240 THE STRUCTURE OF EVOLUTIONARY THEORY
more diversified in habits and structure the descendants of our carnivorous
animals became, the more places they would be enabled to occupy
(Darwin, 1859, p. 113).Darwin continually invokes the calculus of individual success by number of
descendant taxa (see also 1859, p. 116, and Stauffer, ed., 1975, p. 228): "As a
general rule, the more diversified in structure the descendants from any one species
can be rendered, the more places they will be enabled to seize on, and the more
their modified progeny will be increased" (Darwin, 1859, p. 119).
Often, he mixes both criteria—the adaptive success of extreme variants in
struggle, and the calculus of descendant taxa—in a single statement.
Here in one way comes in the importance of our so-called principle of
divergence: as in the long run, more descendants from a common parent
will survive, the more widely they become diversified in habits,
constitution and structure so as to fill as many places as possible in the
polity of nature [organismic level], the extreme varieties and the extreme
species will have a better chance of surviving or escaping extinction, than
the intermediate and less modified varieties or species [taxon level]. But if
in a large genus we destroy all the intermediate species, the remaining
forms will constitute sub-genera or distinct genera, according to the almost
arbitrary value put on these terms (in Stauffer, ed., 1975, p. 238).But are these two statements really equivalent? Does the lower level claim for
organismic advantage imply the higher level phenomenon of species proliferation
without reference to any higher level causes?
In his most striking passage, indicating that he did grasp the need for higher
level sorting based upon such group properties as the range of variation, Darwin
attributes the success of introduced placentals in Australia not, as we might
anticipate from ordinary natural selection, to the adaptive biomechanical
superiority of placental design (honed in the refiner's fire of more severe
competition in Eurasia and America), but to a greater range of placental variation
across taxa, produced by their later stage in the historical process of divergence.
A set of animals, with their organization but little diversified, could hardly
compete with a set more perfectly diversified in structure. It may be
doubted, for instance, whether the Australian marsupials, which are divided
into groups differing but little from each other, and feebly representing, as
Mr. Waterhouse and others have remarked, our carnivorous, ruminant, and
rodent mammals, could successfully compete with these well-pronounced
orders. In the Australian mammals, we see the process of diversification in
an early and incomplete stage of development (Darwin, 1859, p. 116).The causes of trends
Trends represent the primary phenomenon of evolution at higher levels and longer
time scales. Trends therefore pose the key challenge, the ultimate making