|a. evolution of a group of organisms predetermined to occur in a particular direction|
|b. the theory that proposes such a development|
|2.||the theory that there is a series of stages through which all cultures pass in the same order|
theory that successive members of an evolutionary series become increasingly modified in a single undeviating direction. That evolution frequently proceeds in orthogenetic fashion is undeniable, though many striking features developed in an orthogenetic group appear to have little if any adaptive value and may even be markedly disadvantageous. A variety of theories have attempted to explain orthogenesis, but with only partial success.
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