Pattern and Progress on the Geological Stage 473
seed of the dandelion, and in the flattened and fringed legs of the water
beetle, the relation seems at first confined to the elements of air and water.
Yet the advantage of plumed seeds no doubt stands in the closest relation to
the land being already thickly clothed by other plants; so that the seeds may
be widely distributed and fall on unoccupied ground. In the water-beetle,
the structure of its legs, so well adapted for diving, allows it to compete
with other aquatic insects, to hunt for its own prey, and to escape serving as
prey to other animals (p. 77).And epidemic extirpations, not generally attributed to microorganisms in Darwin's
day and therefore an apparently clear case of regulation by non-competitive forces,
may be caused by "parasitic worms"—an organism large enough to engender
thoughts about overt and visible competition between parasite and host: "When a
species, owing to highly favorable circumstances, increases inordinately in
numbers in a small tract, epidemics—at least, this seems generally to occur with
our game animals—often ensue: and here we have a limiting check independent of
the struggle for life. But even some of these so-called epidemics appear to be due
to parasitic worms, which have from some cause, possibly in part through facility
of diffusion amongst the crowded animals, been disproportionably favored: and
here comes in a sort of struggle between the parasite and its prey" (p. 70).
Wedging and the causes of extinction
Darwin's most striking metaphor for biotic competition, invoked from his very first
jotting about natural selection (after his Malthusian insight of October 1838) to the
Origin of Species, imagines a surface packed tightly with wedges, representing
nature chock full to its carrying capacity. Such a maximally crowded world
provides only one path for entry—by forcing ("wedging") another creature out.
Biotic competition rules with a vengeance: "The face of Nature may be compared
to a yielding surface, with ten thousand sharp wedges packed close together and
driven inwards by incessant blows, sometimes one wedge being struck, and then
another with greater force" (1859, p. 67).
The longer version of Natural Selection, Darwin's original manuscript,
presents an even more revealing characterization, replete with almost frantic
images of crowding, an explicit focus on species (in other passages, Darwin
construes the wedges as individual organisms), and the relegation of physical
limitation to an underlying layer, usually not penetrated, while the real work of
nature proceeds by biotic struggle in the visible region above (Natural Selection,
1856 - 1858,1975 edition, edited by Stauffer, p. 208):
Nature may be compared to a surface covered with ten-thousand sharp
wedges, many of the same shape and many of different shapes representing
different species, all packed closely together and all driven in by incessant
blows: the blows being far severer at one time than at another; sometimes a
wedge of one form and sometimes another being struck; and one driven
deeply in forcing out others; with the jar and shock often