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.” Another difficulty which DeVries claims to have solved by his theory, is the supposed contradiction between the physicist and the biologist as to the time allowed by the former and the time required by the latter, for the evolution of animals. Lord Kelvin asserted the age of the earth to be between twenty and forty million years. George Darwin estimates the separation of the moon from the earth as having taken place some fifty-six million years ago. Gekie estimated the existence of the solid crust of the earth as at most hundred million years. Joly, by calculating the amount of dissolved salts, and Dubois by the amount of lime, estimated the age of the rivers, Joly giving as probable fifty-five and Dubois thirty-six millions of years. “All in all,” concludes DeVries, “it seems evident that the duration of life does not comply with the demands of the conception of very slow and continuous evolution.” Mutation, with its sudden leaps, has no such difficulty, and,--“The demands of the biologists and the results of the physicists are harmonized on the ground of the theory of mutation.”