FILOGENESIS. – Within the framework of the theory of evolution, this denotes the historical order according to which the various groups of living beings are presumed gradually to have changed and evolved. All animal and plant species, both living and extinct, could be united in an enormous genealogical or phylogenetic tree, in which common trunks, corresponding to more ancient ancestors, repeatedly divide into secondary branches, representing the more recent forms.
The construction of a phylogenetic tree is based on documents derived from the most diverse disciplines (paleontological, anatomical, embryological, biogeographical, physiological, and genetic data), and therefore represents a complete synthesis of our zoological and botanical knowledge. It is above all, however, paleontological research (Gaudry) that makes it possible to recognize the succession of variations in related species belonging to successive geological strata and thus to different eras; it is therefore possible to specify, in a highly objective manner, the probable evolutionary filiations of species in chronological series, and to recognize the extent of progressive adaptations.
Comparative anatomy (Gegenbaur) and embryological studies likewise help to clarify phylogenetic affinities. According to a conception of Fritz Müller and Ernesto Haeckel (fundamental biogenetic law), the development of the individual organism (ontogeny) would represent a brief recapitulation of phylogeny, and therefore the evolutionary history of the species could be reconstructed from the examination of successive embryonic stages. For example, the fact that all higher metazoans pass at an early stage through the embryonic phase of the gastrula would indicate their descent from the porifers and coelenterates, which in adulthood present an overall appearance similar to that of this developmental stage (theory of the gastraea, Haeckel); and the fact that in the embryo of higher vertebrates, with pulmonary respiration, the branchial apparatus is sketched out, destined to remain nonfunctional, would indicate their descent from lower vertebrates, in which this apparatus is functional.
A succession of species derived from the same original stock by direct filiation is called a phylum. It has often been noted (Bergson, Rosa, Cuénot) that 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 the protostomes and the deuterostomes (Grobben). From the main trunks of the genealogical tree of living beings, the various systematic groups with the most pronounced differentiation generally branch off, however, as lateral branches, which exhaust or attenuate their further possibilities of evolution by becoming highly specialized. Cases have also been brought to light of a phylum continuing its evolution in a given direction, apparently independently of any tendencies toward progressive adaptation (Eimer’s orthogenesis), so that the characters involved in this gradual transformation may sometimes even exceed favorable dimensions and become manifestly harmful and exaggerated structures (phenomena of hypertely), thereby foreshadowing the extinction of the species. Different phyla, originating from the same stock but diverging from one another, may subsequently evolve in the form of parallel lines, in each of which common characteristics may then appear with remarkable regularity (morphological parallelisms of Colosi; homomorphies of Novikoff). In other phyla, unrelated to one another but secondarily adapted to the same environment, strong morphological resemblance may arise through a phenomenon of convergent adaptation and evolution.
In the present state of our knowledge, the f. both of the major and of the minor groups of living beings remains for the most part only a tentative morphological synthesis. Nor has the controversy between the advocates of a monophyletic or polyphyletic nature of evolution yet been resolved: the former maintain that all living beings derive from a single ancestral stock, the latter from several stocks. Modern systematics is nevertheless constructed on a phylogenetic basis; it seeks to group animal or plant species according to their evolutionary relationships, and to establish a hierarchy and succession of the various groups thus obtained, reflecting the historical order of their genealogical derivation, if experience were to demonstrate the theory of evolution to be true. In this way, it would seek a rational “natural system,” as opposed to the arbitrary “artificial systems” characteristic of the evolution (v.).