Structural Constraints, Spandrels, and Exaptation 1219
as a full topic in its own right, he does grant special prominence to functional shift as
a solution to a range of issues, including the overtly opposite pairing of "organs of
extreme perfection" and "organs of small importance." He even dignifies the
principle with a rarity in his own prose conventions, an adjective of intensification:
"In considering transitions of organs, it is so important to bear in mind the probability
of conversion from one function to another" (1859, p. 191, my italics).
But Darwin only came to appreciate the centrality of this principle when the
book that he considered most cogent as a general critique of natural selection—St.
George Mivart's On the Genesis of Species (1871)—led him to compose, for the 6th
and final edition of the Origin of Species (1872), the only chapter ever added to his
book, largely as a point by point refutation of Mivart's claims: the interpolated
Chapter 7 entitled "Miscellaneous objections to the theory of natural selection." As a
further observation on stylistic questions, I'm sure that Mivart's decision to name his
own book with a parody on Darwin's title must have caught Darwin's special
attention, and perhaps his ire. In calling his work On the Genesis of Species (rather
than the Origin), Mivart needled Darwin with the common taunt of the times (see pp.
139 - 140): that natural selection could play a minor and negative role in eliminating
the unfit, but that some other "positive" force must generate the fit. (I suspect that
most of us would prefer to have our ideas rejected as dangerously wrong, but at least
interesting and worthy of anathematization, rather than dismissed as correct, but
trivial.)
Mivart expresses the thoroughness of his condemnation (speaking of himself in
the third person) with a common rhetorical strategy in Victorian science: claiming the
ultimate fairness of an initially favorable impression, only dispelled and reversed by
careful and objective consideration of a catalogue of empirical evidence. In this
passage, Mivart asserts the "secondary and subordinate" role of natural selection,
while claiming that another "positive" mechanism, able to generate the fit (of his
book's title), must be sought (1871, p. 225):
He was not originally disposed to reject Mr. Darwin's fascinating theory.
Reiterate endeavours to solve its difficulties have, however, had the effect of
convincing him that that theory as the one or as the leading explanation of the
successive evolution and manifestation of specific forms is untenable. At the
same time he admits fully that "Natural Selection" acts and must act, and that
it plays in the organic world a certain though a secondary and subordinate
part.
The one modus operandi yet suggested having been found insufficient, the
question arises, Can another be substituted in its place? If not, can anything
that is positive, and if anything, what, be said as to the question of specific
origination?St. George Mivart (1817-1900) became something of a tragic figure in Victorian
biology. He devoted much of his career to reconciling biology and religion in terms
of his unconventional attitudes in each discipline—only to meet