Ornithology, Evolution, and Philosophy 123

University Student in Greifswald and Berlin 27
Evidently he had followed up on certain topics discussed with Stresemann during
earlier visits to the Berlin museum. Most of this letter is here included because of
its historical interest (my translation; letters refer to the notes at the end; see also
Haffer 1994b, 1997):
Dear Dr. Stresemann!
[…] Now I would like to ask you to undertake several studies. You told me in
Berlin that [A. B.] Meyer mentioned in his BirdsofCelebes[A] that everything
in ornithology will soon be accomplished with the help of mathematical formulas.
How about you setting up principles of an ornithological mathematics?
Thiswouldsaveusfromaquaternarynomenclature(whichappearsnecessary
from a purely theoretical point of view). (1) Intensity index. Compare Parus
atricapillus rhenanus and subrhenanus, Motacilla flava rayi and thunbergi,
Carduelis l. linaria and cabaret. (2) a geographical, respectively climatic, factor
(desert, steppe, polar climate, island, humid coast line, etc.), (3) the individual
variation needs to be taken into account, and (4) the nomenclature of intermediateforms(Fig.1.9)
Five strongly differentiated forms are to be expected in this region where one
has to distinguish between the intermediate forms between 1 and 2, 2 and 3,
3 and 4, 5 and 6 on one hand and between 4 and 5 on the other hand. For
instance, the transition from 5 to 6 will be gradual so that a [b, c, etc.] will be
rather uniform. However, hybridization will take place between 4 and 5 and
therefore a strong individual variation will occur (see Long-tailed Tit [B]).
Therefore intermediate forms between 5 and 6 have to be labeled 6–5, by contrast
those between 4 and 5 should be designated 4 × 5, etc [C]. Isolation needs
to be taken into consideration too. If a form does not continuously receive fresh
blood [gene flow] from the parent form, it will enter into a totally aberrant
evolution. This, however, probably cannot be expressed mathematically. So
many subspecies have been described within recent years that the time is ripe
to write a “general Hartert” (or “The Theory of Geographical Variation and
of the Species”) [emphasis added].
The facts mentioned above will have to be discussed as well as modifying factors,
i.e., active factors (climate, etc., partly based on Görnitz [D]) and passive
factors (color, size, biology, etc.). In addition the following topics need to be
treated: (1) How is it possible that members of the same Formenkreis overlap
their ranges without hybridizing? and: (2) May similar and geographically
representative species nevertheless be members of different Formenkreise? An
example: If Ficedula hypoleuca and F. albicollis would exclude each other
geographically, they would surely be included in the same species. Conversely,
someone may say: I can no longer recognize the forms of the Yellow Wagtail
only as subspecies, although they mostly exclude one another geographically;
just as the delimitation of genera is more or less a matter of taste.
Furthermorethephylogeneticconnectionsofthedifferentspeciesmustbeclarified.
With the help of mutations and several representative forms like Sprosser
[Luscinia luscinia] and the Short-toed Tree-creeper (C[erthia] brachydactyla)