236 THE STRUCTURE OF EVOLUTIONARY THEORY
Parental forms will then tend to succumb because natural selection favors their
extreme and divergent descendants in competition: "As in each fully stocked
country natural selection necessarily acts by the selected form having some
advantage in the struggle for life over other forms, there will be a constant
tendency in the improved descendants of any one species to supplant and
exterminate in each stage of descent their predecessors and their original parent"
(1859, p. 121).
All evolutionists know that the Origin of Species contains only a single figure.
This statement has been endlessly repeated in textbooks and lectures, but the true
significance of this figure remains obscure, because we nearly always misinterpret
the diagram (Fig. 3-5). We read this sole figure as Darwin's basic illustration of
evolution as a branching process. But Darwin did not construct his diagram for
such a general purpose. Rather, he devised this unique figure to provide a
surgically precise description of the principle of divergence, accompanied by
several pages of explanatory text (pp. 116-126). Note how only two species of the
original array (A—L) ultimately leave descendants— the left extreme A and the
near right extreme I. Note how each diversifying species first generates an upward
fan of variants about its modal form, and how only the peripheral populations of
the fan survive to diversify further. Note that the total morphospace (horizontal
axis) expands by divergence, even though only two of the original species leave
descendants. Darwin writes (1859, p. 121): "In each genus, the species, which are
already extremely different in character, will generally tend to produce the greatest
number of modified descendants; for these will have the best chance of filling new
and widely different places in the polity of nature: hence in the diagram I have
chosen the extreme species (A) and the nearly extreme species (I), as those which
have largely varied, and have given rise to new varieties and species." Darwin also
states that the success of extremes records the action of natural selection in its
usual mode of organismic struggle: "And here the importance of the principle of
benefit being derived from divergence of character comes in; for this will generally
lead to the most different or divergent variations (represented by the outer dotted
lines) being preserved and accumulated by natural selection" (1859, p. 117).
THE FAILURE OF DARWIN'S ARGUMENT AND THE NEED FOR
SPECIES SELECTIONI apologize to readers for this laborious (though, I trust, not uninteresting)
exposition of Darwin on divergence, but I have now reached the crux both of this
argument and, in one sense, of this entire book as well. I am advocating both the
necessity and importance of a hierarchical expansion of the theory of natural
selection, defending this position by combining the standard techniques of
validation in science and scholarship: empirical example, the logic of argument,
and historical illustration. Why, then, should so much space be accorded to
Darwin's views on divergence of character—especially since I have just
documented Darwin's attempt to render this second keystone