Evolution Social and Organic

by Arthur M. Lewis

It was around this question of the “causes” of variation that the Neo-Lamarckians and the Weismannians fought their battle, the former insisting, as we have seen, that variation was caused by the hereditary transmission of acquired characters, while Weismann maintained that variation arose solely through the combining of two portions of differing germ-plasm contributed by two different individuals, and producing a new individual unlike either,—a “variation” from both. While whatever there was of victory fell to Weismann, neither side has experimentally proven its case, and we are still in the dark as to the “causes of variation.” Our ignorance is still cloaked in the convenient word “spontaneous;” to Darwin’s “spontaneous variation” we now add DeVries’ “spontaneous mutation.”

It is another tribute to Darwin’s caution and insight that he recognized the possibility of variations arising either suddenly, as DeVries asserts they do, or gradually as DeVries denies.

Not only did Alfred Russell Wallace seek to limit the operation of natural selection in certain fields, in order to make room for his spiritualist theories—an adventure which failed dismally—but he denied the sudden appearance of new species or sub-species, thereby restricting Darwinism, as he understood it, to the origin of new species by the gradual accumulation of those almost imperceptible variations usually described as “fluctuations.” Whatever conflict there may be between Darwinism and mutation must be ascribed to Wallace. As DeVries clearly recognizes, Darwin is in no way responsible. “Darwin,” says DeVries, “recognized both lines of evolution.”

The difference between “fluctuations” and “mutation” is illustrated by DeVries recalling Galton’s simile of a polyhedron—an example of which is a solid piece of glass covered with many small flat faces. When it comes to rest on any particular face, it is in stable equilibrium. Small disturbances may make it oscillate, but it returns always to the same face. These oscillations are like fluctuating variations. A greater disturbance may cause the polyhedron to roll over on to a new face, where it comes to rest again, only showing the ever present fluctuations around the new center. The new position corresponds to a mutation. One of the disabilities of this illustration is that some fluctuations represent a greater disturbance from the given position than some mutations. The essential difference is that in the fluctuation it rocks back again while in the mutation it remains on a new base.

Everybody has heard something of the famous evening primrose which gave DeVries his first and most conclusive evidence of mutation. At Hilversum near Amsterdam, he discovered a large number of the plants of the evening primrose, named Lamarckiana after Lamarck. It is an American plant imported to Europe. It often escapes from cultivation and in this case DeVries says it had escaped from a park. It had run wild ten years. A year after first noticing them DeVries observed two new forms which he at once recognized as two new elementary species.

In the test conditions of his own garden, in an experiment covering thirteen years, he observed over fifty thousand of the Lamarckiana spread over eight generations and of these eight hundred were mutations divided among seven new elementary species. These mutations, when self-fertilized, or fertilized from plants like themselves, bred true to themselves, thus answering the test of a real species. DeVries also watched the field from which his original forms were taken, and saw that similar mutations occurred there so that they were not in any way due to cultivation.

Thus has the modest mutating primrose contributed its quota to the solution of that riddle of the universe which, until it is solved, will always command a paramount position in the thoughts of men.

DeVries discourages the notion that mutations are always occurring everywhere, which might seem to be one of the inferences from his theory, and his twenty-fourth lecture of the series, delivered before the University of California is entitled “The Hypothesis of Periodic Mutations.” The common primrose, he says, seems to be immutable at present, and argues that it must have had a mutatory period sometime in the past, when, perhaps, the evening primrose was not mutating. He says: “All the facts point to the conclusion that these periods, of stability and mutability, alternate more or less regularly with one another.”

He deals the Neo-Lamarckians a heavy blow by his denial of “direct” adaptation, and he greatly strengthens their opponents when he asserts that mutation takes place, not only in useful directions, but in all directions, leaving natural selection to destroy the unfit. This is a restatement of Darwin’s conception, followed by Weismann, of “fortuitous” variations, and is contrary to the notion of Spencer and Haeckel, that variations are mainly in the direction of adaptation to environment, as a result of animals exerting themselves in that direction.

This point is well stated by DeVries in the following passage,—“This failure of a large part of the productions of nature deserves to be considered at some length. It may be elevated to a principle, and may be made use of to explain many difficult points of the theory of descent. If in order to secure one good novelty nature must produce ten or twenty or perhaps more bad ones at the same time, the possibility of improvements coming by pure chance must be granted at once. All hypotheses concerning the direct causes of adaptation at once become superfluous, and the great principle enunciated by Darwin once more reigns supreme.”