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In no other group of colonial organisms has the form of the colony played such a prominent role in taxonomy as in the graptolites. The regularity of the colony in respect of branching and gross rhabdosomal characters, the occurrence of progressive thecal modification (thecal gradients), and, in cyrtograptids, the production of cladia with their integrated thecal gradients, imply an unusual degree of morphogenetic control which appears fully to justify the use of colonial characters in taxonomy. The main outlines of the evolutionary history of the group have been made familiar by the work, especially, of Lapworth and Elles; but a greater emphasis is here placed on the importance of abrupt rhabdosomal modifications followed by extensive thecal differentiation. It is suggested that many of the major rhabdosomal changes could be interpreted as evolutionary grades (J.S. Huxley) with subsequent thecal differentiation representing the ensuing cladogenesis, though the extension of these concepts to colonial organisms is tentative. The main evolution of the graptolites is briefly surveyed and possible taxonomic implications are discussed. Jaanusson’s suborder Didymograptina is accepted for the dichograptids, leptograptids, and dicranograptids; his separation of the suborder Glossograptina from the Diplograptina is welcomed despite its limited cladogenetic differentiation; and the magnitude of the advance represented by the monograptid grade is considered to warrant the recognition of a fourth suborder (Monograptina) for the scandent uniserial rhabdosomes. The processes of thecal differentiation and the taxonomic significance and practical utility of various genera and subgenera that have been proposed are discussed with reference to the diplograptids and monograptids. An outline classification at and above family level is appended.
