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On the Origin of Species

Hybridism
Distinction between the sterility of first crosses and of hybrids -- Sterility various in
degree, not universal, affected by close interbreeding, removed by domestication -- Laws
governing the sterility of hybrids -- Sterility not a special endowment, but incidental on
other differences, not accumulated by natural selection -- Causes of the sterility of first
crosses and of hybrids -- Parallelism between the effects of changed conditions of life and
of crossing -- Dimorphism and trimorphism -- Fertility of varieties when crossed and of
their mongrel offspring not universal -- Hybrids and mongrels compared independently
of their fertility -- Summary.
The view commonly entertained by naturalists is that species, when intercrossed, have
been specially endowed with sterility, in order to prevent their confusion. This view
certainly seems at first highly probable, for species living together could hardly have
been kept distinct had they been capable of freely crossing. The subject is in many ways
important for us, more especially as the sterility of species when first crossed, and that of
their hybrid offspring, cannot have been acquired, as I shall show, by the preservation of
successive profitable degrees of sterility. It is an incidental result of differences in the
reproductive systems of the parent-species.
In treating this subject, two classes of facts, to a large extent fundamentally different,
have generally been confounded; namely, the sterility of species when first crossed, and
the sterility of the hybrids produced from them.
Pure species have of course their organs of reproduction in a perfect condition, yet when
intercrossed they produce either few or no offspring. Hybrids, on the other hand, have
their reproductive organs functionally impotent, as may be clearly seen in the state of the
male element in both plants and animals; though the formative organs themselves are
perfect in structure, as far as the microscope reveals. In the first case the two sexual
elements which go to form the embryo are perfect; in the second case they are either not
at all developed, or are imperfectly developed. This distinction is important, when the
cause of the sterility, which is common to the two cases, has to be considered. The
distinction probably has been slurred over, owing to the sterility in both cases being
looked on as a special endowment, beyond the province of our reasoning powers.
The fertility of varieties, that is of the forms known or believed to be descended from
common parents, when crossed, and likewise the fertility of their mongrel offspring, is,
with reference to my theory, of equal importance with the sterility of species; for it seems
to make a broad and clear distinction between varieties and species.
DEGREES OF STERILITY.
First, for the sterility of species when crossed and of their hybrid offspring. It is
impossible to study the several memoirs and works of those two conscientious and
admirable observers, Kolreuter and Gartner, who almost devoted their lives to this
 
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