Species

204 Species

Another contributor also regards Turesson’s approach as a populational one.^73 In
any event, it is clear that Turesson’s conception of species taxa included both a “bio-
logical” (i.e., a genetic) and an ecological aspect. It may be that Turesson’s scheme
is itself “typological,” but its implementation and later inuence is not, in itself,
necessarily so.

German Thinkers: Isolation Is the Key

Species concepts played an increasing role in the thinking of several German-

speaking biologists in the early part of the century. In particular, the views of

Erwin Stresemann, curator of birds at the Berlin Museum, and mentor to both

Ernst Mayr and Bernhard Rensch, were inuential. In 1919, he had written that

morphology as a criterion of species had been abandoned by the late 1890s among

ornithologists in favor of physiological divergence, as evidenced by reproductive

isolation. He said:

forms of the rank of species have physiologically diverged from each other to such

an extent, that they can come together again without mixing with each other. ...

Morphological divergence is thus independent of physiological divergence.^74

Bernhard Rensch was strongly inuenced by this approach, and he dened sev-
eral terms to deal with the reproductive isolation of species and within species in the
case of races that do not interbreed although overall the species has a shared gene
pool.^75 He called these Rassenkreise, or “race circles,” and complexes of incipient
species that replaced each other geographically, he called Artenkreise, or “species
circles.”^76 Although he doesn’t dene species in the 1959 work, he does talk fre-
quently about “good species” being those that are isolated by sexual or genetic dif-
ferences when in contact.
A review of the state of play at the end of the period before the Synthesis began
was published by British zoologist Guy C. Robson,^77 and much of the modern
debate was pregured there—polymorphisms, reproductive isolation, allopatry
(under another name, of course), and genetic variance are all discussed. At that
time, however, it was unclear whether or not the neo-Lamarckian view of the
inheritance of acquired characters was a viable view or not. Robson tended to
think not, but he allowed that later research may show otherwise. For him, species
are not the necessary outcome of evolution, and are recognized by correlations of
differentiated characters that “hang together.”^78 He notes that sampling of speci-
mens fails to reproduce the wider diversity of characters of the larger assemblage
of the taxon. Races are localized more or less homogeneous groups in what is a
continuous distribution of forms. He rejects inability or disinclination to hybridize

(^73) Salisbury 1940, 332.
(^74) Stresemann 1919, 64, 66, quoted in Mayr and Provine 1980, 415.
(^75) Rensch 1980, 294f.
(^76) Rensch 1928, Rensch 1929, Rensch 1947, Rensch 1959.
(^77) Robson 1928.
(^78) Robson 1928, 223–224.