ABSTRACT
An apparent non-congruence in the distribution of two classificatory characters may result from the secondary loss of one of them in some forms, as well as from their non-contemporaneous origins; this may be called 'secondary non-congruence', to distinguish it from the primary kind. The general criterion is that, if the absence of a normal character of a group in some member of it is attributable to primary non-congruence, then that member should show other indications of being unusually distant from the rest of the group and should possess other exceptional and primitive features. The effects of the non-congruence principle are frequently manifest when different systematists propose classifications of the same group of organisms based on different characters, and they have underlain many protracted classificatory controversies in the past. Examples of non-congruence in the incidence of individual characters could be multiplied indefinitely from either the plant or the animal kingdom.
