334 THE STRUCTURE OF EVOLUTIONARY THEORY
of the relative frequency tradition—see pp. 255-256). Natural selection builds a
feature, and others follow by correlational linkage to this generating cause.
Darwin clearly defines correlation of growth as a category contrary to natural
selection, for he explicitly excludes the common case of taxonomically correlated
structures that arise by separate selection on each feature, with later joint
propagation by simple inheritance (wings and beaks in birds). He defines
correlation of growth, on the other hand, as structurally forced association
independent of immediate selection: "We may often falsely attribute to correlation
of growth, structures which are common to whole groups of species, and which in
truth are simply due to inheritance; for an ancient progenitor may have acquired
through natural selection some one modification in structure, and, after thousands
of generations, some other and independent modification; and these two
modifications, having been transmitted to a whole group of descendants with
diverse habits, would necessarily be thought to be correlated in some necessary
manner" (1859, p. 146).
Darwin's genuine interest in correlations of growth arose from several
sources, including the mystery surrounding the subject. "The nature of the bond of
correlation is very frequently quite obscure... What can be more singular than the
relation between blue eyes and deafness in cats, and the tortoise-shell color with
the female sex?" (1859, p. 144). Again, as for the larger issue of variation in
general, Darwin follows a fruitful operational strategy: when you can't specify
causes, at least gather overt phenomena into reasonable categories. The short
epitome of the Origin (where correlation of growth receives seven pages of
discussion) lists four major categories:
- Adaptive modifications of early ontogeny that propagate effects into later
growth: "The most obvious case is, that modifications accumulated solely for the
good of the young or larvae, will, it may safely be concluded, affect the structure
of the adult; in the same manner as any malconformation affecting the early
embryo, seriously affects the whole organization of the adult" (1859, p. 143). - Correlated variation in serially homologous and symmetrical structures of
the body. (Note how Darwin, following the popular structuralist theory of the
vertebral archetype, viewed correlated variation of jaws and limbs as potentially
homologous): "The several parts of the body which are homologous, and which, at
an early embryonic period, are alike, seem liable to vary in an allied manner: we
see this in the right and left sides of the body varying in the same manner; in the
front and hind legs, and even in the jaws and limbs, varying together, for the lower
jaw is believed to be homologous with the limbs" (1859, p. 143).
Although he cited the argument of ontogenetic correlation as first and most
obvious, Darwin devoted more interest and attention to this second category of
homologous variation. He presents (especially in the longer version of 1868—see
pp. 336-341) a variety of wide-ranging and intriguing cases (not all correct, of
course). For example, in regarding teeth and hair as homologous, Darwin
conjectures: "I think it can hardly be accidental, that if we pick