The occurrence of spontaneous atavisms (rare atavistic anomalies in individual specimens) and phylogenetic character reversals (expressed in all members of a given clade) are manifestations of the same phenomenon. The potential of the former to lend insight into underlying formative processes is generally recognized, but this is not the case with the latter. The way in which taxic atavisms (phylogenetic character reversals) are distributed within clades is of considerable interest. Part of this interest derives from the fact that within any lineage there is (potentially) a vast reservoir of plesiomorphic morphologies which may be re-expressed at various levels. The manner in which these features can and do reappear, often after protracted periods of time and over large taxonomic distances, should be of the greatest interest to evolutionary biologists. In addition to illuminating aspects of evolutionary transformation it is suggested that taxic atavism may be a mechanism of considerable importance in generating morphological variation within clades.
