Dear and much honoured Sir,

Your letter appears to me full of the most interesting and important points, and I hope that some day you may publish a full discussion on these subjects.¹ I have noticed observations in your works on the long persistence of certain varieties; & this is a subject which has perplexed me during many years. The cases which I had in my mind were chiefly drawn from the animal kingdom, such as the fossil land-shells of Madeira & of certain marine tertiary shells, which formerly presented the same range of variation as they now do.² With respect to sub-species, races, varieties etc., I dare say you are acquainted with Nägeli’s discussion on Hieracium which interested me greatly;³ and if I had been a younger man I would have experimentally investigated such closely related forms. The subject of one species varying in such a manner as to resemble another allied form, whether called a species, race etc. has long been familiar to me, and I have given a few instances in several of my works under the heading of “analogous variation”.⁴

With respect to the sudden development of dicotyledenous plants, which view Heer likewise maintains, I confess that I am sceptical.⁵ It is of course a mere conjecture, but I imagine that this great group of plants must have been slowly developed in some part of the globe which was formerly more completely isolated from all other regions than any part of the land now is. I have always felt the keenest interest in your observations on the very gradual change of species during the later Tertiary periods; and I observe that A. de Candolle has likewise been struck with these observations which are strongly opposed to Heer’s belief of great & abrupt specific changes.⁶ I venture to repeat my hope that you may induced to write a special essay on these difficult & very interesting points.

With much respect, I remain | Dear Sir, | Yours very faithfully | | Charles Darwin