have been
experimentised on experimentised on 1859 1860 |
profoundly modified. Moreover, most of the varieties which 1861 |
experimented on 1866 1869 1872 |
have been
produced produced 1859 1860 1866 1869 1872 |
experimented on have been produced 1861 |
under domestication; and as domestication
apparently apparently 1859 |
(I do not mean mere confinement) apparently 1860 1861 |
(I do not mean mere confinement) almost certainly 1866 1869 1872 |
tends to eliminate
sterility, sterility, 1859 1860 1861 1866 1869 |
that sterility which, judging from analogy, would have affected the parent-species if intercrossed, 1872 |
we ought not to expect
it also to produce sterility. it also to produce sterility. 1859 1860 1861 1866 1869 |
that domestication would likewise induce sterility in their modified descendants when crossed. 1872 |
↑10 blocks not present in 1859 1860 1861 1866 1869; present in 1872 | This elimination of sterility apparently follows from the same cause which allows our domestic animals to breed freely under diversified circumstances; and this again apparently follows from their having been gradually accustomed to frequent changes in their conditions of life.
A double and parallel series of facts seems to throw much light on the sterility of species, when first crossed, and of their hybrid offspring.
On the one side, there is good reason to believe that slight changes in the conditions of life give vigour and fertility to all organic beings.
We know also that a cross between the distinct individuals of the same variety, and between distinct varieties, increases the number of their offspring, and certainly gives to them increased size and vigour.
This is chiefly owing to the forms which are crossed having been exposed to somewhat different conditions of life; for I have ascertained by a laborious series of experiments that if all the individuals of the same variety be subjected during several generations to the same conditions, the good derived from crossing is often much diminished or wholly disappears.
This is one side of the case.
On the other side, we know that species which have long been exposed to nearly uniform conditions, when they are subjected under confinement to new and greatly changed conditions, either perish, or if they survive, are rendered sterile, though retaining perfect health.
This does not occur, or only in a very slight degree, with our domesticated productions, which have long been exposed to fluctuating conditions.
Hence, when we find that hybrids produced by a cross between two distinct species are few in number, owing to their perishing soon after conception or at a very early age, or if surviving that they are rendered more or less sterile, it seems highly probable that this result is due to their having been in fact subjected to a great change in their conditions of life, from being compounded of two distinct organisations.
He who will explain in a definite manner why, for instance, an elephant or a fox will not breed under confinement in its native country, whilst the domestic pig or dog will breed freely under the most diversified conditions, will at the same time be able to give a definite answer to the question why two distinct species, when crossed, as well as their hybrid offspring, are generally rendered more or less sterile, whilst two domesticated varieties when crossed and their mongrel offspring are perfectly fertile.
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The sterility of hybrids is a
very very 1859 1860 1861 | very 1866 1869 |
different case from that of
first first 1859 1860 | a first 1861 1866 1869 |
crosses, crosses, 1859 1860 | cross, 1861 1866 1869 |
for
their their 1859 1860 1861 | the 1866 1869 |
reproductive organs
are are 1859 1860 1861 |
of hybrids are 1866 1869 |
more or less functionally impotent; whereas in first
crosses crosses 1859 1860 | crosses, 1861 1866 1869 |
the organs
on on 1859 1860 | of 1861 1866 1869 |
both
sides sides 1859 1860 | species 1861 1866 1869 |
are
in in 1859 1860 1861 |
of course in 1866 1869 |
a perfect condition. As we continually see that organisms of all kinds are rendered in some degree sterile from
their constitutions having been disturbed by slightly different and new conditions of life, their constitutions having been disturbed by slightly different and new conditions of life, 1859 1860 1861 1866 |
being exposed to slightly changed conditions, 1869 |
we need not feel surprise at hybrids being in some degree sterile, for their constitutions can hardly fail to
have been have been 1859 1860 1861 1866 | be 1869 |
disturbed from being compounded of two distinct
organisations. organisations. 1859 1860 1861 1866 |
organisations; but whether this is the true cause of their sterility I will not pretend to decide. 1869 |
This This 1859 1860 1861 1866 | The above 1869 |
parallelism is supported by another parallel, but directly opposite, class of
facts; facts; 1859 1860 | facts, 1861 1866 1869 |
namely, that the vigour and fertility of all organic beings are increased by slight changes in their conditions of life, and that the offspring of slightly modified forms or varieties
acquire from being crossed acquire from being crossed 1859 1860 1861 |
when crossed acquire 1866 1869 |
increased vigour and fertility. So that, on the one hand,
considerable considerable 1859 1860 1861 1866 | a considerable 1869 |
changes changes 1859 1860 1861 1866 | change 1869 |
in the conditions of life and crosses between greatly modified forms, lessen fertility; and on the other hand, lesser changes in the conditions of life and crosses between less modified forms, increase fertility. |
Turning to geographical distribution, the difficulties encountered on the theory of descent with modification are
grave grave 1859 1860 | serious 1861 1866 1869 1872 |
enough. All the individuals of the same species, and all the species of the same genus, or even higher group,
must have must have 1859 1860 1861 1866 1869 | are 1872 |
descended from common parents; and therefore, in however distant and isolated parts of the world they
are are 1859 1860 1861 | may 1866 1869 1872 |
now
found, found, 1859 1860 1861 | be found, 1866 1869 1872 |
they must in the course of successive generations have
passed passed 1859 1860 1861 1866 | travelled 1869 1872 |
from some one
part part 1859 1860 1861 | point 1866 1869 1872 |
to
the the 1859 1860 1861 | all the 1866 1869 1872 |
others. We are often wholly unable
|