While it follows from Weismann's theory that an adaptation acquired by an individual during his lifetime cannot be transmitted to his offspring, it remained for De Vries to show authoritatively that evolution can, and does, take place without this. Once this was established, biologists cheerfully abandoned the earlier notion. Lester Ward and the biologists of his day in general not only believed in the transmission of acquired characters, but they filled the obvious gaps which occurred in trying to apply this theory to the observed facts by placing a fantastic emphasis upon sexual selection. That is, much progress was accounted for through the selection by the females of the superior males. This, as a prime factor in evolution, has since been almost "wholly discredited" (Kellogg's phrase) by the careful experiments of Mayer, Soule, Douglass, Dürigen, Morgan and others. The belief in sexual selection involved a long string of corollaries, of which biology has about purged itself, but they hang on tenaciously in sociological and popular literature. For instance, Ward believed in the tendency of opposites to mate (tall men with short women, blonds with brunettes, etc.), although Karl Pearson had published a statistical refutation in his _Grammar of Science_, which had run through two editions when the _Pure Sociology_ appeared. The greater variability of males than females, another gynæcocentric dogma had also been attacked by Pearson on statistical evidence in 1897 (in the well-known essay on Variation in Man and Woman, in _Chances of Death_) and has become increasingly unacceptable through the researches of Mrs. Hollingworth[6,7,8]. The idea of a vanished age of mother-rule in human society, so essential to the complete theory, has long since been modified by anthropologists. De Vries' experiments showed that a moderately simple fact practically makes all these complicated theories unnecessary. No two living things are exactly alike--that is, all living matter is more or less variable. Some variations are more fortunate than others, and these variants are the ones which survive--the ones best adapted to their environment. Given this fact of the constant variation of living matter, natural selection (i.e., survival of the fittest and elimination of the unfit) is the mechanism of evolution or progress which best accounts for the observed facts. Such variation is called "chance variation," not because it takes place by "chance" in the properly accepted sense of the term, but because it is so tremendously varied--is evidently due to such complicated and little-understood circumstances--that it can best be studied mathematically, using statistical applications of the "theory of probabilities."