a change in the time of establishment and rate of development of organs in the descendants of animals and plants as compared with that of their ancestors. Heterochronism can be expressed in the earlier establishment and accelerated development of an organ (acceleration) or in later establishment and slower development (retardation); this depends upon the time when the organ begins to function and, consequently, the environmental conditions in which ontogenetic development of the organism takes place. As adaptations of organisms to changing conditions of their development, heterochronism is of vital significance in the historical development of species (phylogeny).

The term “heterochronism” was introduced into biology by the German natural scientist E. Haeckel to signify temporary deviations from the biogenetic law. Heterochronism is studied as one of the basic processes of how the organization of animals and plants is transformed under the influence of changing life conditions during speciation. An example of heterochronism is the early development in mammals of the tongue muscles, owing to which a newborn child is capable of producing sucking motions. Early maturity and late maturity are also related to the phenomena of heterochronism that refer to the organism as a whole.