Biological classification has been successively shaped by
Neoplatonism, ideas of plenitude and one-dimensional continuity (the Great
Chain of Being), two-dimensional continuity (Linnaeus's map), idealized
comparative morphology and development and, through
phylogenetic theory, Darwinian descent with modification. Concepts of
Algae have thus evolved within a succession of very different
paradigms. Algae have been imperfect plants; a segment of the Great
Chain; the least-developed members of vegetative form-series; and
primitive forms from which plants (and even animals) can be elaborated.
The idea of a single grouping of simple organisms, formalized by
Bory de Saint-Vincent and others in the early nineteenth century,
was developed in a phylogenetic framework by Haeckel as Protista.
Most macrophytic algae, however, remained Plants until Copeland merged
eukaryotic non-green algae into Protista. Arrangements of
algal taxa within form-series, and recognition of Algae and Protista as
taxa, are incompatible with Hennigian phylogenetic systematics
(cladistics). Developing genomic perspectives threaten to undermine
established concepts of organismal lineage and higher-level biological
classification.