Notes:
In properly applying Lankester's dual concepts of homogeny and homoplasy in evolutionary biology, we can provide definitions of our terms which no longer suffer from the confusion which has befallen homology for over 100 years. This is especially significant in light of the fact that the definition of homology should be limited to hypotheses involving Owen's archetype theory and not evolutionary theory. Hypotheses involving the latter theory are best accommodated by the terminology offered by Lankester. I suggest the following taxonomy of terms:
We can readily maintain the concept of homologue, as it was originally intended by Owen in 1843, defined as hypotheses accounting for our perceptions of similar properties among organisms. This is denoted by our activity of applying the same names to the features of individuals.
As it is the case that causal hypotheses are inferred to account for observed effects, the recognition that phylogenetic inference is abductive makes the dual recognition of homogeny and homoplasy far more relevant than has been presented in the past.
Lankester's homogeny is an accurate place holder for causal hypotheses accounting for homologous features, as homogens, by way of descent with modification or common ancestry. By referring to homogeny in evolutionary biology, we avoid the historical baggage of misinterpretations associated with homology. Which, as Lankester noted, had its origin in an entirely different philosophy.
Lankester's term homoplasy remains as he originally intended, as an ad hoc hypothesis accounting for homologous features, as homoplasts, by separate, intraspecific events of either parallelism or convergence. More generally, homoplasy simply recognizes the ad hoc nature of the hypothesis, as a statement that causal events other than common ancestry must be invoked to account for homologues.
