See page in:
1859
1860
1861
1866
1869
1872

Compare with:
1859
1860
1861
1866
1872

has been asserted by so many authors to be universal, this cannot 1861 1866 1869 1872
cannot 1859 1860

correct after the facts given on the authority of Gärtner and Kölreuter. 1861 1866 1869
as universal; nor is their very general fertility surprising when we remember that it is not likely that either their constitutions or their reproductive systems should have been profoundly modified. 1859 1860
as quite correct after the facts given on the high authority of Gärtner and Kölreuter. 1872

Moreover, most of the varieties which 1859 1860 1866 1869
Nor is the very general fertility of varieties, when crossed, surprising, when we remember that it is not likely that either their constitutions or their reproductive systems should 1861
Most of the varieties which 1872

experimented on 1866 1869 1872
experimentised on 1859 1860
profoundly modified. Moreover, most of the varieties which 1861

produced 1859 1860 1866 1869 1872
experimented on have been produced 1861

(I do not mean mere confinement) almost certainly 1866 1869 1872
apparently 1859
(I do not mean mere confinement) apparently 1860 1861

sterility, 1859 1860 1861 1866 1869
that sterility which, judging from analogy, would have affected the parent-species if intercrossed, 1872

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.

of hybrids are 1866 1869
are 1859 1860 1861

of course in 1866 1869
in 1859 1860 1861

being exposed to slightly changed conditions, 1869
their constitutions having been disturbed by slightly different and new conditions of life, 1859 1860 1861 1866

organisations; but whether this is the true cause of their sterility I will not pretend to decide. 1869
organisations. 1859 1860 1861 1866

when crossed acquire 1866 1869
acquire from being crossed 1859 1860 1861

of their mongrel offspring has been asserted by so many authors to be universal, this cannot be considered correct after the facts given on the authority of Gärtner and Kölreuter. Nor is the very general fertility of varieties, when crossed, surprising, when we remember that it is not likely that their reproductive systems should have been profoundly modified. Moreover, most of the varieties which have been experimented on have been produced under domestication; and as domestication (I do not mean mere confinement) almost certainly tends to eliminate sterility, we ought not to expect it also to produce sterility.
The sterility of hybrids is a
very
....
different case from that of
first
a first
crosses,
cross,
for
their
the
reproductive organs of hybrids are more or less functionally impotent; whereas in first
crosses
crosses,
the organs
on
of
both
sides
species
are of course in a perfect condition. As we continually see that organisms of all kinds are rendered in some degree sterile from being exposed to slightly changed conditions, we need not feel surprise at hybrids being in some degree sterile, for their constitutions can hardly fail to
have been
be
disturbed from being compounded of two distinct organisations; but whether this is the true cause of their sterility I will not pretend to decide.
This
The above
parallelism is supported by another parallel, but directly opposite, class of
facts;
facts,
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 when crossed acquire increased vigour and fertility. So that, on the one hand,
considerable
a considerable
changes
change
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