The Structure of Evolutionary Theory

(Michael S) #1

Internalism and Laws of Form 339


And yet, though Darwin now presents more examples of correlated variability
than he had provided in 1859, the main thrust of the 1868 volumes, as we shall see
in the next section, only accentuates the dominance of selection over these
exceptions to its domination.


The "Quite Subordinate Position" of Constraint to Selection


TO SELECTION

Paradoxically perhaps, an absence of strong concern for a subject may be better
expressed by inadequacy of treatment than by total evasion. Had Darwin simply
omitted the subject of correlated variability altogether, we would know little about
his attitude toward constraint (however much we might infer from his
indifference). But the weakness of his limited discussion reveals far more. As E. S.
Russell rightly remarks (1916, p. 240): "Darwin's conception of correlation was
singularly incomplete. As examples of correlation he advanced such trivial cases as
the relation between albinism, deafness and blue eyes in cats, or between the
tortoise-shell color and the female sex. He used the word only in connection with
what he called 'correlated variation.' ... He took it for granted that the 'correlated
variations' would be adapted to the original variation which was acted upon by
natural selection." (I find Russell a bit harsh in this particular claim. Correlated
variability may be "adapted" to the primary target of selection in the sense that
organic coherence in growth must be maintained, for the correlation marks an
inherited pathway. But Darwin clearly designates the realized feature as potentially
independent of utility, though admittedly not harmful, for selection would then
work to eliminate it.)
Darwin's discussion of variation in the Origin clearly illustrates his conviction
about the primacy of selection over any internal drive in supply of variation or
constraint. Variation (with its three key properties) must be available as raw
material, but all shaping and change arise by natural selection. In those rare cases
when change can be traced directly to variability itself—that is, when a
phenomenon caused by variability becomes visible as a property of a population,
rather than the oddity of an individual—we must seek an explanation in the
relaxation of selection's usual vigilance and control. Rudimentary parts, for
example, become free from selective control, and may therefore manifest the
influence of subsidiary factors, including constraint, that selection would ordinarily
mask: "Rudimentary parts, it has been stated by some authors, and I believe with
truth, are apt to be highly variable... their variability seems to be owing to their
uselessness, and therefore to natural selection having no power to check deviations
in their structure. Thus rudimentary parts are left to the free play of the various
laws of growth, to the effects of long continued disuse, and to the tendency to
reversion" (1859, pp. 149-150).
The concluding statement in Chapter 5 on "Laws of Variation" clearly
expresses the domination of selection: "Whatever the cause may be of each

Free download pdf