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circumstances and rules 1859 1860 1861 1866 1869
laws 1872

chiefly drawn up 1859 1860 1861 1866 1869
drawn up chiefly 1872

of the same genus applied to the stigma of some one species, 1859 1860 1861 1866
of the same genus applied to the stigma of some one of the species, 1869
applied to the stigma of some one species of the same genus, 1872

that some degree of sterility, both in first crosses and in hybrids, is an extremely general result; but that it cannot, under our present state of knowledge, be considered as absolutely universal.
Laws
Laws
governing
governing
the
the
Sterility
Sterility
of
of
first
first
Crosses
Crosses
and
and
of
of
Hybrids .
Hybrids .—
We will now consider a little more in detail the circumstances and rules governing the sterility of first crosses and of hybrids. Our chief object will be to see whether or not
these laws
the rules
indicate that species have
been specially
specially been
endowed with this quality, in order to prevent their crossing and blending together in utter confusion. The following
rules and
rules and
conclusions are chiefly drawn up from
Gärtners
Gärtner's
admirable work on the hybridisation of plants. I have taken much pains to ascertain how far
they
the
rules
rules
apply to animals,
and,
and
considering how scanty our knowledge is in regard to hybrid animals, I have been surprised to find how generally the same rules apply to both kingdoms.
It has been already remarked, that the degree of fertility, both of first crosses and of hybrids, graduates from zero to perfect fertility. It is surprising in how many curious ways this gradation can be
shown;
shown
to exist;
to exist;
but only the barest outline of the facts can here be given. When pollen from a plant of one family is placed on the stigma of a plant of a distinct family, it exerts
on
no
more influence than so much inorganic dust. From this absolute zero of fertility, the pollen of
differ- ent
different
species of the same genus applied to the stigma of some one species, yields a perfect gradation in the number of seeds produced, up to nearly complete or even quite complete fertility; and, as we have seen, in certain abnormal cases, even to an excess of fertility, beyond that which the
plants
plant's
own pollen
will
will
produces.
produce.
So in hybrids themselves, there are some which never