The eclipse of Darwinism


used the phrase "the eclipse of Darwinism" 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. 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.
Four major alternatives to natural selection were in play in the 19th century:
  • Theistic evolution, the belief that God directly guided evolution
  • 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.

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. Six objections were raised to the theory in the 19th century:
  1. The fossil record was discontinuous, suggesting gaps in evolution.
  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.
  3. It was argued that many structures were nonadaptive, so they could not have evolved under natural selection.
  4. Some structures seemed to have evolved on a regular pattern, like the eyes of unrelated animals such as the squid and mammals.
  5. Natural selection was argued not to be creative, while variation was admitted to be mostly not of value.
  6. The engineer Fleeming Jenkin correctly noted in 1868, reviewing The Origin of Species, that the blending inheritance favoured by Charles Darwin would oppose the action of natural selection.
Both Darwin and his close supporter Thomas Henry Huxley freely admitted, too, that selection might not be the whole explanation; Darwin was prepared to accept a measure of Lamarckism, while Huxley was comfortable with both sudden change and directed evolution.
By the end of the 19th century, criticism of natural selection had reached the point that in 1903 the German botanist,, 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." 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." 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. 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.

Motivation for alternatives

A variety of different factors motivated people to propose other evolutionary mechanisms as alternatives to natural selection, some of them dating back before Darwin's Origin of Species. Natural selection, with its emphasis on death and competition, did not appeal to some naturalists because they felt it was immoral, and left little room for teleology or the concept of progress in the development of life. Some of these scientists and philosophers, like St. George Jackson Mivart and Charles Lyell, who came to accept evolution but disliked natural selection, raised religious objections. Others, such as Herbert Spencer, the botanist George Henslow, and Samuel Butler, felt that evolution was an inherently progressive process that natural selection alone was insufficient to explain. Still others, including the American paleontologists Edward Drinker Cope and Alpheus Hyatt, had an idealist perspective and felt that nature, including the development of life, followed orderly patterns that natural selection could not explain.
Another factor was the rise of a new faction of biologists at the end of the 19th century, typified by the geneticists Hugo DeVries and Thomas Hunt Morgan, who wanted to recast biology as an experimental laboratory science. They distrusted the work of naturalists like Darwin and Alfred Russel Wallace, dependent on field observations of variation, adaptation, and biogeography, considering these overly anecdotal. Instead they focused on topics like physiology, and genetics that could be easily investigated with controlled experiments in the laboratory, and discounted natural selection and the degree to which organisms were adapted to their environment, which could not easily be tested experimentally.

Anti-Darwinist theories during the eclipse

Theistic evolution

British science developed in the early 19th century on a basis of natural theology which saw the adaptation of fixed species as evidence that they had been specially created to a purposeful divine design. The philosophical concepts of German idealism inspired concepts of an ordered plan of harmonious creation, which Richard Owen reconciled with natural theology as a pattern of homology showing evidence of design. Similarly, Louis Agassiz saw Ernest Haeckel's recapitulation theory, which held that the embryological development of an organism repeats its evolutionary history, as symbolising a pattern of the sequence of creations in which humanity was the goal of a divine plan. In 1844 Vestiges adapted Agassiz's concept into theistic evolutionism. Its anonymous author Robert Chambers proposed a "law" of divinely ordered progressive development, with transmutation of species as an extension of recapitulation theory. This popularised the idea, but it was strongly condemned by the scientific establishment. Agassiz remained forcefully opposed to evolution, and after he moved to America in 1846 his idealist argument from design of orderly development became very influential. In 1858 Owen cautiously proposed that this development could be a real expression of a continuing creative law, but distanced himself from transmutationists. Two years later, in his review of On the Origin of Species, Owen attacked Darwin while at the same time openly supporting evolution, expressing belief in a pattern of transmutation by law-like means. This idealist argument from design was taken up by other naturalists such as George Jackson Mivart, and the Duke of Argyll who rejected natural selection altogether in favor of laws of development that guided evolution down preordained paths.
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. 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."
A secular version of this methodological naturalism was welcomed by a younger generation of scientists who sought to investigate natural causes of organic change, and rejected theistic evolution in science. By 1872 Darwinism in its broader sense of the fact of evolution was accepted as a starting point. Around 1890 only a few older men held onto the idea of design in science, and it had completely disappeared from mainstream scientific discussions by 1900. There was still unease about the implications of natural selection, and those seeking a purpose or direction in evolution turned to neo-Lamarckism or orthogenesis as providing natural explanations.

Neo-Lamarckism

had originally proposed a theory on the transmutation of species that was largely based on a progressive drive toward greater complexity. Lamarck also believed, as did many others in the 19th century, that characteristics acquired during the course of an organism's life could be inherited by the next generation, and he saw this as a secondary evolutionary mechanism that produced adaptation to the environment. Typically, such characteristics included changes caused by the use or disuse of a particular organ. It was this mechanism of evolutionary adaptation through the inheritance of acquired characteristics that much later came to be known as Lamarckism. Although Alfred Russel Wallace completely rejected the concept in favor of natural selection, Darwin always included what he called Effects of the increased Use and Disuse of Parts, as controlled by Natural Selection in On the Origin of Species, giving examples such as large ground feeding birds getting stronger legs through exercise, and weaker wings from not flying until, like the ostrich, they could not fly at all.
File:Blind insects in Packard's Mammoth Cave and its Inhabitants 1872.jpg|thumb|Alpheus Spring Packard's 1872 book Mammoth Cave and its Inhabitants used the example of cave beetles that had become blind to argue for Lamarckian evolution through inherited disuse of organs.
In the late 19th century the term neo-Lamarckism came to be associated with the position of naturalists who viewed the inheritance of acquired characteristics as the most important evolutionary mechanism. Advocates of this position included the British writer and Darwin critic Samuel Butler, the German biologist Ernst Haeckel, the American paleontologists Edward Drinker Cope and Alpheus Hyatt, and the American entomologist Alpheus Packard. They considered Lamarckism to be more progressive and thus philosophically superior to Darwin's idea of natural selection acting on random variation. Butler and Cope both believed that this allowed organisms to effectively drive their own evolution, since organisms that developed new behaviors would change the patterns of use of their organs and thus kick-start the evolutionary process. In addition, Cope and Haeckel both believed that evolution was a progressive process. The idea of linear progress was an important part of Haeckel's recapitulation theory. Cope and Hyatt looked for, and thought they found, patterns of linear progression in the fossil record. Packard argued that the loss of vision in the blind cave insects he studied was best explained through a Lamarckian process of atrophy through disuse combined with inheritance of acquired characteristics.
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.
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.