340 THE STRUCTURE OF EVOLUTIONARY THEORY
slight difference in the offspring from their parents, and a cause for each must
exist, it is the steady accumulation, through natural selection, of such differences,
when beneficial to the individual, that gives rise to all the more important
modifications of structure, by which the innumerable beings on the face of this
earth are enabled to struggle with each other, and the best adapted to survive"
(1859, p. 170).
But the fuller treatment of the 1868 work—Darwin's longest book, and
primarily written as a treatise on variation, not selection—asserts this fundamental
claim even more forcefully. As in the Origin, Darwin does allow (1868, vol. 2, p.
320 and p. 355, for example) that correlations of growth produce features
(including taxonomically important markers) independent of utility. But the
domination of selection, by arguments of relative frequency and importance, now
becomes even more explicit, as in this statement from the introductory chapter: "I
shall in this volume treat, as fully as my materials permit, the whole subject of
variation under domestication. We may thus hope to obtain some light, little
though it be, on the causes of variability.... During this investigation we shall see
that the principle of Selection is all important. Although man does not cause
variability and cannot even prevent it, he can select, preserve, and accumulate the
variations given to him by the hand of nature in any way which he chooses; and
thus he can certainly produce a great result" (1868, vol. 1, p. 3).
Darwin, as I argued previously, may not be a consistently brilliant writer in
the tradition of Huxley or Lyell. But he did exceed his more stylish colleagues in a
literary gift for inventing metaphors that capture the essence of complex ideas.
Most of Darwin's enduring lines fall into this category—the face of nature bright
with gladness, struggle for existence, survival of the fittest, wedging as an image
for competition. The Origin introduces strikingly appropriate and beautifully
crafted metaphors in crucial places—the tree of life in the summary of natural
selection at the end of Chapter 4 (pp. 129-130), and the entangled bank of the
book's final flourish (p. 489).
Darwin also developed a remarkable metaphor to summarize his conviction
about the relative importance of selection and variation. He introduced this long
passage at the end of his chapter on selection in his 1868 work—the "interloper"
chapter, if you will, where the dominating force surveys an entire volume devoted
to subservients. We might label this image as the metaphor of building stones for
the house of morphology. No other passage in all Darwin's writing so strongly
illustrates the domination of selection over raw material.
All components for the primacy of selection, and for the inconsequentiality of
constraint (and other internal factors), flow together in this striking image:
Selection may depend upon variation, but the character of variation hardly matters
(so long as appropriate amounts and styles be present), given the power of
selection. Variation cannot be truly random, and we should interest ourselves in its
particular forms and biases (the shapes of stones used by the mason). But, in the
deepest sense, these preferred forms exert no influence upon the final building
when selection (the architect) takes charge. For laws