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Charlotte Weissman
“The First Evolutionary Synthesis: August Weismann and the Origins of
Neo-Darwinism”
Abstract
In this thesis I describe and analyze the development of the evolutionary ideas of
August Weismann, one of the most influential evolutionary biologists of the 19th
century, generally considered second only to Darwin in importance. He was an
uncompromising selectionist, challenging prevalent, seemingly self-evident ideas, the
most significant of which was the inheritance of acquired characters. By doing so, he
polarized the scientific community and forced a reorganization of the boundaries of
the evolutionary discourse. Weismann’s work attracted much attention during his
lifetime, although many objections were raised to his views and theories.
Nevertheless, his ideas, particularly his extreme selectionism, became very influential,
contributing to the shaping of genetics and to the Modern Evolutionary Synthesis of
the 20th century.
Although excellent papers on Weismann’s theories and research exist, the historical
study of Weismann’s ideas, unlike those of Darwin, is not systematic, and there is no
book-length discussion of his contributions and intellectual history. In particular, his
early work and the developments that led to his grand heredity-development-evolution
synthesis have received relatively little analysis. In this thesis I focus on of his early
work, between the 1860s and the mid-1890s. I analyze his early embryological
research, and follow the development of his ideas about evolution, development and
heredity, trying to uncover the roots of his theories and point to the turning points in
his intellectual history. I suggest that three major themes were central to Weismann’s
intellectual edifice: the principle of developmental segregation, selectionism, and a
cytology-focused view of heredity. These three themes were the pillars on which
Weismann’s scientific work was erected, and were the basis of his ever-developing
views.
Weismann’s most enduring work was theoretical. There were progressive changes in
his ideas as he developed his theory of heredity, because he was continuously re I
formulating his theories in the light of the empirical investigations, especially
cytological and embryological research. In this thesis I provide an analysis of the
origins of Weismann’s synthesis, and follow Weismann’s thought from the beginning
of his career in zoology. I argue that his theory of heredity and evolution started its
development with his first empirical embryological observations in 1862.
Weismann was well-trained in histology and comparative anatomy. After qualifying
in medicine, an experience that contributed to his first successful embryological
investigations, he made important contributions to the field of invertebrate
embryology, and later to evolutionary biology. His work was influenced by Darwin’s
Origin of Species, which captivated him from the moment he read it, and guided his
thoughts thereafter. I also show how studying the processes of differentiation during
the embryogenesis of dipterans led Weismann to discover developmental segregation,
a principle that became constitutive for his ideas about the continuity of the germ
plasm and his future mature theory of heredity.
In 1893 Weismann suggested the first synthesis of Darwinism with prevailing
biological knowledge, a synthesis that rejected the inheritance of acquired characters
and became known as neo-Darwinism. His synthesis challenged and polarized the
opinions of biologists at the end of the 19th century, because it was based on the view
that selection alone was driving adaptive evolutionary changes. This view differed
from that of Darwin, who even in his early writing (and even more so later) added the
inheritance of acquired characters to natural selection as a factor in adaptive
evolution. As I show in this thesis, natural selection became the self-conscious core of
Weismann’s thinking, starting with his inaugural lecture, The Legitimation of
Darwin’s Theory, which was delivered in Freiburg University in 1868.
Natural selection was an overarching explanatory principle in Weismann’s thinking,
and became more important as he applied it and interpreted biological phenomena in
its light. Since the transformation of species was central to evolution, but its
mechanism was controversial, Weismann engaged in a debate with Moritz Wagner
over the role of isolation in speciation, arguing for the primacy of natural selection in
species formation. I analyze this debate, which started in 1868 as a reaction to
Wagner’s theory of migration and continued into the 1870s.
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Following Weismann’s career, I discuss his embryological experiments and outline
the development of his theory of the continuity of germ plasm, for which the principle
of developmental segregation and division of labor was crucial. I describe how
Weismann’s theory became consolidated as the dramatic and foundational discoveries
about the structure and behavior of cells were made in the third part of the 19th
century.
On the basis of cytological discoveries, Weismann formulated a theory of heredity
and development, which put together and systematized existing ideas about biological
inheritance. He suggested that the germ plasm had a definite hierarchical
organization, and was made up of vital particles, the “determinants,” and their
constitutive part, the “biophores.” I discuss the origins of this theory, and suggest that
the principle of segregation of formative components, which Weismann formulated
following his early embryological studies of dipterans, became one of its foundations.
I argue that his cytologically-oriented development-heredity theory led to the rejection
of the possibility of inheritance of acquired characters – to what became known as
Weismannism, or neo-Darwinism.
I conclude by discussing the way the publication of Weismann’s neo-Darwinian
theories affected biology, following his debates with other leading theoreticians and
empiricists. He faced much opposition to his views, and eminent scientists who held
other beliefs tried to refute his ideas. However, most of the debate over evolutionary
change was delineated, and to a large extend dominated, by Weismann’s neoDarwinian ideas. Even when his neo-Darwinism was rejected, the boundaries of the
discourse were drawn by his theories. The reaction to Weismann’s synthesis triggered
the development of alternative theories, such as new versions of neo-Lamarckism and
mutationism, and his neo-Darwinism was one of the foundations on which the
Moderns Evolutionary Synthesis that was constructed in the 20th century was erected.
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