whose statement I can trust, procured eggs of
Aylesbury ducks from that town, where they are kept in houses, and are
reared as early as possible for the London market; the ducks bred from
these eggs in a distant part of England, hatched their first brood on
January 24th, while common ducks, kept in the same yard and treated in
the same manner, did not hatch till the end of March; and this shows
that the period of hatching was inherited. But the grandchildren of
these Aylesbury ducks completely lost their habit of early incubation,
and hatched their eggs at the same time with the common ducks of the
same place.
Many cases of non-inheritance apparently result from the conditions of
life continually inducing fresh variability. We have seen that when the
seeds of pears, plums, apples, etc., are sown, the seedlings generally
inherit some degree of family likeness. Mingled with these seedlings,
a few, and sometimes many, worthless, wild-looking plants commonly
appear, and their appearance may be attributed to the principle of
reversion. But scarcely a single seedling will be found perfectly to
resemble the parent-form; and this may be accounted for by constantly
recurring variability induced by the conditions of life.
STEPS BY WHICH DOMESTIC RACES HAVE BEEN PRODUCED.
[Origin of
Species,
page 22.]
Some effect may be attributed to the direct and definite action of the
external conditions of life, and some to habit; but he would be a bold
man who would account by such agencies for the differences between a
dray and race horse, a greyhound and blood-hound, a carrier and tumbler
pigeon. One of the most remarkable features in our domesticated races
is that we see in them adaptation, not, indeed, to the animal’s or
plant’s own good, but to man’s use or fancy. Some variations useful to
him have probably arisen suddenly, or by one step; many botanists, for
instance, believe that the fuller’s teasel, with its hooks, which can
not be rivaled by any mechanical contrivance, is only a variety of the
wild _Dipsacus_; and this amount of change may have suddenly arisen in
a seedling. So it has probably been with the turnspit-dog; and this is
known to have been the case with the ancon sheep. But when we compare
the dray-horse and race-horse, the dromedary and camel, the various
breeds of sheep fitted either for cultivated land or mountain-pasture,
with the wool of one breed good for one purpose, and that of another
breed for another purpose; when we compare the many breeds of dogs,
each good for man in different ways; when we compare the game-cock, so
pertinacious in battle, with other breeds so little quarrelsome, with
“everlasting layers” which never desire to sit, and with the bantam, so
small and elegant; when we compare the host of agricultural, culinary,
orchard, and flower-garden races of plants, most useful to man at
different seasons and for different purposes, or so beautiful in his
eyes--we must, I think, look further than to mere variability. We can
not suppose that all the breeds were suddenly produced as perfect and
as useful as we now see them; indeed, in many cases, we know that this
has not been their history. The key is man’s power of accumulative
selection: Nature gives successive variations; man adds them up in
certain directions useful to him. In this sense he may be said to have
made for himself useful breeds.
* * * * *
[Page 23.]
If selection consisted merely in separating some very distinct variety,
and breeding from it, the principle would be so obvious as hardly to be
worth notice; but its importance consists in the great effect produced
by the accumulation in one direction, during successive generations, of
differences absolutely inappreciable by an uneducated eye--differences
which I for one have vainly attempted to appreciate. Not one man in
a thousand has accuracy of eye and judgment sufficient to become an
eminent breeder. If gifted with these qualities, and he studies his
subject for years, and devotes his lifetime to it with indomitable
perseverance, he will succeed, and may make great improvements; if
he wants any of these qualities, he will assuredly fail. Few would
readily believe in the natural capacity and years of practice requisite
to become even a skillful pigeon-fancier.
UNCONSCIOUS SELECTION.
[Origin of
Species,
page 25.]
A man who intends keeping pointers naturally tries to get as good dogs
as he can, and afterward breeds from his own best dogs, but he has no
wish or expectation of permanently altering the breed. Nevertheless,
we may infer that this process, continued during centuries, would
improve and modify any breed, in the same way as Bakewell, Collins,
etc., by this very same process, only carried on more methodically, did
greatly modify, even during their lifetimes, the forms and qualities
of their cattle. Slow and insensible changes of this kind can never be
recognized unless actual measurements or careful drawings of the breeds
in question have been made long ago, which may serve for comparison.
In some cases, however, unchanged or but little changed individuals
of the same breed exist in less civilized districts, where the breed
has been less improved. There is reason to believe that King Charles’s
spaniel has been unconsciously modified to a large extent since the
time of that monarch. Some highly competent authorities are convinced
that the setter is directly derived from the spaniel, and has probably
been slowly altered from it. It is known that the English pointer has
been greatly changed within the last century, and in this case the
change has, it is believed, been chiefly effected by crosses with the
fox-hound; but what concerns us is, that the change has been effected
unconsciously and gradually, and yet so effectually, that, though the
old Spanish pointer certainly came from Spain, Mr. Borrow has not
seen, as I am informed by him, any native dog in Spain like our pointer.
By a similar process of selection, and by careful training, English
race-horses have come to surpass in fleetness and size the parent
Arabs, so that the latter, by the regulations for the Goodwood races,
are favored in the weights which they carry. Lord Spencer and others
have shown how the cattle of England have increased in weight and in
early maturity, compared with the stock formerly kept in this country.
* * * * *
[Page 26.]