336 THE STRUCTURE OF EVOLUTIONARY THEORY
(1859, p. 144). (Note Darwin's interesting linguistic choice in designating this
example as so well "adapted" to illustrate constraint.)
Darwin also allowed that taxonomically important, and not just trivial,
characters could be shaped by correlations of growth: "Hence we see that
modifications of structure, viewed by systematists as of high value, may be wholly
due to unknown laws of correlated growth, and without being, as far as we can see,
of the slightest service to the species" (1859, p. 146).
Darwin, as all professional evolutionists know, had been writing a much fuller
version of his evolutionary views (a book that would have been about as long as
Lyell's three-volume Principles of Geology) when Wallace's note from Ternate
arrived in 1858. The hurried Origin of Species (1859) is an epitome (of 490 pages!)
without formal references. Darwin intended to complete and publish the longer
version, but never realized this project. Instead, he took most of the material
designated for the early part of Natural Selection (Darwin's putative title for the
full treatment), expanded his coverage, and published his longest book in 1868, the
two-volume Variation of Animals and Plants Under Domestication. (Incidentally,
the fourteen-page introductory chapter to volume 1 presents Darwin's clearest and
most cogent general summary of his evolutionary views and methodological
principles. This largely unread essay should be assigned to all students of
evolution.)
Volume one provides a chapter-by-chapter treatment of various domesticated
species (with Chapter 5 on "domestic pigeons," unsurprisingly given Darwin's
interest, as the most illuminating). This material largely represents an expansion of
Chapter 1 in the Origin, "Variation under domestication." Volume two then
presents Darwin's general ideas on variation and inheritance (an expansion of
Chapters 4 and 5 of the Origin, with much added material). Darwin actually
published more about inheritance than about natural selection, thus refuting the
common statement that he neglected the subject of heredity—a myth engendered
by the retrospective fallacy, given his success with selection and his limited impact
in resolving the principles of heredity. Volume two includes sets of chapters on
four subjects: inheritance, crossing, selection, and causes of variation, all capped
with the chapter that would become (along with his geological error on the
"parallel roads" of Glen Roy) his nemesis—Chapter 27 on "the provisional
hypothesis of pan-genesis" (Darwin's Lamarckian conjecture about the nature of
inheritance). Darwin included his material on structural constraint within this
volume, providing an expanded version of his views in Chapters 25 and 26 on
"correlated variability," a term that he now regards as preferable to "the somewhat
vague expression of correlation of growth" (1868, vol. 2, p. 319), used previously.
However, despite extensive elaboration and addition of examples, Darwin's
treatment scarcely differs from the short version in the Origin. He presents the
same taxonomy for modes of "correlation of growth." Again, early modification
with propagating effects through ontogeny wins first place, although Darwin now
adds some interesting examples: "with short-muzzled races of the dog certain
histological changes in the basal elements of the