CHAPTER IX. HYBRIDISM.
1. DISTINCTION BETWEEN THE STERILITY OF FIRST CROSSES AND OF HYBRIDS.
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.