ORTHOGENESIS, a zoological term introduced by Eimer to express the view that the variations from the normal form of an animal species, on the occurrence of which evolution de pends, do not arise at hazard but have a definite direction in all closely allied species ; and that these variations may be repeated over a long series of generations, always in the same direction. The cause of these definite variations is left uncertain but is supposed to lie within the animal and not in its environment. Thus evolution would follow a direct course, which need have no positive adaptive significance. This view was founded on a study of the patterns of butterfly wings.
The phylogenies built up by palaeontologists, as E. D. Cope was the first to observe, do show that evolution has generally proceeded in definite directions, each member of a series differing from its predecessor in the same ways as its successor differs from it. In most cases the changes take place in such a way that greater mechanical efficiency is secured, and they can thus, theoretically, be accounted for by natural selection, exercised by a constant or auto-orthogenetic environment. In some cases however the changes may proceed in a definite direction despite very great changes in habits and habitat amongst the animals.
The essential point, that variation is not indefinite in direction, has been established for those heritable variations known as mutations by the observation that definite mutants appear time after time at a definite rate amongst the members of a species bred under controlled conditions ; and that identical mutants occur in allied species and genera. The implication of these facts is that the mechanism which determines the course of development of the fertilized egg, and is hence responsible for heredity, is capable of modification only in certain definite ways, and that this mechanism is of essentially the same structure in allied animals. This view is in harmony with the fact, observed by palaeontologists, that closely allied stocks (phyla or lineages) exposed to similar conditions pursue parallel evolutionary courses. It is not inconsistent with the view that the evolution, at least of certain structures, may follow a definite trend independently of its environment. See EVOLUTION. (D. M. S. W.)