The Modern Synthesis as a Limited Consensus 533
the flow of nature. (We name species, under this view, only because our poor
minds can't handle continuity.)
Dobzhansky didn't deny the importance of Morgan's questions. But he argued
that evolution operates on a series of levels, and that the primary gaze of natural
history must not be focused upon these lower levels, but upon the broader
phenomenon of the origin of species itself (Darwin's title, after all). Diversity
represents the primary fact of nature (and the first topic of chapter 1 in
Dobzhansky's book). Diversity arises by the splitting of lineages—that is by
speciation. Speciation produces discontinuity in nature. How can a continuous
process of genetic change yield such bounded separations? The origin of
discontinuities between species must therefore be recast as the key problem in
evolutionary theory. Only a naturalist (better yet, a trained systematist) could have
reset the stage for synthesis in such a fruitful way.
The origin of hereditary variations is, however, only a part of the
mechanism of evolution... These variations may be compared with building
materials, but the presence of an unlimited supply of materials does not in
itself give assurance that a building is going to be constructed ... Mutations
and chromosomal changes are constantly arising at a finite rate, presumably
in all organisms. But in nature we do not find a single greatly variable
population of living beings which becomes more and more variable as time
goes on; instead, the organic world is segregated into more than a million
separate species, each of which possesses its own limited supply of
variability which it does not share with the others... The origin of species... constitutes a problem which is logically distinct from that of the origin of
hereditary variation (Dobzhansky, 1937, p. 119).
Mayr (1942) then furthered Dobzhansky's program by dedicating an entire
book to modes of speciation, and to realigning taxonomic practice with insights of
the developing Synthesis. He even formulated his title in conscious parallel to
Dobzhansky's (while both, of course, also claim and honor Darwin)—and as a
manifesto for the centrality of his field: Systematics and the Origin of Species.
Mayr's first paragraph (1942, p. 3) sets his theme and tone:
The rise of genetics during the first thirty years of this century had a rather
unfortunate effect on the prestige of systematics. The spectacular success of
experimental work in unraveling the principles of inheritance and the
obvious applicability of these results in explaining evolution have tended to
push systematics into the background. There was a tendency among
laboratory workers to think rather contemptuously of the museum man,
who spent his time counting hairs or drawing bristles, and whose final aim
seemed to be merely the correct naming of his specimens. A welcome
improvement in the mutual understanding between geneticists andsystematists has occurred in recent years.^
Mayr (1942) follows the characteristic pluralism of the early synthesis in
listing all valid evolutionary principles that can explain the data of systematics. His
major aim therefore follows the program of "healthy restriction"—