For a translation of this letter, see Appendix I. It appears to be a resumé of a much longer (twelve-page) letter.

CD had responded to Nägeli’s theory of species formation (Nägeli 1865) in his letter of 12 June [1866] (Correspondence vol. 14). Nägeli regarded natural selection as one mechanism of species development, but maintained that it was insufficient by itself to explain various features of plants. He argued that ‘higher’ species evolved through the operation of a ‘perfectibility principle’ (‘Vervollkommnungsprincip’): a tendency within each organism to develop more complicated structures. Nägeli claimed that this principle operated in conjunction with the natural selection of useful variations (see Nägeli 1865, pp. 16–17, 28–30). CD admitted that he could not explain many morphological peculiarities, such as the arrangement of leaves or position of ovules, but added, ‘I hardly see how they support the doctrine of some law of necessary development for it is not clear to me that a plant with its leaves placed at some particular angle or with its ovules in some particular position, thus stands higher than another plant.’

Nägeli had been working on Hieracium (the genus of hawkweeds) for four years (see Correspondence vol. 15, letter to J. D. Hooker, 5 April [1867]). In a paper on intermediate forms in plant species (Nägeli 1866d), Nägeli recognised that some plant genera seemed to have an abundance of intermediate forms, which pointed to the relatedness of these hawkweed species and showed that species in general were not absolutely differentiated and could have developed from a common source or one from another (Nägeli 1866d, p. 190). Nägeli further tried to develop a method of distinguishing between intermediate species and hybrids.

Nägeli discussed variation in Hieracium in ‘Ueber die systematische Behandlung der Hieracien rücksichtlich der Mittelformen’ (On the systematic treatment of Hieracium with regard to intermediate forms; Nägeli 1866a), ‘Ueber die systematische Behandlung der Hieracien rücksichtlich des Umfanges des Species’ (On the systematic treatment of Hieracium with regard to the range of species; Nägeli 1866b), and ‘Ueber die Innovation bei den Hieracien und ihre systematische Bedeutung’ (On innovation in Hieracium and its systematic significance; Nägeli 1866c). See n. 6, below.

In his letter to Joseph Dalton Hooker of 5 April [1867] (Correspondence vol. 15), CD passed on Nägeli’s request for a complete set of British forms of Hieracium in exchange for a large set of German and Alpine forms.

Nägeli had included several of his papers on intermediate species of Hieracium in the second volume of his Botanische Mittheilungen (Nägeli 1863–81; included were Nägeli 1866a, 1866b, and 1866d). CD’s heavily annotated copy of Nägeli 1863–81 vol. 2, with the first two papers, 16 and 17, removed, is in the Darwin Library–CUL (see Marginalia 1: 625–30). Nägeli also sent the second edition of his Entstehung und Begriff der naturhistorischen Art (The origin and concept of natural historical species; Nägeli 1865) and his photograph (see Correspondence vol. 15, letter to C. W. von Nägeli, [after 8 April 1867]).