254 THE STRUCTURE OF EVOLUTIONARY THEORY
(and even privileges) another interpretation. Suppose that Unity of Type records no
mysterious groundplan of created design, but only the actual, retained form of a
common ancestor at the base of a bush of descent? Then homology can be simply
explained as passive retention in the genealogy of diversified descendants—not an
archetype of intelligent design, but only the signature of history.
In this context of evolutionary reform, we may then inquire about the causes
of these common ancestral structures in distant pasts. And Darwin now makes his
fundamental choice by affirming fealty to the English lineage of adaptationist
thought. He argues that ancestral structures, forming the great homologies of Unity
of Type, initially arose, by natural selection, as adaptations to "organic and
inorganic conditions of life" in ancestral environments. Thus, the dichotomous
poles of Unity of Type and Conditions of Existence achieve a single and unified
explanation under natural selection— as immediate adaptations to present
environments (Conditions of Existence), or as adaptations to ancient environments,
transmitted by inheritance to diversified descendants (Unity of Type). The old
dichotomy, in fact, expresses no clash of opposites at all, but only marks the
temporally sequential representations of one dominant principle in evolution—
adaptation by natural selection. Thus, since adaptation embodies the principle of
Conditions of Existence, and since adaptation builds both ends of the old
dichotomy, Conditions of Existence becomes the victorious pole of the old
contrast, in Darwin's own words the "higher law; as it includes, through the
inheritance of former adaptations, that of Unity of Type."
Yet Darwin, far too sophisticated a thinker to embrace extreme positions,
could not claim that natural selection and adaptation—though responsible for both
poles of the old dichotomy—reigned exclusively in nature. Darwin knew that
primary judgments in natural history must be rendered in terms of relative
frequencies. Indeed, he had written as his last line in the Introduction to the Origin
of Species, first edition: "I am convinced that Natural Selection has been the main
but not exclusive means of modification" (1859, p. 6). He also reacted as strongly
as his genial temperament ever permitted against those who charged him with false
claims of exclusivity. In such cases, he usually cited this line from the Origin in
vindication—as in his famous, almost rueful statement {Origin, 6th ed., 1872b, p.
395): "As my conclusions have lately been much misrepresented, and it has been
stated that I attribute the modification of species exclusively to natural selection, I
may be permitted to remark that in the first edition of this work, and subsequently,
I placed in a most conspicuous position—namely, at the close of the Introduction
the following words: 'I am convinced that natural selection has been the main but
not the exclusive means of modification.' This has been of no avail. Great is the
power of steady misrepresentation."
Thus, while extending natural selection to cover both poles of the old
dichotomy between unity of type and conditions of existence, Darwin also listed
the main supplements to selection among causes of evolutionary change: use and
disuse, direct action of external conditions, and laws of