This article concerns Ontogeny in biology. Not to be confused with the philosophy concepts of ontology, or the medical term oncology, and odontology.
Ontogeny (also ontogenesis or morphogenesis) is the origin and the development of an organism: for example, from the fertilizedegg to mature form. In time frame, it can cover the study of an organism's lifespan. The word ontogeny comes from the Greek ὄντος, ontos, present participle singular of εἶναι, "to be"; and from the suffix -geny, which expresses the concept of "mode of production". In more general terms, ontogeny is defined as the history of structural change in a unity, which can be a cell, an organism, or a society of organisms, without the loss of the degree and type of organization which allow that unity to exist. More recently, the term ontogeny has been used in cell biology to describe the development of various cell types within an organism.
Within biology, ontogeny pertains to the developmental history of an organism within its own lifetime, as distinct from phylogeny, which refers to the evolutionary history of species. In practice, writers on evolution often speak of species as "developing" traits or characteristics. This can be misleading. While developmental (i.e., ontogenetic) processes can influence subsequent evolutionary (e.g., phylogenetic) processes (see evolutionary developmental biology), individual organisms develop (ontogeny), while species evolve (phylogeny).
A seminal paper named ontogeny as one of the four primary questions of biology, along with Huxley's three others: causation, survival value and evolution. Tinbergen emphasized that the change of behavioral machinery during development was distinct from the change in behavior during development. "We can conclude that the thrush itself, i.e. its behavioral machinery, has changed only if the behavior change occurred while the environment was held constant...When we turn from description to causal analysis, and ask in what way the observed change in behavior machinery has been brought about, the natural first step is to try and distinguish between environmental influences and those within the animal...In ontogeny the conclusion that a certain change is internally controlled (is "innate") is reached by elimination. " (p. 424) Tinbergen was concerned that the elimination of environmental factors is difficult to establish, and the use of the word "innate" often misleading.