On the Origins of New Forms of Life
7.2: Darwin and Saltation
EUGENE M. MCCARTHY, PHD GENETICS
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De Vries was wrong, however, in asserting that Darwin was ignorant of ordinary breeding techniques. Darwin carried on an extensive correspondence with breeders and was well aware that sudden change occurred in domestic stocks. In particular, he made a special study of what he called “bud-variation,” in which a plant produces a bud that grows into an offspring individual with highly distinctive characteristics. But variation of this sort receives little attention in the Origin. Darwin gave it significant treatment only in Variation of Animals and Plants under Domestication. For example, in Variation (1868: vol. I, 406) he writes
In addition to using the term single variation, Darwin referred to such variation by a variety of other names (“sports,” “spontaneous variations,” “sudden variations”). The following passage from Variation (1868: vol. I, 213) shows that Darwin's notions of breeding were not always so far from those of de Vries:
Elsewhere in the same book he states his opinion that pigeon “fanciers can act by selection on excessively slight individual differences, as well as on those greater differences which are called sports” (Darwin 1868: vol. I, 224). He also says "it is probable that some breeds, such as the semi-monstrous niata cattle, and some peculiarities, such as being hornless, etc., have appeared suddenly from what we may call a spontaneous variation” (Darwin 1868: vol. I, 92). But he rarely expressed such opinions in the far more widely read Origin. What seems to be the only affirmative reference to saltation in that book comes in the first chapter on variation under domestication (Darwin 1859: 29–30):
Certainly, elsewhere in the Origin, Darwin consistently emphasizes gradual change, not saltation, especially in connection with natural, as opposed to domestic, variation. As has already been noted (Sec. 6.2), Darwin placed an increasing emphasis on the importance of continuous variation in later editions of the Origin. Certainly, de Vries is correct in saying that Darwin, at least Darwin as he expressed himself in the Origin, “left out of account” the part played by hybridization in the origin of new forms of life. In the Origin he stressed gradual divergence. Hybridization's role in breeding is almost entirely dismissed.
For example, in the Origin Darwin expresses his belief that all the various breeds of domestic pigeons are descended by divergence from a single ancestral stock. There (Darwin 1859: 23) he claims, “it is impossible to make the present domestic breeds by the crossing of any lesser number: how, for instance could a pouter be produced by crossing two breeds unless one of the parent-stocks possessed the characteristic enormous crop?” In a letter to the American naturalist Asa Gray, Darwin (1887: vol. III, 85) went so far as to assert that
But, in fact, Darwin apparently didn't know anything about how the pouter had been bred. Such knowledge was available. If he had searched English libraries, he might have found John Moore's Columbarium (1735), in which Moore explicitly reports the pouter was first obtained by hybridizing the “Dutch Cropper” pigeon with a breed known as the “Horseman.”1
De Vries would certainly have been right if he had merely said Darwin had had little personal experience with breeding prior to the publication of the Origin. Before that book appeared in 1859 Darwin seems to have bred only pigeons. He did not become interested even in these until the summer of 1855 (Desmond and Moore 1991: 427). Moreover, within three years, he dropped pigeon breeding entirely (ibid 1991: 459). During this period, from 1855 to 1858, he kept a wide variety of breeds. Many of these unfortunate birds he poisoned and boiled in order to compare their skeletons (ibid 1991: 427, 459). Indeed, his actual breeding experience even during this period must have been quite meager. Three years is hardly enough time to accomplish much in the way of pigeon breeding. And comparing skeletons could not have done much to educate him about breeding techniques. According to his biographers, Desmond and Moore (ibid: 428), his description in the Origin of pigeon breeding had a specific motivation:
And yet, it should be said that outside the pages of the Origin, Darwin actually embraced hybridization as a source of saltatory change, especially in Variation and in his later correspondence, where he made saltatory claims rather similar to those of de Vries (see Appendix H).
However, de Vries' views were clearly in conflict with those Darwin expressed in the Origin, the views most people identified with the name Darwin. When de Vries' theory first appeared, many biologists of the day considered it more consistent with observation than Darwin's. As Provine (1986: 220) notes, “by the turn of the century a growing number [of biologists] were supporting the idea that natural selection could not be the primary mechanism of speciation because too many differences between closely related species were apparently nonadaptive.” Thus, the American geneticist T. H. Morgan (1903: 299) remarked,
Independently of de Vries, the Russian botanist Korschinsky (1899, 1901), a disciple of Kölliker, brought together a vast mass of data demonstrating that saltational change had been widely reported.
Darwin's claims of gradual evolution were also opposed by taxonomists, many of whom felt that any process creating new types of organisms by gradual selection would never work because their experience had, in their opinion, shown populations treated as species have insufficient individual variability to support such a process (Harte 1994: 100). Taxonomic method was based on the fact that characters are typically constant among specimens treated as conspecific and that such characters are reproduced in every generation. Darwin claimed that traits show significant individual variation within populations treated as species and that gradual change is ongoing in such populations. Taxonomists called a population a “good species” when it showed little or no variability. Not surprisingly, such conflicting views were hard to reconcile.
Others pointed to the absence of intermediates. They believed it should be possible, if preexisting forms gradually evolve into new ones through intermediate forms, to find “numerous, fine, intermediate varieties” (Darwin's words) connecting them, both in the fossil record and in nature. When this prediction did not fit observation, many scientists questioned Darwin’s theory. NEXT PAGE >>
Note:
1. (Histoire Naturelle, XVI, 547–548) expressed the opinion that most of the various breeds of domestic pigeons were derived from intercrossing of three naturally occurring forms. Thus, he says: “It is therefore quite possible, as we have already suggested, that the Rock Dove [Columba livia], the Wood Pigeon [Columba palumbus], and the Turtle Dove [Streptopelia turtur], species that appear to keep themselves separate in a natural state, may nevertheless unite frequently in captivity and that from their union may be produced the majority of our domestic pigeon breeds, of which some are of the size of the Wood Pigeon, and others resemble the Turtle Dove in their smallness, their shape, etc., and others resemble the Rock Pigeon, or show affinity to all three.” Translated by E. M. McCarthy. Original French: “Il est donc fort possible, comme nous l’avons déjà insinué, que les bisets, les ramiers et les tourterelles, dont les espèces paroissent se soutenir séparément et sans mélange dans l’état de nature, se soient néanmoins souvent unies dans celui de domesticité ; et que de leur mélange, soient issues la plupart des races de nos pigeons domestiques, dont quelques-uns sont de la grandeur du ramier, et d’autres ressemblent à la tourterelle par la petitesse, par la figure, etc. et dont plusieurs enfin tiennent du biset ou participent de tous trois.”