M Database Inspector (cheetah)
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|OriginOfSpecies - 475 Rows|
|01 - Variations Under Domestication||01-01 - Causes of Variability||25||
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.
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 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.
|01 - Variations Under Domestication||01-01 - Causes of Variability||30||
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, &c., 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 conditions 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.
|01 - Variations Under Domestication||01-04 - Inheritance||30||
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.
|01 - Variations Under Domestication||01-07 - Origin of Domestic Varieties from one or more Species||30||
Since the discovery of flint tools in the superficial formations of many parts of the world, all geologists believe that barbarian man existed at an enormously remote period;
and we know that at the present day there is hardly a tribe so barbarous, as not to have domesticated at least the dog.
The origin of most of our domestic animals will probably for ever remain vague.
But I may here state, that, looking to the domestic dogs of the whole world, I have, after a laborious collection of all known facts, come to the conclusion that several wild species of Canidae have been tamed, and that their blood, in some cases mingled together, flows in the veins of our domestic breeds. In regard to sheep and goats I can form no decided opinion.