The author introduces his paper by giving a brief review of the writings of the ancient Greeks, especially those of Aristotle, and indicating how they forecast certain modern points of view. He develops the subject under the following heads: The biomechanist replaces the mechanist; Linnaean classification the expression of adaptation; the mathematics of limb adaptation; the static and kinetic engineer; the origin of new adaptive biomechanisms; revolutionary progress in phylogeny since Darwin''s time; elephant ridge crests; new aristogenes added to inferior grinders in the period estimated at 20,000,000 years; philosophical inductions; and the biology of 1890 and of 1933. The author''s present position is summarized as follows: "In biomechanical evolution there are two distinct processes. The one, long known, consists in the alloiometric modification of existing adaptations as in changes of proportion and of function. The other, discovered in course of researches on the phylogeny of the horses, titanotheres and proboscideans, consists in the gradual geneplasmic origin of new and distinct adaptations; it is to the latter originative and creative process that the term Aristo-genesis is applied. Both processes become part of the hereditary equipment of the organism.".