142 THE STRUCTURE OF EVOLUTIONARY THEORY
sufficient raw material would then be setting a primary limit upon the rate and
style of evolutionary change, and selection would not occupy the driver's seat.
Darwin responds by denying this interpretation, and arguing that differing
intensities of selection, rather than intrinsically distinct capacities for variation,
generally cause the greater or lesser differentiation observed among domestic
species. I regard this argument as among the most forced and uncomfortable in the
Origin—a rare example of Darwinian special pleading. But Darwin realizes the
centrality of copiousness to his argument for the creativity of natural selection, and
he must therefore face the issue directly:
Although I do not doubt that some domestic animals vary less than others,
yet the rarity or absence of distinct breeds of the cat, the donkey, peacock,
goose, etc., may be attributed in main part to selection not having been
brought into play: in cats, from the difficulty in pairing them; in donkeys,
from only a few being kept by poor people and little attention paid to their
breeding; in peacocks, from not being very easily reared and a large stock
not kept; in geese, from being valuable only for two purposes, food and
feathers, and more especially from no pleasure having been felt in the
display of distinct breeds (p. 42).Second, copiousness must also be asserted in the face of a powerful argument
about limits to variation following modal departure from "type." To use Fleeming
Jenkin's (1867) famous analogy: a species may be compared to a rigid sphere, with
modal morphology of individuals at the center, and limits to variation defined by
the surface. So long as individuals lie near the center, variation will be copious in
all directions. But if selection brings the mode to the surface, then further variation
in the same direction will cease—and evolution will be stymied by an intrinsic
limitation upon raw material, even when selection would favor further movement.
Evolution, in other words, might consume its own fuel and bring itself to an
eventual halt thereby. This potential refutation stood out as especially serious—not
only for threatening the creativity of natural selection, but also for challenging the
validity of uniformitarian extrapolation as a methodology of research. Darwin re-
sponded, as required by logical necessity, that such limits do not exist, and that
new spheres of equal radius can be reconstituted around new modes: "No case is
on record of a variable being ceasing to be variable under cultivation. Our oldest
cultivated plants, such as wheat, still often yield new varieties: our oldest
domesticated animals are still capable of rapid improvement or modification" (p.
8).
I cannot here provide a full history for the subsequent odysseys of these key
Darwinian precepts. But a few cursory comments indicate how these claims have
remained central and contentious throughout the history of post-Darwinian
thought, and how they continue to underlie important debates within Darwinism
today.
The argument about copiousness, particularly as expressed in the claim for
limits to further variability after intense selection, dogged the 19th century