The multiplication of living organisms is based upon the reproduction of the program, subject to an extremely low error rate at the individual level, so as to ensure that the identity of the offspring remains practically unchanged. The phylogenetic mechanisms are fundamentally non-deterministic, with the mutation and recombination rate providing a major source of diversity. This diversity is indispensable for the survival of living species, for their continuous adaptation to a changing environment, and for the appearance of new species. The successive divisions of the mother cell, the zygote, with each newly formed cell possessing a copy of the original genome, is followed by a specialization of the daughter cells in accordance with their surroundings, i.e., their position within the ensemble. This latter phase is known as cellular differentiation.
Ontogeny is thus the developmental process of a multicellular organism.
The ontogenetic program is limited in the amount of information that can be stored, thereby rendering the complete specification of the organism impossible. Therefore, upon reaching a certain level of complexity, there must emerge a different process that permits the individual to integrate the vast quantity of interactions with the outside world. This process is known as epigenesis , and primarily includes the nervous system, the immune system, and the endocrine system. The epigenetic processes can be loosely grouped under the heading of learning systems. The distinction between the axes cannot be easily drawn where nature is concerned, indeed the definitions themselves may be subject to discussion.