We have in New Zealand a good number of what Darwin called “polymorphic genera” in which “species present an inordinate amount of variation”43 such as
[Footnote] 43 Origin: 40.
he had observed in the genus Rubus and in several general of Brachiopoda (lamp shells) where “hardly two naturalists can agree which forms to rank as species and which as varieties”. Darwin suspected that such variations “are of no service or disservice to the species and …have not been seized on and rendered definite by natural selection”. Systematists are still plagued by such polymorphic swarms, and New Zealand paleontologists have still to rationalize the nature of such variability in certain Tertiary Brachiopoda; in one case the late Dr. J. A. Thomson named 10 species of one genus in a single thin bed, but admitted in a paper44 read before this Society in 1919 that “there are so many intermediates that it is obvious that evolution either had taken place only a short time previously or was still in progress”. Modern workers would not be so prodigal of specific names, I suspect, but would adopt Darwin's explanation that selection, in such cases, was failing to keep such populations confined rigidly to the narrow range of variation we are used to.
[Footnote] 44 Thomson, J. A., 1920. Trans. Roy. Soc. N. Z., 52: 370.