Archallaxis

one of the means of evolution in which a change in the initial foundations of an organ occurs at an early stage of embryogenesis and changes the whole subsequent course of its development. The term “archallaxis” was introduced by A. N. Severtsov (1910) in his theory of phylembryogenesis. In archallaxis a relatively sharp, abrupt change in organ structure takes place, and new organs are formed which were absent in the organism’s progenitors. Archallaxis is a means of adapting to quickly changing environmental conditions. W. Garstang (1922) named evolution by means of change in the early stages of ontogenesis “neogenesis.” B. Rensch (1954) differentiates archallaxis of an organ from total, or general archallaxis. Examples of a type of archallaxis include the development of the foundations of ventral fins in the higher bony fishes near the head and not, as in the lower fishes, near the anal orifice and the foundations of hair in mammalian embryos, instead of the corneous scales of their ancestors.