It was the merit of de Vries[292] to have pointed out that fluctuating variations are not hereditary and hence could not have played the rôle assigned to them by Darwin, while discontinuous variations as they appear in the so-called “sports” or mutations are inherited. This was an important step in the history of the theory of evolution. It did not touch the foundation of Darwin’s work, namely the substitution of the idea of an accidental evolution for that of a purposeful creation; it only modified the conception of the possible mechanism of evolution. According to de Vries, there are special species or groups of species which are in a state of mutation. He considers the evening primrose on which he made his observations as one of these forms. Morgan and his pupils have observed over 130 mutations in a fly _Drosophila_. From our present limited knowledge we must admit the possibility that the tendency toward the production of mutants is not equally strong in different forms. Whether this part of de Vries’s idea is or is not correct there can be no doubt that variations occur which consist in the loss and apparently, though in rarer cases, in the gain or a modification of a Mendelian factor. If we wish to visualize the basis of such a change we may do so by imagining well-defined chemical constituents in one or more of the chromomeres undergoing a chemical change. [292] de Vries, H., _The Mutation Theory_, translated by Farmer, J. B., and Darbishire, A. D., Chicago, 1909. _Species and Varieties_. Chicago, 1906. _Gruppenweise Artbildung_. Berlin, 1913. This way of looking at the origin of variation has had the effect of putting an end to the vague speculations concerning the evolution of one form from another. We demand today the experimental test when such a statement is made and as a consequence the amount of mere speculation in this field has diminished considerably. It is possible that any further progress concerning evolution must come by experimental attempts to bring about at will definite mutations. Such attempts have been reported but they are not all beyond the possibility of error.[293] The most remarkable among them are those by Tower who by a very complicated combination of effects of temperature and moisture claims to have produced definite mutations in the potato beetle. The conditions for these experiments are so expensive and complicated that a repetition by other investigators has not yet been possible. [293] For a critical discussion of the details, see Bateson, W., _Problems of Genetics_, New Haven, 1913, Chapter X. It is, however, still uncertain whether the mere addition or loss of Mendelian characters can lead to the origin of new species. Species specificity is determined by specific proteins (Chapter III.), while some Mendelian characters at least seem to be determined by hormones or substances which need neither be proteins nor specific for the species. CHAPTER XIV DEATH AND DISSOLUTION OF THE ORGANISM