TACHYGENESIS, (Nco-Lat., from Gk. null-, tachys, swift + }freers, genesis, origin, production, generation). A term proposed by Hyatt for rapid evolution, or evolution by leaps, i.e. without the vast series of intermediate forms postulated by Darwin. In his own words Hyatt defines tachygenesis as "the law of acceler ation in the inheritance of characters." It has been found, for example, that characteristics are inherited in a series of species in a given stock at earlier and earlier stages in the ontogeny of each member of the series. These characteristics, as a rule, altogether disappear from the ontogeny, through lapse of heredity in the last members of a series, and thus the terminal forms become very distinct in their development. Rapid evo lution was also very marked at the beginning of the evolution of any stock. In the dawn of geological history, as soon as divergent evolution set in, each type had a more or less free field, and its first steps in evolution were obviously not affected by natural selection. Afterwards evo
lution became much slower. On the other hand, when the type began to decline there was a sen sible quickening of evolution. Da11 suggested the term `saltatorial evolution'; Galion, De Vries, and others, and especially the paleontologists, have all supposed that evolution has often been by leaps or spurts. Consult: Hyatt. "Cycle in the Life of the Individual (ontogeny) and in the Evolution of Its Own Group (phylogeny)," in Proceedings of the American Academy of Arts end Sciences, vol. xxxii. (Boston, 1807) ; DalI. "On a Hypothesis of Saltatory Evolu tion," in American. Naturalist, vol. xi. (Boston, 1877).