The development or evolution in a definite direction.
Suggested mechanism to explain how the process of evolution works. Literally, “orthogenesis” means “straight line generation,” from two Greek words: orthos, meaning “a straight line,” and genesis, meaning “origination.” Orthogenetic organisms have their entire evolutionary possibilities preprogrammed into their genes. Proponents of orthogenesis believed that some uncontrollable evolutionary tendencies can lead to the extinction of species; for instance, 19th-century scientists believed that the Irish elk became extinct because its huge antlers (a secondary sexual characteristic) grew so large that the animals were no longer able to lift their heads.
Another example was given by British paleontologist A. E. Trueman, who published a paper in the 1930s stating that the coiled shell evolved by oysters of the genus Gryphaea led to their extinction. Trueman cited fossil evidence to support his theory that the oysters were unable to stop their shells from coiling. Eventually, the scientist concluded, the Gryphaea oysters had coiled so much that they were unable to open their shells, and so they became extinct.