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A Role for August Weismann In The Darwinian
Revolution
S.Subramanya Sastry
1 Introduction
In 1859, Charles Darwin published the Origin Of Species in which he outlined a theory of evolution
based on the mechanism of natural selection. Initially, it was well-received both by the public as
well as the scientific community. This was in large part due to Darwin’s presentation of the theory
as well as the efforts of Darwin’s friends and colleagues to publicize the theory. Though Darwin’s
book ensured that a large number of people got converted to evolutionism, his mechanism of
natural selection was not very popular. Yet, Darwinism gained widespread popularity in spite of
the objections (technical and otherwise) raised against natural selection. Bowler ascribes this to
the flexibility on the part of Darwin and Darwinism1 . Though Darwin proposed natural selection
as the primary evolutionary mechanism, he accepted other auxiliary mechanisms too. The most
important of these was the transmission of acquired characteristics (those acquired by use-disuse)
first proposed by Lamarck in the early 1800s. In the rest of this essay, when Darwinism is referred
to, it encompasses all these auxiliary evolutionary mechanisms in addition to natural selection.
Between the 1860s to 1880s, Darwinism faced increasing objections that led to the proposal of
a number of alternative evolutionary mechanisms. In this melee, August Weismann, a German naturalist and a staunch Darwinian, published his theory of the germ plasm in the 1880s. He proposed
this theory to solve the problem of heredity, one of the outstanding problems of Darwinism 2 . As
part of this theory, he attempted to purge Darwinism of its Lamarckian trimmings and expounded
natural selection as the sole evolutionary mechanism. This stricter form of Darwinism has been
dubbed Neo-Darwinism or Ultra-Darwinism3. This precipitated matters for Darwinism by attract1 Peter
Bowler The Eclipse Of Darwinism, pages 13-15, 28
2 Ibid., page 41
3 People differ in
their use of the term Neo-Darwinism. In one usage, Neo-Darwinism is used to indicate the
Darwinian theory of evolution without resorting to any kind of indirect inheritance (Ralf-Dietmar Hegel, August Weismann: One of the First Synthetic Theorists of Evolutionary Biology, page 264). However, another usage of Neo-
1
ing much greater opposition than before.
Around this time, Mendelian genetics was being revived in the context of discontinuous mutation that scientists were working on. In the period between 1890s and 1930s, even though
Mendelian genetics was viewed as being contradictory with Neo-Darwinism, by the 1930s, both
these theories were successfully reconciled to yield a theory that was able to explain evolution
and heredity much more satisfactorily than all the other theories. Thus, around this time, Darwin’s theory of evolution achieved paradigm status in the field of evolutionary biology. Following
Dobzhansky, in the rest of this essay, this revised Darwinian theory of evolution will be referred to
as the synthetic theory of evolution4 .
In this essay, I will examine August Weismann’s role in this success story of Darwin’s evolutionary theory (albeit in the form of the synthetic theory of evolution). I will initially portray
Weismann as an ardent Darwinian. It is not very hard to support this claim if one examines his
writings. More importantly, I will examine the role he played in laying down the groundwork
for Mendelian genetics to take off which ultimately was responsible for a successful reception of
Darwin’s theory in the 1930’s.
Before proceeding to analyze Weismann’s role, I will provide a context for doing so. This is
based on Bowler’s analysis of the Eclipse Of Darwinism in the 1900’s which examines the status
of Darwinism around this period and the reasons for the eclipse.
1.1 Eclipse of Darwinism
As mentioned previously, in the period between the 1860s and 1880s, a number of technical objections were raised against the mechanism of natural selection. There were also objections that
were raised on theological grounds since Darwin’s theory reduced the status of man to yet another
animal on this earth, and, more significantly, eliminated the need for divine intervention in evolution. As a result of these objections, a number of anti-Darwinian theories (orthogenesis, theistic
evolution, Lamarckism) were proposed. Many of these were taken seriously by a large number of
professional biologists, who were able to provide empirical arguments to support their position5.
Darwinism refers to the evolutionary theory that emerged from the modern synthesis between the classical Darwinian
theory of natural selection and Mendelian genetics (Timothy D.Johnston, The influence of germ-plasm theory on the
distinction between learned and innate behavior, page 116).
4 Theodosius Dobzhansky: Evolution, Genetics and Man, pages 109-110. Discussing the conflicting use of the
term Neo-Darwinism, he suggests the use of either synthetic theory or the biological theory of evolution to refer to the
evolution theory of the 1930’s
5 Peter Bowler, The Eclipse of Darwinism, page 10
2
However, because of the flexibility and lack of dogmatism in early Darwinism and its allowance
for a number of partially divergent views, it was fairly popular well into the 1880s. This flexibility
was shared both by Darwin as well as by Darwin’s followers, including August Weismann. As a
result, there were many naturalists who considered themselves Darwinians but did not necessarily
believe in the mechanism of natural selection.
In the 1880s, August Weismann published his theory of the germ plasm. Without going into the
details of this theory at this point, it suffices to say that, if true, the theory implied that Lamarckism
was infeasible. Also, Weismann viewed his theory as a solution to the problem of heredity and the
source of heritable variations and thus expounded natural selection as an all-sufficient mechanism
for evolution.
This is viewed by Bowler as a decisive event in the debate between the Darwinians and antiDarwinians. Previously, people who straddled the fence without taking a stance with respect to
selectionism were now forced to either go with or against Darwinism since Weismann made natural
selection the sole evolutionary mechanism of Darwinism. This gave rise to another source of
opposition for Darwinism because those who were explicitly committed to Lamarckism were now
forced to oppose Weismann and hence Darwinism6 . Furthermore, geneticists found his theory to
be incompatible with Mendelian laws of heredity, naturalists found his theory highly artificial and
in the absence of experimental proof, opponents found his theory highly speculative7 .
Thus, on the face of it, it appears that by publishing his theory of the germ plasm, Weismann
actually harmed Darwinism. As Bowler puts it, “To a large extent, it can be argued that the eclipse
of Darwinism was precipitated by movements within Darwinism that created a situation in which
lukewarm supporters of the theory were turned into active opponents. 8 ”
Given this viewpoint and the fact that Mendelian geneticists and Neo-Darwinians were not
on the friendliest of terms in the early 1900s, why has it been claimed that Weismann, a NeoDarwinian, paved the way for Mendelian genetics? More to the point, even though the Mendelian
geneticists initially opposed Darwinism, ultimately, they would have realized that genetics and
Darwinism were complementary rather than contradictory. Hence, Darwinism would have come
out of its eclipse of the 1900s and achieved paradigm status in spite of Weismann. Given this, what
was Weismann’s role in the establishment of the Darwinian paradigm? I hope to provide some
answers to these questions in this essay.
6 Ibid., page
7 Ibid., page
8 Ibid.
42
41
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2 August Weismann: An Ardent Darwinian
Ernst Mayr mentions that Darwin’s evolutionary theory had a very great impact in Germany9 . This
might partially explain Weismann’s keen interest in Darwin’s theory. All his early evolutionary
studies (on caterpillars and butterflies) of the 1870s were done assuming the truth of Darwin’s
theory. In the following sections, I hope to show that Weismann was an ardent Darwinian, one
who never relinquished his faith in Darwinian evolution and natural selection.
2.1 Weismann and Natural Selection
Unlike many biologists of his time, Weismann was a firm believer in selectionism right from the
beginning. Mayr cites Weismann’s first evolutionary publication of 1868 as a proof of this claim10 .
He also considers Weismann to be the most consistent selectionist in the nineteenth century and
quotes a number of Weismann’s statements as evidence for such a belief11 .
In the 1870s, Weismann conducted a number of studies on butterflies and sphingid caterpillars
to examine the efficacy of natural selection12 . These studies convinced him of the usefulness of
natural selection in evolution. In his book, The Evolution Theory, Vol I, Weismann discusses the
phenomena of coloration of animals in great detail13 . As part of this discussion, he presents numerous examples of color adaptation in a number of insects, eggs, caterpillars and other organisms.
He explains how all these adaptations can be readily explained by resorting to natural selection.
Dismissing alternative explanations for simple cases, he argues:
But, all attempts at any other explanation are even more decidedly excluded when
we turn our attention to more complicated cases of colour-adaptation, which are not
confined to the simple, general coloration, but are helped by markings and colourpatterns, that is, by schemes of colour14 .
That he strongly believed in natural selection as the sole explanation for color-adaptation can be
seen from the following claim he makes:
That all these extremely favourable protective colorations find their explanation in the
9 Ernst
Mayr, Weismann and Evolution, page 296
305
11 Ibid., page 306
12 Ibid.; Frederick B. Churchill: August Weismann and a Break from Tradition, page 91
13 August Weismann: The Evolution Theory: Vol I, pages 57-90
14 Ibid. page 66
10 Ibid., page
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slow and gradually cumulative effects of natural selection cannot be disputed; it is
beyond doubt that they cannot be explained, so far as we know, in any other way15 .
He also presents examples from the phenomena of mimicry (wherein certain organisms mimic the
characteristics of other organisms in order to survive) to explain how only natural selection can
satisfactorily explain this adaptation of organisms16 .
He was also able to successfully address some objections raised against natural selection. For
example, the presence of nonadaptive structures in organisms posed a problem for Darwinism
since natural selection was a strictly utilitarian mechanism. An example of this was the loss of
eyes of animals that inhabit dark caves. Weismann solves this problem by invoking the power of
conservation exerted by natural selection. As long as an organ is under selection pressure, only
those individuals that have the best organs survive. However, once the selection pressure is relaxed
(for example, when the presence of sharp eyes is no longer crucial to the survival of an organism,
as in the case of cave dwellers), perfection of that organ is no longer maintained (conserved)
by natural selection. Coupled with competition for tissue substrate for other essential organs,
he argued that, this can lead to degeneration of organs. Thus, Weismann was able to provide a
satisfactory solution to the problem that was based solely on natural selection.
Speaking of natural selection in general, Weismann goes so far as to claim that
There is no part of the body of an individual or of any of its ancestors, not even the
minutest and most insignificant part, which has arisen in any other way than under the
influence of the conditions of life17 .
In the light of the preceding discussion, it is accurate to characterize August Weismann as
a devoted selectionist. However, one need not stop at that. He did more for Darwinism than
merely defend natural selection. He set about to solve what he perceived were unsolved problems
with Darwin’s theory of evolution. The foremost problem of these was the problem of source of
variations that is so crucial to Darwin’s evolutionary theory. A related problem was that of heredity.
The following subsection discusses how Weismann addressed this problem.
15 Ibid., page 87-88
16 Ibid., pages 91-118
17 Ibid., page
260; Ernst Mayr, Weismann and Evolution, page 307
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2.2 Weismann and the problem of heredity
In Darwin’s theory of evolution, speciation and introduction of varieties required that organisms
undergo some form of heritable variations. However, Darwin did not have a satisfactory answer
for what caused these variations. Given that such variations were introduced in organisms, natural
selection could successfully explain adaptation and speciation in organisms. To some extent, the
Lamarckian mechanism of inheritance of acquired characters provided a partial solution to this
problem of variations18 . According to this Lamarckian mechanism, variations were introduced in
organisms based on use/disuse of organs. Furthermore, the organism could transmit these acquired
characters to its offspring. Darwin, probably, for lack of a better solution, accepted this and went
so far as to come up with his theory of pangenesis to explain the mechanism of the inheritance of
acquired characters.
Like all other Darwinians, Weismann started his career by accepting the mechanism of inheritance of acquired characters. In all of his evolutionary writings prior to 1882, he believed it to
be the major source of variation 19 . However, by 1883, he was ready to purge Darwinism of all
Lamarckian connections and assert natural selection as the sole evolutionary mechanism. Having
done this, he recognized the need to come up with an alternative mechanism for the production
of genetic (heritable) variations20. Towards this end, he proposed his theory of the germ plasm.
Before examining this theory and its impact, I briefly present Weismann’s attack on the mechanism
of inheritance of acquired characters.
2.3 Weismann’s attack on Lamarckism
Weismann did not disagree with Lamarck in that use/disuse of organs can be a source of variations
in an organism’s lifetime. He says: “ Lamarck’s idea is at once clear: It is true that exercising
an organ strengthens it, that disuse makes it weaker. ... Indeed, we may now go so far as to
admit that every organ is strengthened by use and weakened by disuse, ...
21
”. What he did not
agree with was that this was the only source of variations22 . More importantly, he did not agree
that these variations were heritable23 . He claims that inheritance of acquired characters has never
18 Ernst
Mayr, Weismann and Evolution, page 312
19 Ibid.
20 Ibid., page
315, August Weismann, Essays Upon Heredity, pages 252-253
Weismann, The Evolution Theory, Vol I, page 20
22 Ibid., page 21; August Weismann, Essays Upon Heredity, “It is well known that Lamarck attempted to explain
the structure of the organism as almost entirely due to this principle alone”, page 84
23 August Weismann, The Evolution Theory, Vol I, page 21; August Weismann, Essays Upon Heredity, pages 71-105
21 August
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been proved, either by means of direct observation or by experiment. He was also very skeptical
of descriptions where mutilations, scars and such suffered by an organism are transmitted to its
offspring. Of these, he says, “... in these descriptions, the previous history is invariably obscure,
and hence the evidence loses all scientific value”24 .
Weismann argues that though exercise strengthens organs, perfection of an organism does not
depend on the amount of exercise, but rather on the predisposition of the germ cells (reproductive
cells) to produce a perfect organ. Thus, with the same amount of exercise, the organ that is destined
to be strong will attain a higher degree of functional activity than one that is destined to be weak.
Further, natural selection, by selecting the organism that possessed the stronger organ weeds out
the germ cells that had a predisposition to produce the weaker organ 25 . By arguing thus, Weismann
undercuts the importance of the Lamarckian mechanism because it is the germ cell coupled with
natural selection that gives the impression that the acquired characters have been inherited. On a
different note, this means that, in Weismann’s theory, natural selection operates on the genotypes
(germ cells) rather than the phenotypes (external manifestations of the germ-cell traits). This is
clear from what Weismann says: “... natural selection, while it apparently decides between individuals of various degrees of strength, is in truth operating upon the stronger and weaker germs”26 .
Weismann brought arguments from color adaptations and mimicry to bear upon Lamarckism
(use/disuse as well as inheritance of acquired characters). By doing so, he attempted to demonstrate
that Lamarckism would be at a loss explaining these phenomena. To quote him: “But, even if we
recognize the Lamarckian factor27 as a vera causa, it is easy to show that there are a great many
characters which it is not in a position to interpret 28 ”. For example, speaking of the adaptation
wherein certain insects possess exactly the same shade green as the plants that they feed on, he argues: “How could this green colour of the skin have been brought about by the activity of the skin,
since the colour of the surroundings does not usually stimulate the skin to activity at all?29 ” Darwin
had first explained color adaptation and mimicry in insects using natural selection. Bates claimed
that these phenomena served as valuable proof of natural selection 30 . However, Weismann used
these very same phenomena to claim that Lamarckism was not a complete theory since it could
24 August Weismann,
25 Ibid., page 84
Essays Upon Heredity, page 81
26 Ibid., page 85
27 He means the inheritance of
28 August
29 Ibid.
30 Peter
acquired characters
Weismann: The Evolution Theory: Vol I, page 21.
Bowler The Eclipse Of Darwinism, pages 29-30
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not explain these phenomena. There were a number of other cases (for example, how could certain
complex behavior that is exhibited only once in an entire lifetime be learnt through use/disuse?)
that Weismann used to demonstrate that Lamarckism was implausible. Since it was hard to obtain
direct proof of a case of natural selection producing adaptations, Weismann hoped to best support
natural selection by the principle of exclusion, i.e. by refuting all competing theories31 . Mayr best
sums up Weismann’s strategy as follows:
Weismann’s strategy was to show not only that an inheritance of acquired characters
encounters formidable difficulties, but also that cases cited in its favor could be explained quite well through the theory of natural selection 32 .
From the preceding discussion, it should be clear that Weismann went about supporting Darwinism by refuting all competing theories. Moreover, in publishing the theory of the germ plasm,
he intended to solve the existing problems of Darwinism and make it a more perfect theory than
what it was before. In the process, based on his germ-plasm theory, he also set about refuting
competing theories of evolution.
I am now ready to present some details of the germ plasm theory and how it influenced the
outcome of the Darwinian Revolution.
3 The Germ Plasm Theory
In a nutshell, the germ plasm, as opposed to somatoplasm, (ordinary body fluid), was responsible
for transmitting the information of heredity. More importantly, the body of an organism served only
as a host and could never affect this germinal material and hence could not affect its offspring. If
true, this implied that Lamarckism was infeasible since characteristics that were acquired by an
organism during its lifetime could not be transmitted to its offspring. But, as mentioned earlier,
Weismann was now forced to come up with an alternative mechanism for producing heritable
variations. Sexual reproduction came to his rescue here. Based on the progress of cytology in the
1870s and 1880s, he knew that during sexual reproduction, two distinct germ cells come together
to “restablish the diploidy of the zygote”33 . Since these individual germ cells carried separate
hereditary characteristics, he viewed this coming together of germ cells as a significant source of
31 Ernst
Mayr, Weismann and Evolution, page 307
33 Ernst
Mayr, Weismann and Evolution, page 316
32 Ibid., pages 313-314
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genetic variation34 .
For a long time, Weismann maintained in the “continuity of the germ plasm” wherein the
germ plasm is perceived as a stable substance that has virtually unchanged through the evolution
of multicellular organisms35. Thus, genetic recombination was the only means of introducing
heritable variations in organisms. However, in later years, he realized that this limited the amount
of variation possible and was hardpressed to come up with a source of novel genetic variations36.
As a result, he allowed for an evolutionary change of the germ plasm. However, he was very
hesitant to allow for “internalist” changes of the germ plasm (mutations) since he had consistently
opposed orthogenesis37 .
However, his theory faced a lot of criticism because of its many internal inconsistencies and
also because most of his theory was based on pure speculation. Weismann very well recognized
the speculative nature of his theories and says:
... even if it should have to be abandoned at a later period, it seems to me that, at the
present time, it is a necessary stage in the advancement of our knowledge, and one
which must be brought forward and passed through, whether it prove right or wrong,
in the future.38
In any case, the most important aspect of his theory was his insistence on the constancy of
germinal material and its isolation from the body. Further, he recognized the importance of sexual
reproduction as a source of genetic variation. These ultimately made their appearance in the final
synthetic theory of evolution of the 1930s.
As discussed previously, in Weismann’s theory, natural selection acts on germinal traits rather
than the external manifestations of these traits, i.e. he distinguished between the phenotypes and
genotypes of organisms and argued that natural selection acted on genotypes only. This was different from earlier theories of evolution where this distinction was not made. Again, this distinction
was crucial and was present in the synthetic theory of evolution.
Mayr recognizes all these factors and also mentions that Weismann was also responsible for the
“firm establishment of particulate inheritance” (as opposed to blending inheritance). He mentions
34 August
Weismann, Essays Upon Heredity, pages 272-273; Ernst Mayr, Weismann and Evolution, pages 315-316
Weismann, Essays Upon Heredity, pages 165-248; Timothy D.Johnston, The influence of germ-plasm
theory on the distinction between learned and innate behavior, page 117
36 Ernst Mayr, Weismann and Evolution, page 317
37 Ibid.
38 August Weismann, Essays Upon Heredity, page 174
35 August
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that Weismann’s postulates on reduction division laid the foundation for Mendelian genetics39 .
4 Conclusions
Based on the preceding discussion, it should be clear that simply characterizing Weismann as a
Darwinian presents an incomplete picture of the man. He was much more than a Darwinian. He
was responsible for sowing the seeds for the evolutionary theories that would ultimately come to
be accepted. Though Weismann might have got many of the details of his theory wrong, he got
many of the principles right which ultimately found their way in the synthetic theory of evolution. Churchill claims that more than simply achieving a breakthrough in the problem of heredity,
Weismann was also responsible for shaking the assumptions of the scientific community of that
time40 .
It is instructive to examine what has been said about this subject before. According to Ernst
Mayr, even though Weismann failed to solve the problem of heredity that he set out to solve, the intellectual preparation he gave to the area enabled the Mendelian theory to prosper41 . Ralf-Dietmar
Hegel claims: “With the rejection of the inheritance of acquired characteristics and the assumption
of the plasma as a substrate of inheritance of relatively high stability, Neo-Darwinism prepared the
theoretical ground for the rediscovery of Mendelian laws of inheritance by Tschermak, Correns,
and Bateson around 190042 .” These opinions (there are many more than just these two) should
show the importance of Weismann and the role his theory had to play in the subsequent establishment of the Darwinian paradigm in the field of evolutionary biology.
On a concluding note, here is what Michael Ruse has to say about Darwin’s role in the Darwinian Revolution:
Had Darwin (and Wallace) never existed, scientists would still have become evolutionists – but it was Darwin who precipitated and expedited the change43 .
A similar case can be made for August Weismann and the synthetic theory of evolution.
39 Ibid., page
40 Frederick
326
B. Churchill, The Weismann-Spencer Controversy Over the Inheritance Of Acquired Characters, page
457
41 Ernst
Mayr, Weismann and Evolution, page 327
Weismann: One of the First Synthetic Theorists of Evolutionary Biology, page 264
43 Michael Ruse, The Darwinian Revolution: Science Red in Tooth and Claw, page 265
42 Ralf-Dietmar Hegel, August
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References
[1] Essays Upon Heredity, August Wiesmann
[2] The Evolution Theory: Volume I, August Wiesmann
[3] The Eclipse of Darwinism, Peter Bowler
[4] Weismann and Evolution, Ernst Mayr, In the Journal of the History of Biology, Vol 18, No. 3,
pages 295-329.
[5] The Weismann-Spencer Controversy Over the Inheritance Of Acquired Characters, Frederick
B. Churchill, In the Proceedings of the 15th International Congress of the History of Science,
pages 451-468.
[6] August Weismann and a Break from Tradition, Frederick B. Churchill, In the Journal of the
History of Biology, 1968:1, pages 91-112.
[7] The influence of germ-plasm theory on the distinction between learned and innate behavior,
Timothy D.Johnston, In Journal of the History of the Behavioral Sciences, 1995, Vol 31, page
116
[8] August Weismann: One of the First Synthetic Theorists of Evolutionary Biology, Ralf-Dietmar
Hegel, In World Views and Scientific Discipline Formation, page 264
[9] The Darwinian Revolution: Science Red in Tooth and Claw, Michael Ruse
[10] Evolution, Genetics and Man, Theodosius Dobzhansky
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