Thanks GdeS for his "Études sur la végétation" [Ann. Sci. Nat. (Bot.) 5th ser. 15 (1872): 277–315]. "Nothing can be more important . . . than your evidence of the extremely slow and gradual manner in which specific forms change."
Hopes GdeS will shed light on whether polymorphic forms like Rubus and Hieracium are generating new species at present; CD doubts this.