A ‘natural’ taxonomic system, inherent in organisms that themselves demonstrate their ‘affinity’, is to be recognized perhaps only in the biospecies. The concept of the biospecies as a comprehensive taxon is, however, only notional amongst the vast majority of living organisms, and it is not directly applicable to fossils. ‘Natural’ systems of Linnaean kind rest on assumptions made a priori and are imposed by the systematist. The graded time-sequence of the lineage and the clade introduces factors into a systematics that cannot well be accommodated under pre-Darwinian assumptions or be formulated by a Linnaean nomenclatural method.
The fossil evidence of ‘affinity’ is best served by a system of classes defining intrinsic relationships that are phylogenetic. Multiple demic variants, clinal bioseries, variable rates of evolutionary change, pulses of biased selection pressure in expanded and restricted palaeodemes, and permutations of character-expression in the evolutionary plexus impose a need for a palaeontologically-orientated systematics under which (in evolutionary descent) could be subsumed the taxa of the neontological moment.
Environmentally controlled morphs, biofacies variants, migrating variation fields, and typological segregants are sources of ambiguity in a distinction between phenetic and genetic fossil grades. Trends in variants are not to be confused with trends in transients; and sequential demes may (in time and space) show trend reversals as a sign of nongenetic (phenetic) selection. The morphospecies of the local palaeodeme is then to be distinguished from the holomorphospecies of the lineage segment, both in kind and in taxonym; and the association of variant morphotypes in a unitary deme is no justification for a splitting of the deme into several morphospecies.
Criteria of cladal disjunction are multiple. They may rest in one or a few ‘differential’ characters, or (in a ‘numerical’ taxonomy) on an Adansonian equal-weighting of ‘all’ characters. A consequence is to impose on an inferred phylogeny taxonomic incompatibilities deriving from contrasted interpretations of homology and homoeomorphy. Clinal disjunction as an incipient sign of cladal branching presents other problems in nomenclature and in the definition of taxon-range. Accelerated and retarded rates of bioserial change in sub-parallel lineages further complicate systematics notably in mosaic evolution. A basic distinction between Linnaean (morphotypic) and evolutionary systematics rests in the relevance of the question that asks whether the class determines the characters or the characters the class.