Seeds of Hierarchy 247
of nature; for although the structure of each organism stands in the most
direct and important relation to many other organic beings, and as these
latter increase in number and diversity of organization, the conditions of the
one will tend to become more and more complex, and its descendants might
well profit by a further division of labor; yet all organisms are
fundamentally related to the inorganic conditions of the world, which do
not tend to become infinitely more varied (ibid., p. 247).But Darwin also recognizes that ecological limits may not be sufficient to
restrain diversification, and he advances two other mechanisms, based on species
selection against traits of small populations. First, Darwin argues that long before
diversity reaches a physical limit (as quoted just above), species selection against
finely divided taxa with consequently small populations will balance ordinary
natural selection for further diversification: "If there exist in any country, a vast
number of species (although a greater amount of life could be supported) the
average number of individuals of each species must be somewhat less than if there
were not so many species; and any species, represented by but few individuals,
during the fluctuation in number to which all species must be subject from
fluctuations in seasons, number of enemies, etc., would be extremely liable to total
extinction" (ibid., pp. 247-248).
Finally, Darwin cites another reason for species selection against populations
of unusually small size. Such populations are not only more prone to extinction;
they are also less subject to further speciation because such a restricted number of
individuals per population implies an insufficiency of opportunity for the origin of
rare favorable variants: "Lastly we have seen... that the amount of variations, and
consequently of variation in a right or beneficial direction for natural selection to
seize on and preserve, will bear some relation within any given period, to the
number of individuals living and liable to variation during such period:
consequently when the descendants from any one species have become modified
into very many species, without all becoming numerous in individuals,... there
will be a check amongst the less common species to their further modification"
(ibid., p. 248).
In a lovely closing metaphor, Darwin provides a fine description of the central
hierarchical concept of balance produced by negative feedback between levels.
Number of species will equilibrate at a stable level of diversification when positive
selection at the organismal level (Darwin's argument for the advantages of extreme
variants) becomes balanced by negative selection at the species level
(disadvantages of small population size): "the lesser number of the individuals,"
Darwin writes, "serving as a regulator or fly-wheel to the increasing rate of further
modification, or the production of new specific forms" (ibid., p. 248).
In summary, I have documented both Darwin's discomfort with his forced
attempt to explain the primary species-level phenomenon of diversity by natural
selection of extreme organismal variants, and his inability to complete the
argument without an explicit invocation of species selection against taxa with
small populations. His attempt to render his "principle of divergence"