252 THE STRUCTURE OF EVOLUTIONARY THEORY
agreement in structure, which we see in organic beings of the same class, and
which is quite independent of their habits of life." In another critically placed
passage, introducing the subject of "Morphology" in Chapter 13, Darwin waxes
almost poetic about unity of type (p. 434): "This is the most interesting department
of natural history, and may be said to be its very soul. What can be more curious
than that the hand of a man, formed for grasping, that of a mole for digging, the leg
of the horse, the paddle of the porpoise, and the wing of the bat, should all be
constructed on the same pattern, and should include the same bones in the same
relative proportions."
These two principles have always dwelled together in exquisite tension. Any
complete account of morphology must call upon both phenomena, for most
organisms are well adapted to their immediate environments, but also built on
anatomical ground plans that transcend any particular circumstance. Yet the two
principles seem opposed in a curious sense—for why should structures adapted for
particular ends root their basic structure in homologies that do not now express any
common function (as in Darwin's example of mammalian forelimbs)?
The designation of one principle or the other as the causal foundation of
biology virtually defines the position of any scientist towards the organic world
and its causes of order (see, especially, Russell's superb 1916 book on this
dichotomy). Shall we regard the plan of high-level taxonomic order as primary,
with local adaptation viewed as a set of minor wrinkles (often confusing) upon an
abstract majesty? Or do local adaptations build the entire system from the bottom
up? This dichotomy set the major debate of pre-Darwinian biology: does God
reveal himself in nature primarily by the harmony of taxonomic structure, or by the
intricacies of particular adaptations (see Section II, this chapter)? This dichotomy
continues to define a major issue in modern evolutionary debates: does functional
adaptation or structural constraint maintain priority in setting evolutionary
pathways and directions (see Chapters 10-11)?
This issue of primacy between the two principles has held the central stage of
natural history for so long that national traditions have developed, with continental
preferences usually emphasizing unity of type (despite important exceptions like
Georges Cuvier), and mainstream anglophonic science generally favoring
adaptation (with exceptions for a few important pluralists like Richard Owen, or
dissenters like William Bateson or D'Arcy Thompson). We often blunder in our
historical understanding by assuming that evolution must be an ultimate watershed,
marking a complete break between a bad before and an enlightened after. In fact,
much continuity pass right through Darwin's rupture of history, with evolution only
providing a different explanation for unaltered principles and phenomena. The
good ship Dichotomy— Unity of Type vs. Conditions of Existence—entered the
Darwinian current by converting its terms from a debate about God's primary mode
of self-expression in nature to an argument about constraint and adaptation in
evolution.
We cannot understand Darwin without grasping this fundamental continuity