Darwin's Theory of Evolution by the Selection of Minor Saltations

Abstract

Darwin did not so sharply distinguish in language as Weismann and Mendel have taught us to between the different kinds of variation and different factors of evolution, yet in his observations and the eases he cites his perception is absolutely clear between heredity, ontogeny, environment and selection as interoperating factors. In fact, he remarks on the frequent difficulty of distinguishing between the inextricably mingled factors of ontogeny (e. g., effects of use and disuse), and of correlated or coadapted variability and of spontaneous variations. A comparison of all the various kinds of variation cited by Darwin in his two great volumes shows that they fall into the following four classes: I. "Individual variations," "spontaneous variations," new suddenly appearing heritable characters, practically equivalent to the minor mutations of De Vries, believed by Darwin to be the chief material of natural selection and evolution. II. Sports, or major saltations, such as the "mauchamp," "ancon" and "niata" breeds, believed by Darwin not to occur in a state of nature. III. Fluctuations of proportion, congenital and hence transmissible, equivalent to the quantitative variation of Bateson, best illustrated by Darwin in his theory of the evolution of the long neck of the giraffe: IV. Fluctuating variability, dearly distinguished by Darwin from II and not especially connected by him with the process of evolution.