The Structure of Evolutionary Theory

386 THE STRUCTURE OF EVOLUTIONARY THEORY

for evolution at all scales and times). He buttressed natural selection by noting that
breeders had produced this extensive range of results by propagating favored forms
from a cornucopia of essentially isotropic and undirected variation.
Whitman, of course, accepted the first contention, but refuted the second by
challenging one of Darwin's smaller claims. Darwin had observed two major
patterns of coloration within Columba livia—(1) "two-barred," with two black
bands on the front edges of the wings and uniform gray color elsewhere (Fig. 5-6);
and (2) "checkered" (spelled "chequered" by both Darwin and Whitman), with
black splotches on some or all wing feathers (Fig. 5-7), but also retaining the two
bars (usually in more indistinct form). Darwin regarded the two-barred state as
ancestral, and the checkered pattern as derived. Whitman reversed this sequence,
writing:

The wild rock pigeons, universally regarded as the ancestral stock of all our

domestic pigeons, exhibit two very distinct color patterns, one consisting of

black chequers uniformly distributed to the feathers of the wing and the

back, the other consisting of two black wing bars on a slate-gray ground.

The latter was regarded by Darwin as the typical wing

5 - 6. Whitman's figure of the two-barred wing pattern, which Darwin regarded as ancestral and

Whitman interpreted as an advanced stage in his orthogenetic sequence. From Whitman, 1919.

5 - 7. The checkered pattern, viewed by Darwin as derived and by Whitman as the primitive state

in the evolution of pigeon wing colors. From Whitman, 1919.