132 THE STRUCTURE OF EVOLUTIONARY THEORY
incidental or dependent on unknown differences, chiefly in the reproductive
systems, of the species which are crossed (p. 260).In what I regard as Darwin's most brilliant use of his favorite device—
argument by analogy—he then compares hybrid sterility with incompatibility in
hybrid grafts (whereas grafts between varieties of the same species usually "take").
I find this comparison particularly compelling because we would not be tempted to
construct an argument about species selection to explain the incompatibility of
grafts—as no advantage for the integrity of species accrues thereby, especially
since the "experiment" of grafting between two species almost never occurs in
nature. Yet the logical structures of these two arguments about grafting and
sterility, as well as the attendant results, share an identical logic—joining within
species, and maintenance of separation between species, based upon incidental
effects wrought by increasing degrees of difference evolved for other reasons:
It will be advisable to explain a little more fully by an example what I mean
by sterility being incidental on other differences, and not a specially
endowed quality. As the capacity of one plant to be grafted or budded on
another is so entirely unimportant for its welfare in a state of nature, I
presume that no one will suppose that this capacity is a specially endowed
quality, but will admit that it is incidental on differences in the laws of
growth of the two plants... The facts by no means seem to me to indicate
that the greater or lesser difficulty of either grafting or crossing together
various species has been a special endowment; although in the case of
crossing, the difficulty is as important for the endurance and stability of
specific forms, as in the case of grafting it is unimportant for their welfare
(pp. 261-263).Darwin then drives the point home with a lovely prose flourish (and a memorable
visual image!) in explicitly rejecting an appeal to supraorganismal selection.
Nature knows no explicit principle of higher-level order. "There is no more reason
to think that species have been specially endowed with various degrees of sterility
to prevent them crossing and blending in nature, than to think that trees have been
specially endowed with various and somewhat analogous degrees of difficulty in
being grafted together in order to prevent them becoming inarched in our forests"
(p. 276).
THE DEVELOPMENT OF DARWIN'S VIEWS ON ORGANISMIC SELECTION. If the first
edition of the Origin only marked a waystation in fluctuation or degree of
commitment, then Darwin's stand on organismic selection, however strongly
expressed in this initiating volume, might not be deemed so central to his
worldview. But Ruse (1980) has documented Darwin's continuing and increasing
attention to this issue—particularly as he argued with Wallace (see also Kottler,
1985) about the principle of incidental effects to explain hybrid sterility as a side
consequence of natural selection rather than a direct product of species selection.
Ruse writes (1980, p. 620): "By the end of the decade [the 1860's] with respect to
the animal and plant worlds, there was