3 The New York Years (1931–1953)

History of Biology 353
The concept of organic evolution includes two independent processes, (1) transformation
(anagenesis, phyletic evolution, the “vertical,” adaptive component)
and (2) diversification (cladogenesis, speciation, the “horizontal” component of
change in differentiating populations and incipient species). At first Darwin was
aware of this difference but later did not sufficiently stress the independence of
these processes. He developed the theory of geographic speciation on islands and
believed that his “principle of character divergence” (ecological differentiation
supposedly explaining sympatric speciation) would overcome the difficulty that
on continents barriers are apparently lacking. Darwin more or less retained his
theoretical views from the 1840s on, even though he changed his mind regarding
the importance of certain factors, e.g., geographical isolation and soft inheritance.
He reversed himself with respect to geographical isolation during the 1850s, when
he accepted sympatric speciation through ecological, seasonal or behavioral specialization
treating speciation increasingly as a process of adaptation and no longer
considered species as reproductively isolated populations.
Darwin’s theses on evolution are summarized by Mayr in a small book, One
Long Argument. Charles Darwin and the Genesis of Modern Evolutionary Thought
(1991g) and related to the development of the modern theory of evolution. Darwin’s
Origin was “one long argument” against special creation, not one in favor of
natural selection. His scientific method was the time-honored method of the best
naturalists going continually back and forth between making observations, posing
questions, establishing hypotheses or models, testing them by making further
observations, and so forth (the hypothetico-deductive method). Perhaps Darwin
was the first naturalist who used this method so consistently and with as much
success. With his talents and interests he became a bridge-builder between various
scientific fields.
Soon after 1859 most scientists accepted Darwin’s theses of evolution as such,
common descent and multiplication of species which Mayr (1991g) termed the
“First Darwinian Revolution” (the “Second Darwinian Revolution” refers to the
Evolutionary Synthesis of the 1940s, when the geneticists and naturalists reached
a consensus, p. 183) 4 .TheOrigin was a superb treatment of the thesis of common
descent and a great plea for the efficacy of natural selection, but, according to
Mayr, it was vague and contradictory both on the nature of species and on the
mode of speciation. Darwin’s theses challenged the traditional religious and philosophical
views of his time, although not every one of his theses was in conflict
with all of them. The concept of natural selection was evidently not a reflection
of the industrial revolution and the socioeconomic situation because this concept
was almost unanimously rejected by Darwin’s contemporaries. Ideological
factors, like the philosophy of essentialism, also had a powerful effect on the rejection
of several of his theses. Darwin accepted the strict working of “natural
laws” at the physiological level, but was aware of chance (stochastic) processes
4 Occasionally, Mayr (1977a, 1982d: 116–117) also referred to Darwin’s theories of common
descent and natural selection as the “first” and “second” Darwinian revolution,
respectively.