Fig.. 3.—Differences in mean characters between population samples of two consecutive chrono-subspecies of scallop (Pecten) in the Castlecliffian Stage of Wanganui Basin. The population intermediate in time is intermediate in the three characters measured: A, height/length ratio B, percentage of sample with concentric lamellae on flanks. C, right valve inflation (%). (Open oblong indicates observed range; arrow heads are 2σ each side of mean.)

Fleming. But a population intermediate in time between these two forms is also intermediate in several of its characters that can be measured. “On the theory of descent,” Darwin wrote, “the full meaning of the fact of fossil remains from closely consecutive formations, though ranked as distinct species, being closely related, is obvious”.⁶⁸ The consecutive Pecten populations are now ranked as subspecies, not species as they would have been classed in Darwin's time, later in life he “speculated on what nomenclature would come to and concluded it would be trinomial”⁶⁹.

Many gradual transitions in fossils like the ones illustrated can now be cited to answer what Darwin thought “difficulties of the gravest nature”.