PHYLOGENY. — Within the framework of the theory of evolution, it denotes the historical order according to which the various groups of living beings would have gradually modified and evolved. All present and extinct animal and plant species could be united in an immense genealogical tree or phylogenetic tree, in which common trunks, corresponding to more ancient ancestors, repeatedly subdivide into secondary branches, which indicate the more recent forms.
The construction of a phylogenetic tree is based on evidence drawn from the most diverse disciplines (paleontological, anatomical, embryological, biogeographical, physiological, and genetic data), and thus represents a complete synthesis of our zoological and botanical knowledge. It is, however, especially paleontological research (GAUDRY) that enables us to recognize the succession of variations in related species belonging to successive geological strata and hence to different eras; it is thus possible to determine, in a highly objective manner, the probable evolutionary lineage of species in chronological order and to recognize the extent of progressive adaptations.
Comparative anatomy (GEGENBAUER) and embryological studies also contribute to clarifying phylogenetic affinities. According to a conception of FRITZ MÜLLER and ERNST HAEKEL (fundamental biogenetic law), the development of the individual organism (ontogeny) represents a brief recapitulation of phylogeny, and thus from the examination of successive embryonic stages one could trace back to the evolutionary history of the species. For example, the fact that all higher metazoans early pass through the embryonic gastrula stage would indicate their descent from poriferans and coelenterates, which in their adult stage present an overall appearance similar to that of this developmental stage (the gastrea theory, HAEKEL); and the fact that in the embryo of higher vertebrates, with pulmonary respiration, the branchial apparatus begins to form—destined to remain non-functional—would indicate in them a descent from lower vertebrates, in which this apparatus is functional.
A phylum is said to be a succession of species derived from the same original stock through direct lineage. It has often been noted (BERGSON, ROSA, CUÉNOT) how every major phylum tends to divide one or more times by dichotomy. Thus, in the animal kingdom, a primitive trunk comprising the coelenterates divides into the two great branches of protostomes and deuterostomes (GROBBEN). From the main trunks of the genealogical tree of living beings, the various systematic groups with more specific differentiation generally detach themselves as lateral branches, which exhaust or attenuate their further evolutionary possibilities by becoming highly specialized. Cases have also been highlighted of a phylum that continues its own evolution in a given direction, with apparent independence from any tendencies toward progressive adaptation (EIMER’s orthogenesis), in which the characters involved in this gradual transformation may sometimes even exceed favorable dimensions and become manifestly harmful and exaggerated devices (phenomena of hypertrophy), which foreshadows the extinction of the species. Different phyla, born from the same stock but differentiated from one another in a divergent sense, may subsequently evolve in the form of parallel lines, in each of which highly regular common characteristics may then appear (morphological parallelisms of COLOSI; homomorphies of NOVIKOFF). In other phyla, unrelated to one another but secondarily adapted to the same environment, a strong morphological resemblance may arise through a phenomenon of convergent evolution.
At the present state of our knowledge, the phylogeny of both major and minor groups of living beings remains, for the most part, at the stage of an attempt at morphological synthesis. Nor is the controversy between supporters of a monophyletic or polyphyletic nature of evolution resolved: the former admit that all living beings derive from a single ancestral stock, the latter from multiple stocks. Modern systematics, however, is constructed on a phylogenetic basis, which aims to group animal or plant species according to evolutionary kinship, to establish a hierarchy and a succession of the various groups thus obtained, reflecting the historical order of their genealogical derivation if experience were to demonstrate the truth of the theory of evolution. Thus, a rational “natural system” would be achieved, to be contrasted with the arbitrary “artificial systems” characteristic of the period preceding the theory of evolution (cf.).