Species

272 Species

of identication, he was not a nominalist, unlike several of his followers. He was
a pluralist as to the degree of difference between, and causes of, species. It is sig-
nicant that he thought that sexual constitutions were what caused the isolation of
species, not form or adaptive traits. Darwin’s evolving views are signicant, as his is
perhaps the rst and the most complete attempt to deal with the implications of the
transmutation of species.
The adaptationists such as Wallace, Romanes, and several others, however, were
monists. Species were formed solely by selection. Weismann is an interesting excep-
tion here—he appears to have allowed for stochastic causes as well as deterministic
selectionist causes of species. Poulton’s essay, discussing these turn-of-the-20th-
century arguments, effectively set the stage for the modern debates by introducing
the notion of a genetic population as the boundary of species. Several contemporary
writers such as Lotsy, Karl Jordan, and Turesson elaborated on that theme under the
recent introduction of Mendelian genetics, with Turesson reintroducing the older
notion of ecological habitat affecting form (albeit in a much more limited way).
Several authors treated asexuals as a different kind of entity than species; Fisher
even thinks asexuality is not a real phenomenon. This will become signicant later
in the century.
One thing that is obvious, even at this early stage in the debates over the “species
problem,” is that there is a distinction to be made between “universalist” species
concepts, which are intended to apply to all organisms, especially in regard to spe-
ciation mechanisms after Darwin, and those proposing more limited notions, which
apply only to some sorts of organisms. In the twentieth century, most conceptions of
species are universalist conceptions, and the properties that mark them out are those
derived from the proponent’s preferred mechanisms of speciation. Biospecies are
formed through the acquisition of reproductive isolating mechanisms in allopatry or
sympatry according to the preference of the author. Saltationists claim that all spe-
cies are formed through macromutations. Punctuated equilibrium theory later treats
speciation as a rapid process followed by a period of stasis, and so species are univer-
sally delimited by this “sudden” event, thus nding the individualism thesis and the
evolutionary conception of species congenial, and so on. Only a few people proposed
limited conceptions, such as ecological conceptions of agamospecies, which had no
general application.
So let us list the historical conclusions of this book in summary form.
Genus and species and their cognates in Greek and other languages are vernacu-
lar terms, logical terms, and biological terms. It is important to read each text in the
right context and understand that when a classical author, including Aristotle, uses
these terms (or their Greek equivalents), they may be using them in a vernacular or
technical logical sense. The biological notion of species did not develop until the
end of the sixteenth and through the seventeenth centuries . Consequently, it is
anachronistic to read the classical writers as making claims about biological species.
Throughout the history of biological thought, species has always been thought to
mean the generation of similar form. That is, a living kind or sort is that which has a
generative power to make more instances of itself. The generative conception of spe-
cies was the common view from the Greeks to the beginnings of Mendelian genetics
around 1900. Prior to this, there had been a “species question”—that is, the question