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1859
1860
1861
1866
1869
1872

Compare with:
1859
1860
1861
1869
1872

which have suffered much extinction; but 1861 1866 1869 1872
but 1859 1860

OMIT 1860 1861 1866 1869 1872
as is proclaimed by the canon, "Natura non facit saltum," 1859

of 1859 1860 1861 1866 1869
in the same community of 1872

in the same community of ants; 1859 1860 1861 1866
ants in the same community; 1869
ants; 1872

distinct trees 1866 1869
trees 1859 1860 1861
distinct kinds of trees 1872

are crossed reciprocally,— 1866
are crossed reciprocally; 1859 1860 1861
reciprocally,— 1869 1872

in no respect from each other 1866 1869
from each other in no respect 1872

It is, no doubt, extremely difficult even to conjecture by what gradations many structures have been perfected, more especially amongst broken and failing groups of organic
beings;
beings,
which have suffered much extinction; but we see so many strange gradations in nature, OMIT that we ought to be extremely cautious in saying that any organ or instinct, or
the
any
whole
structure,
being,
could not have arrived at its present state by many graduated steps. There are, it must be admitted, cases of special difficulty
opposed to
on
the theory of natural selection; and one of the most curious of these is the existence of two or three defined castes of workers or sterile
female
females
in the same community of ants; but I have attempted to show how
this
these
difficulty
difficulties
can be mastered.
With respect to the almost universal sterility of species when first crossed, which forms so remarkable a contrast with the almost universal fertility of varieties when crossed, I must refer the reader to the recapitulation of the facts given at the end of the
ninth
eighth
chapter, which seem to me conclusively to show that this sterility is no more a special endowment than is the incapacity of two distinct trees to be grafted
together,
together;
but that it is incidental on
constitutional
....
differences
in
confined to
the reproductive systems of the intercrossed species. We see the truth of this conclusion in the vast difference in the
results
result,
of crossing
when
the same two species are crossed reciprocally,— that is, when one species is first used as the father and then as the
mother.
mother:
Analogy
analogy
from the consideration of dimorphic and trimorphic plants clearly leads
to
us to
the same conclusion, for when the forms are illegitimately united, they yield few or no seed, and their offspring are more or less sterile; and these forms
belong to
of
the same undoubted
species,
species
and
....
differ in no respect from each other except in their reproductive organs and functions.
The
Although the
fertility of varieties when intercrossed and