Development of Organisms

(1) Individual development, or ontogeny, is the aggregate of successive morphological and physiological changes experienced by every organism from the moment of birth to the end of life. Development is a process of closely related quantitative and qualitative rearrangements. Quantitative changes—growth—consist in increase in weight and size of the organism as a whole and of individual body parts or organs. Qualitative changes—differentiation—consist in rearrangement of structure and function of the organism and its parts and organs.

(2) Historical development, or phylogeny, of organisms and their taxonomic groups (phyla, classes, orders, families, genera, species) throughout their existence on earth. The connection between ontogeny and phylogeny is that phylogeny consists in a historical series of ontogenies that have undergone natural selection. (See alsoONTOGENY.)