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1859
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the crossing of forms only slightly different is favourable to 1859 1860 1861
the crossing of forms only slightly differentiated favours 1866
first, the crossing of forms only slightly differentiated favours 1869
firstly, slight changes in the conditions of life add to 1872

their offspring; and that slight changes in the 1859 1860 1861 1866
their offspring, whilst close interbreeding is injurious; and secondly, that slight changes in the 1869
all organic beings; and secondly, that the crossing of forms, which have been exposed to slightly different 1872

are apparently favourable to the 1859 1860 1861
apparently add to the 1866 1869
or which have varied, favours the size, 1872

all organic beings. 1859 1860 1861 1866
all organic beings, whilst greater changes are often injurious. 1869
their offspring. 1872

3 blocks not present in 1859 1860 1861; present in 1866 1869 1872
But the facts given on the sterility of the illegitimate unions of dimorphic and trimorphic plants and of their illegitimate progeny, render it probable that there is some unknown bond connecting in all cases the degree of fertility of first unions with that of their offspring. The consideration of these facts on dimorphism, as well as the results of reciprocal crosses, drive us to conclude that in all cases the primary cause of sterility, both in the parents and in the offspring, is confined to differences in their reproductive systems. But why in numerous species, descended from a common parent-form, the reproductive system should in all have become more or less modified, leading to their mutual infertility, we do not know in the least; nor whether this has been effected directly, or in correlation with other structural and functional modifications.

generally correspond, though 1859 1860 1861
in most cases correspond, even if 1872

of some kind 1859 1860 1861
OMIT 1872

from it, and 1860 1861
and 1859 1872

attempts to express 1859 1860 1861
includes resemblances of 1872

2 blocks not present in 1859 1860 1861 1872; present in 1866 1869
It is not surprising that the degree of difficulty in uniting two species, and the degree of sterility of their hybrid-offspring, should generally correspond, even if due to distinct causes; for both depend on the amount of difference of some kind between the species which are crossed. Nor is it surprising that the facility of effecting a first cross, and the fertility of the hybrids thus produced, and the capacity of being grafted together—though this latter capacity evidently depends on widely different circumstances—should all run, to a certain extent, parallel with the systematic affinity of the forms which are subjected to experiment; for systematic affinity attempts to express all kinds of resemblance between all species.

as is so often falsely stated, universally 1861
quite universally, 1859
quite uni- versally, 1860
as is so often stated, universally 1866
as is so often stated, invariably 1869 1872

not of differences in the reproductive system. 1859 1860 1861 1866
that they have not been long exposed to uniform conditions of life. 1869 1872

1 blocks not present in 1859 1860 1861; present in 1866 1869 1872
Nor should it be forgotten that long-continued domestication apparently tends to eliminate sterility, and is therefore little likely to induce this same quality.

In 1859 1860 1861
Excluding the subject of fertility, in 1866

although we are profoundly ignorant in every case of the precise cause of sterility, the facts briefly 1861
the facts briefly 1859 1860
although we are profoundly ignorant in every case of the precise cause of the sterility of first crosses and of hybrids, the facts briefly 1866
although we are profoundly ignorant of the precise cause of the sterility of first crosses and of hybrids, the facts 1869
although we are as ignorant of the precise cause of the sterility of first crosses and of hybrids as we are why animals and plants removed from their natural conditions become sterile, yet the facts 1872

frequently affects pure species, when
exposed to
their
unnatural
natural
conditions of
life.
life
have
have
been
been
disturbed.
disturbed.
This view is
strongly supported
supported
by a parallelism of another
kind;—
kind:
kind:—
namely,
that,
that
the crossing of forms only slightly different is favourable to the vigour and fertility of their offspring; and that slight changes in the conditions of life are apparently favourable to the
vigour,
vigour
and fertility of all organic beings. It is not surprising that the
degree of
degree of
difficulty in
crossing any
uniting
two species, and the
sterility
degree
of
sterility of
sterility of
their
hybrid-offspring
hybrid offspring,
hybrid-offspring,
should generally correspond, though due to distinct causes; for both depend on the amount of difference of some kind between the species which are crossed. Nor is it surprising that the facility of effecting a first cross,
and the
the
fertility of the hybrids
produced,
thus produced,
produced
from it, and the capacity of being grafted together— though this latter capacity evidently depends on widely different circumstances— should all run, to a certain extent, parallel with the systematic affinity of the forms
which are
which are
subjected to experiment; for systematic affinity attempts to express all
kinds.
kinds
of
of
resemblance
resemblance
between
between
all
all
species.
species.
First crosses between forms known to be varieties, or sufficiently alike to be considered as varieties, and their mongrel offspring, are very generally, but
not
not,
as is so often falsely stated, universally fertile. Nor is this
almost universal
nearly general
and perfect fertility surprising, when
it is
we
remembered
remember
how liable we are to argue in a circle with respect to varieties in a state of nature; and when we remember that the greater number of varieties have been produced under
domesti- cation
domestication
by the selection of mere external differences, and not of differences in the reproductive system. In all other
respects
respects,
excluding fertility,
excluding fertility,
there is
the
a
closest
close
general resemblance between hybrids and mongrels. Finally, then, although we are profoundly ignorant in every case of the precise cause of sterility, the facts briefly