1222 THE STRUCTURE OF EVOLUTIONARY THEORY
study of natural "sports," both the occasional large variants that can survive in nature,
and the teratological malformations that may not be viable, but that illustrate the
potential pathways of internally coordinated variation, following recognizable
channels of ontogeny, sexual variation, etc. "It is probable therefore that new species
may arise from some constitutional affection of parental forms—an affection mainly,
if not exclusively, of their generative system" (p. 233).
In the added chapter to his 6th and final edition, Darwin refers to Mivart as "a
distinguished zoologist," and admits that he has presented all viable objections to
natural selection "with admirable art and force" (1872b, p. 164). He then summarizes
Mivart's structuralist alternative, describing first the claim for channeling, and then
the argument for saltation. He rejects both, primarily because they lack either a
known mechanism or verified cases; Darwin then reasserts his belief in the efficacy
of gradualistic natural selection, working upon isotropic and undirected variation (pp.
187 - 188):
At the present day almost all naturalists admit evolution under some form. Mr.
Mivart believes that species change through "an internal force or tendency,"
about which it is not pretended that anything is known. That species have a
capacity for change will be admitted by all evolutionists; but there is no need,
as it seems to me, to invoke any internal force beyond the tendency to
ordinary variability, which through the aid of selection by man has given rise
to many well-adapted domestic races, and which through the aid of natural
selection would equally well give rise by graduated steps to natural races or
species...
Mr. Mivart is rather inclined to believe, and some naturalists agree with
him, that new species manifest themselves "with suddenness and by
modifications appearing at once." For instance, he supposed that the
differences between the extinct three-toed Hipparion and the horse arose
suddenly. He thinks it difficult to believe that the wing of a bird "was
developed in any other way than by a comparatively sudden modification of a
marked and important kind"; and apparently he would extend the same view
to the wings of bats and pterodactyls. This conclusion, which implies great
breaks or discontinuity in the series, appears to me improbable in the highest
degree.Darwin acknowledges that Mivart's argument about incipient stages had been
particularly troubling (p. 165): "The one new point which appears to have struck
many readers is, 'natural selection is incompetent to account for the incipient stages
of useful structures.' This subject is intimately connected with that of the gradation of
characters, often accompanied by a change of function—for instance, the conversion
of a swim-bladder into lungs." Darwin notes that he had dealt with this issue in his
original Chapter 6 on "difficulties," admits that he had not paid the subject sufficient
heed, and praises Mivart for an opportunity to correct his previous slighting (p. 185):
"A good opportunity has thus been afforded for enlarging a little on gradations of
structure, often associated with changed functions, —an important