It will be seen that I express doubts as to the validity of Hering’s invocation of molecular vibrations as the mechanism of memory, and suggest as an alternative rhythmic chemical changes. This view has recently been put forth in detail by J. J. Cunningham in his essay on the “Hormone {0f} Theory of Heredity,” in the _Archiv für Entwicklungsmechanik_ (1909), but I have failed to note any direct effect of my essay on the trend of biological thought. Among post-Darwinian controversies the one that has latterly assumed the greatest prominence is that of the relative importance of small variations in the way of more or less “fluctuations,” and of “discontinuous variations,” or “mutations,” as De Vries has called them. Darwin, in the first four editions of the “Origin of Species,” attached more importance to the latter than in subsequent editions; he was swayed in his attitude, as is well known, by an article of the physicist, Fleeming Jenkin, which appeared in the _North British Review_. The mathematics of this article were unimpeachable, but they were founded on the assumption that exceptional variations would only occur in single individuals, which is, indeed, often the case among those domesticated races on which Darwin especially studied the phenomena of variation. Darwin was no mathematician or physicist, and we are told in his biography that he regarded every tool-shop rule or optician’s thermometer as an instrument of precision: so he appears to have regarded Fleeming Jenkin’s demonstration as a mathematical deduction which he was bound to accept without criticism. Mr. William Bateson, late Professor of Biology in the University of Cambridge, as early as 1894 laid great stress on the importance of discontinuous variations, collecting and collating the known facts in his “Materials for the Study of Variations”; but this important work, now become rare and valuable, at the time excited so little interest as to be ‘remaindered’ within a very few years after publication. In 1901 Hugo De Vries, Professor of Botany in the University of Amsterdam, published “Die Mutationstheorie,” wherein he showed that mutations or discontinuous variations in various directions may appear simultaneously in many individuals, and in various directions. In the gardener’s phrase, the species may take to sporting in various directions at the same time, and each sport may be represented by numerous specimens.