The Essence of Darwinism and the Basis of Modern Orthodoxy 113
four—the branchial circulation in young bird and mammalian embryos as
indications of a "community of descent" with an aquatic past. This common
condition in embryonic frogs, birds, and mammals cannot reflect design for current
function: "We can not, for instance, suppose that in the embryos of the vertebrata
the peculiar loop-like course of the arteries near the branchial slits are related to
similar conditions, — in the young mammal which is nourished in the womb of his
mother, in the egg of the bird which is hatched in a nest, and in the spawn of a frog
under water" (p. 440).
The key argument of the section on taxonomy makes the same point in a
different form: if animals had experienced no history of change, and were created
in accord with current needs and functions, then why should similar anatomical
designs include creatures of such widely divergent styles of life? Darwin writes, in
the opening paragraph of his discussion on taxonomy: "The existence of groups
would have been of simple signification, if one group had been exclusively fitted
to inhabit the land, and another the water; one to feed on flesh, another on
vegetable matter, and so on; but the case is widely different in nature; for it is
notorious how commonly members of even the same subgroup have different
habits" (p. 411).
These arguments strike us as most familiar when based on organic form, but
fewer evolutionists recognize that method four also under girds Darwin's two
chapters on biogeography (11 and 12). Darwin uses dissonance between organism
and dwelling place as the coordinating theme of these chapters: the geographic
distributions of organisms do not primarily suit their current climates and
topographies, but seem to record more closely a history of opportunities for
movement. Again, Darwin presents the basic argument in his first paragraph (p.
346): "In considering the distribution of organic beings over the face of the globe,
the first great fact which strikes us is, that neither the similarity nor the
dissimilarity of the inhabitants of various regions can be accounted for by their
climatal and other physical conditions."
Example tumbles upon example throughout these two chapters. Darwin notes
that northern hemisphere organisms of subarctic and north temperate climes
maintain far closer taxonomic similarity than the current geographic separation of
their continents would imply. He therefore interprets these likenesses as vestiges of
history—preserved expressions of the glacial age, when these climatic bands stood
further to the north, near the Arctic Circle where all northern continents virtually
touch (p. 370). He also finds too much organic similarity for the modern range of
climatic differences along lines of longitude from north to south poles, and he
again implicates the climax of glacial ages as a time of formation (with modern
persistence as a vestige), when even a subarctic species might migrate in comfort,
on a cold earth, across the equator from north to south along a single line of
longitude. Invoking a complex and graphic metaphor for history, Darwin writes of
disjunct distributions on opposite hemispheres, and of geographic refugia at high
altitudes of lower latitudes between these endpoints:
The living waters may be said to have flowed during one short period from
the north and from the south, and to have crossed the equator; but