Regrets that F. Hilgendorf proved so greatly in error ["Planorbis Multiformis", Monatsber. K. Akad. Wiss. Berlin (1866): 474–504; "Noch einmal Planorbis Multiformis", Z. Dtsch. Geol. Ges. 29 (1877): 50–62].
Discusses polymorphic species.
Surprised that shell form developed from various different progenitors.
Reminds CD of C. Nägeli’s conclusions on Hieracium.
But still retains belief expressed in first edition of Origin that variation in protean species is neither advantageous nor disadvantageous.
The full text of this letter is not yet available online.