On the Origins of New Forms of Life
6.4: Saltation versus Gradualism
EUGENE M. MCCARTHY, PHD GENETICS
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During the eighty years following the publication of the Origin, many naturalists had saltationist views. Mayr comments that
Prior to Darwin virtually all evolutionists had been saltationists.2 Étienne Geoffroy Saint-Hilaire (1772-1844), for example, thought birds must have arisen from dinosaurs by saltation.3 If anything, Cuvier was a more extreme saltationist than Geoffroy. Geoffroy said that new organs always arise by modification of ones preexisting in ancestral forms. "Cuvier, on the other hand," says Russell, "was always ready to admit Nature's power to form entirely new organs in response to new functional requirements."4 Karl Ernst von Baer (1792-1876), the founder of embryology, was a saltationist.5 In a series of essays6 he criticized Darwin's theory and instead explained evolution in terms of saltations guided by a vaguely defined "tendency to perfection." Richard Owen (1804-1892), the English comparative anatomist and paleontologist, "conceived change to have taken place by abrupt variation, independent of environment and habit."7 In his Anatomy of Vertebrates (1866-1868), he says this change takes place through "departures from parental type, probably sudden and seemingly monstrous, but adapting the progeny inheriting such modifications to higher purposes."8 William Bateson (1861-1926), coiner of the word genetics, wrote, "Species are discontinuous: May not the variation by which species are produced be discontinuous too?"9
Some had more balanced views. For example, Albert von Kölliker (1817-1905), the Swiss zoologist and histologist, entertained notions of evolution through saltation, but thought evolutionary change could also occur gradually. In his critique of Darwin's theory (Über die Darwin'sche Schöpfungstheorie, 1864), he maintained that "no transitional forms between existing species are known"10 and proposed his own theory of "heterogeneous generation." "The fundamental idea of this hypothesis," he said, "is that under the influence of a general law of evolution creatures produce from their germs others which differ from them." 11 But Kölliker did not stress saltation. In fact, he wrote that a
Darwin's friend Thomas Henry Huxley, too, felt that evolution could be either gradual or saltatory. In his review of the Origin, he wrote that
Bateson (1894: 15) also bemoaned the "gratuitous difficulties which have been introduced by this assumption." Indeed, Darwin here adopted the exact expression ("natura non facit saltum") the Scholastics had employed during the medieval era to express the idea that the "natural order" exhibited a continuous range of variation, their so-called "Law of Continuity."15
Others, however, thought Darwin's emphasis on minor change had not gone far enough in eschewing saltation. In 1867, a Scottish engineer, Fleeming Jenkin, attacked Darwin's theory in the June issue of the North British Review. Though Darwin had given saltational changes (which he called "single variations") short shrift in the first four editions of Origin, Jenkin's article convinced him that he had not been stringent enough. In a letter to Alfred Wallace, Darwin wrote,
In the fifth edition of the Origin (1869) Darwin added a section further emphasizing his belief that selection of small individual differences was the paramount evolutionary force. After discussing a hypothetical case involving selection between slim and stocky wolves, he wrote the following:
Notes:
(Works Cited)
1. Mayr (1982: 544).
2. Osborn (1894).
3. Russell (1982: 68), citing Geoffroy Saint-Hilaire (1833: 80).
4. Russell (1982: 305).
5. Russell (1982: 242).
6. In von Baer (1864–1876, vol. II). See: Über den Zweck in den Vorgängen der Natur, Über Zielstrebigkeit in den organischen Körpern inbesondere, and Über Darwins Lehre.
7. Russell (1982: 244).
8. Owen (1866–1868: vol. III, 797).
9. Bateson (1894: 18). Bateson and de Vries were bellwethers of genetic thought during the early years of the twentieth century. Bateson coined many of the terms used in the field of genetics today, such as allele, homozygote, and heterozygote (Cremer 1985: 191).
10. Translated by Huxley in his essay Criticisms on "The Origin of Species" (1864); reprinted in Huxley (1893–1894: vol. II, 88).
11. Translated in Russell (1982: 243).
12. Translated in Russell (1982: 243).
13. Saltationists often referred to vaguely defined laws governing evolution and they were frequently associated with the orthothogenetic, or vitalistic movement. Proponents of orthogenetic evolution claim that evolution is purposeful and directed. Vitalism is a doctrine claiming that the processes of life are not explicable by the laws of physics and chemistry alone. These thinkers made the error of attempting to explain a real, observed phenomenon (saltation) in terms of unobserved, undocumented phenomena (e.g., "vital impulse").
14. Huxley (1860).
15. Lovejoy (1936).
16. Darwin (1887: 107).
17. Darwin (1869: 103–104).