namely, the sterility of two
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 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 has probably 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
have descended from common parents, when intercrossed, and likewise the
fertility of their mongrel offspring, is, on my theory, of equal
importance with the sterility of species; for it seems to make a broad
and clear distinction between varieties and species.
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 subject, without being deeply
impressed with the high generality of some degree of sterility.
Kolreuter makes the rule universal; but then he cuts the knot, for in
ten cases in which he found two forms, considered by most authors as
distinct species, quite fertile together, he
unhesitatingly ranks them as varieties. Gartner, also, makes the rule
equally universal; and he disputes the entire fertility of Kolreuter s
ten cases. But in these and in many other cases, Gartner is obliged
carefully to count the seeds, in order to show that there is any degree
of sterility. He always compares the maximum number of seeds produced
by two species when crossed and by their hybrid offspring, with the
average number produced by both pure parent-species in a state of
nature. But a serious cause of error seems to me to be here introduced:
188