The eclipse of Darwinism

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Julian Huxley used the phrase "the eclipse of Darwinism"^[a] to describe the state of affairs prior to what he called the "modern synthesis". During the "eclipse", evolution was widely accepted in scientific circles but relatively few biologists believed that natural selection was its primary mechanism.^[2][3] Historians of science such as Peter J. Bowler have used the same phrase as a label for the period within the history of evolutionary thought from the 1880s to around 1920, when alternatives to natural selection were developed and explored—as many biologists considered natural selection to have been a wrong guess on Charles Darwin's part, or at least to be of relatively minor importance.^[4][5]

Four major

• Theistic evolution, the belief that God directly guided evolution^[b]
• Neo-Lamarckism
  , the idea that evolution was driven by the inheritance of characteristics acquired during the life of the organism
• Orthogenesis, the belief that organisms were affected by internal forces or laws of development that drove evolution in particular directions
• Mutationism, the idea that evolution was largely the product of mutations that created new forms or species in a single step.

Theistic evolution had largely disappeared from the scientific literature by the end of the 19th century as direct appeals to supernatural causes came to be seen as unscientific. The other alternatives had significant followings well into the 20th century; mainstream biology largely abandoned them only when developments in genetics made them seem increasingly untenable, and when the development of population genetics and the modern synthesis demonstrated the explanatory power of natural selection. Ernst Mayr wrote that as late as 1930 most textbooks still emphasized such non-Darwinian mechanisms.^[6]

Context

Evolution was widely accepted in scientific circles within a few years after the publication of On the Origin of Species, but there was much less acceptance of natural selection as its driving mechanism.^[7] Six objections were raised to the theory in the 19th century:^[8]

1. The
   fossil record was discontinuous, suggesting gaps in evolution.^[8]
2. The physicist Lord Kelvin calculated in 1862 that the Earth would have cooled in 100 million years or less from its formation, too little time for evolution.^[c][8]
3. It was argued that many structures were nonadaptive (functionless), so they could not have evolved under natural selection.^[8]
4. Some structures seemed to have evolved on a regular pattern, like the eyes of unrelated animals such as the
   mammals.^[8]
5. Natural selection was argued not to be creative, while variation was admitted to be mostly not of value.[8]
6. The engineer Fleeming Jenkin^[d] correctly noted in 1868, reviewing The Origin of Species, that the blending inheritance favoured by Charles Darwin would oppose the action of natural selection.^{[e][8][10][11]}

Both Darwin and his close supporter

By the end of the 19th century, criticism of natural selection had reached the point that in 1903 the German botanist, Eberhard Dennert  [de], edited a series of articles intended to show that "Darwinism will soon be a thing of the past, a matter of history; that we even now stand at its death-bed, while its friends are solicitous only to secure for it a decent burial."^[13] In 1907, the Stanford University entomologist Vernon Lyman Kellogg, who supported natural selection, asserted that "... the fair truth is that the Darwinian selection theory, considered with regard to its claimed capacity to be an independently sufficient mechanical explanation of descent, stands today seriously discredited in the biological world."^[14] He added, however, that there were problems preventing the widespread acceptance of any of the alternatives, as large mutations seemed too uncommon, and there was no experimental evidence of mechanisms that could support either Lamarckism or orthogenesis.^[15] Ernst Mayr wrote that a survey of evolutionary literature and biology textbooks showed that as late as 1930 the belief that natural selection was the most important factor in evolution was a minority viewpoint, with only a few population geneticists being strict selectionists.^[6]

Motivation for alternatives

A variety of different factors motivated people to propose other evolutionary mechanisms as

British science developed in the early 19th century on a basis of

Many of Darwin's supporters accepted evolution on the basis that it could be reconciled with design. In particular, Asa Gray considered natural selection to be the main mechanism of evolution and sought to reconcile it with natural theology. He proposed that natural selection could be a mechanism in which the problem of evil of suffering produced the greater good of adaptation, but conceded that this had difficulties and suggested that God might influence the variations on which natural selection acted to guide evolution.^[23] For Darwin and Thomas Henry Huxley such pervasive supernatural influence was beyond scientific investigation, and George Frederick Wright, an ordained minister who was Gray's colleague in developing theistic evolution, emphasised the need to look for secondary or known causes rather than invoking supernatural explanations: "If we cease to observe this rule there is an end to all science and all sound science."^[24]

A

In the late 19th century the term

Many American proponents of neo-Lamarckism were strongly influenced by Louis Agassiz, and a number of them, including Hyatt and Packard, were his students. Agassiz had an idealistic view of nature, connected with natural theology, that emphasized the importance of order and pattern. Agassiz never accepted evolution; his followers did, but they continued his program of searching for orderly patterns in nature, which they considered to be consistent with divine providence, and preferred evolutionary mechanisms like neo-Lamarckism and orthogenesis that would be likely to produce them.[28]^[31]

In Britain the botanist George Henslow, the son of Darwin's mentor John Stevens Henslow, was an important advocate of neo-Lamarckism. He studied how environmental stress affected the development of plants, and he wrote that the variations induced by such environmental factors could largely explain evolution. The historian of science Peter J. Bowler writes that, as was typical of many 19th century Lamarckians, Henslow did not appear to understand the need to demonstrate that such environmentally induced variations would be inherited by descendants that developed in the absence of the environmental factors that produced them, but merely assumed that they would be.^[32]

Polarising the argument: Weismann's germ plasm

Critics of neo-Lamarckism pointed out that no one had ever produced solid evidence for the inheritance of acquired characteristics. The experimental work of the German biologist August Weismann resulted in the germ plasm theory of inheritance. This led him to declare that inheritance of acquired characteristics was impossible, since the Weismann barrier would prevent any changes that occurred to the body after birth from being inherited by the next generation. This effectively polarised the argument between the Darwinians and the neo-Lamarckians, as it forced people to choose whether to agree or disagree with Weismann and hence with evolution by natural selection.^[33] Despite Weismann's criticism, neo-Lamarckism remained the most popular alternative to natural selection at the end of the 19th century, and would remain the position of some naturalists well into the 20th century.^[29][34]

Baldwin effect

As a consequence of the debate over the viability of neo-Lamarckism, in 1896

Orthogenesis was the theory that life has an innate tendency to change, in a unilinear fashion in a particular direction. The term was popularized by

Support for orthogenesis began to decline during the modern synthesis in the 1940s, when it became apparent that orthogenesis could not explain the complex branching patterns of evolution revealed by statistical analysis of the fossil record by paleontologists. A few biologists however hung on to the idea of orthogenesis as late as the 1950s, claiming that the processes of macroevolution, the long term trends in evolution, were distinct from the processes of microevolution.^[10][11]

Mutationism

Mutationism was the idea that new forms and species arose in a single step as a result of large mutations. It was seen as a much faster alternative to the Darwinian concept of a gradual process of small random variations being acted on by natural selection. It was popular with early geneticists such as Hugo de Vries, who along with Carl Correns helped rediscover Gregor Mendel's laws of inheritance in 1900, William Bateson a British zoologist who switched to genetics, and early in his career, Thomas Hunt Morgan.^[38][39]

The 1901 mutation theory of evolution held that species went through periods of rapid mutation, possibly as a result of environmental stress, that could produce multiple mutations, and in some cases completely new species, in a single generation. Its originator was the Dutch botanist Hugo de Vries. De Vries looked for evidence of mutation extensive enough to produce a new species in a single generation and thought he found it with his work breeding the evening primrose of the genus Oenothera, which he started in 1886. The plants that de Vries worked with seemed to be constantly producing new varieties with striking variations in form and color, some of which appeared to be new species because plants of the new generation could only be crossed with one another, not with their parents. DeVries himself allowed a role for natural selection in determining which new species would survive, but some geneticists influenced by his work, including Morgan, felt that natural selection was not necessary at all. De Vries's ideas were influential in the first two decades of the 20th century, as some biologists felt that mutation theory could explain the sudden emergence of new forms in the fossil record; research on Oenothera spread across the world. However, critics including many field naturalists wondered why no other organism seemed to show the same kind of rapid mutation.^[40]

Morgan was a supporter of de Vries's mutation theory and was hoping to gather evidence in favor of it when he started working with the fruit fly