successive varieties in time of the same species of animal.
In 1869 the German scientist W. Waagen introduced the term into paleontology in order to denote the several ammonite-species varieties that succeeded one another in deposits from different geological ages. (The Oppelia subradiata from Jurassic deposits in Germany was used as an example.) The successive appearance of these variations in a single species, according to Waagen, is determined by internal factors influencing the development of the species. External factors only have an insignificant effect on the rate of the process.
Further study of Waagen’s mutations was carried out by the Austrian scientist M. Neumayr (1875, 1880, 1889). His work was based on data about Pliocene mollusks of southeastern Europe. The term “chronocline,” suggested by the American scientist J. Simpson in 1943, has come into use to designate gradual changes in any trait of successive populations or species. [17–401-l]