114 THE STRUCTURE OF EVOLUTIONARY THEORY
to have flowed with greater force from the north so as to have freely in-
undated the south. As the tide leaves its drift in horizontal lines, ... so have
the living waters left their living drift on our mountain summits, in a line
gently rising from the arctic lowlands to a great height under the equator.
The various beings thus left stranded may be compared with savage races
of man, driven up and surviving in the mountain fastnesses of almost every
land, which serve as a record, full of interest to us, of the former inhabitants
of the surrounding lowlands (p. 382).
Everyone cites the Galapagos in a virtual catechism about Darwin's evidence
for evolution, but few biologists can state how he invokes these islands in the
Origin. Most textbooks talk about a diversity of finches, each beautifully adapted
to available resources on different islands, or of variation in tortoise carapaces
from place to place. Both these stories exemplify both diversification and current
adaptive value—but Darwin speaks not a word about either case in the Origin!
In fact, Darwin invokes the Galapagos primarily as an extended example of
method four applied to biogeography: These islands house many endemic species,
necessarily created in situ according to his opponents. But why then should all
these endemics bear close relationship with species on the nearby American
mainland? A creationist might say that God fits creatures to immediate
circumstances, and that the Galapagos Islands, located so near America, must
resemble America in environment, and therefore be best suited to house species of
the same basic design. But now we grasp the beauty of the Galapagos as an almost
uncannily decisive natural experiment for the influence of history. These islands do
lie close to America, but could scarcely resemble the mainland less in climate,
geology and topography—for the Galapagos are volcanic islands in the wake of a
cool current that even permits access to the northernmost species of penguin!
Therefore, if the Galapagos endemics resemble American species, they must be
recording a history of accidental transport and subsequent evolutionary change—
not similar creations for similar environments. Darwin's brilliant argument
deserves citation in extenso:
Here almost every product of the land and water bears the unmistakable
stamp of the American continent. There are 26 land birds, and 25 of these
are ranked by Mr. Gould as distinct species, supposed to have been created
here; yet the close affinity of most of these birds to American species in
every character, in their habits, gestures, and tones of voice, was
manifest... why should this be so? Why should the species, which are
supposed to have been created in the Galapagos Archipelago, and nowhere
else, bear so plain a stamp of affinity to those created in America? There is
nothing in the conditions of life, in the geological nature of the islands, in
their height or climate, or in the proportions in which the several classes are
associated together, which resembles closely the conditions of the South
American coast: in fact there is considerable dissimilarity in these respects.
On the other hand, there is a considerable degree