Difficulty of distinguishing varieties and species. Did HCW suggest a printed list that might help?

Polymorphic genera.

On geographical distribution of plants. Plant systematics and natural classification.

Do the plants that are common to Europe and North America nearly all live north of the Arctic Circle? CD bases his question on HCW’s "capital" comparison between relations of Europe to North America and Europe to E. Asia if the intervening land had been submerged. CD has been led to speculate that in the mid-Pliocene the organisms now living in middle Europe and northern U. S. lived within the Arctic Circle. Subsequent movements of this flora with advance and retreat of glaciers would explain present distribution better than Forbes’s vast submergences.

Discusses the possibility of "convergence" occurring; believes it could be only very limited.

Asks HCW’s help with his experiments on Lythrum salicaria, for which he needs flowers of the rare Lythrum hyssopifolia.

Thanks HCW for Lythrum specimens.

CD has at last finished his Lythrum paper. ["Three forms of Lythrum", Collected papers 2: 106–31.]

Gives CD an instance of facts that can be read either way as to whether a plant (Veronica humifusa) is a species or a variety.

Cover containing some seeds mentioned in the letter to H. C. Watson, 28 May [1864], f.2 (S 4512).

In response to CD’s query, HCW says he cannot supply "any list of species as the flora of a single and sterile soil". Suggests a possible source of information, and provides some figures for Britain, but these apply to diverse soils.

Sends a count of the number of species of flowering plants and ferns on the islands of Fayal and Flores in the Azores.

Returns CD’s list of Azores plants with information on the distribution of the species added. Encloses a list, extracted from CD’s list, of those plants common to Europe and the Azores that were probably not introduced by man.

Is having difficulties marking close species on the list of British plants.

In all his attempts to advance geographical botany he is stopped by the "application and signification of the word ""species"" " the use of which is both "indefinite and variable". He encloses his list of "Categories of Species".

Sends a catalogue of plants [missing] with the close species marked.

Close species in large and small genera.

Artificiality of botanical classification.

Expresses his general opinion on the relative closeness of species in large and small genera. Warns that the size of a genus is dependent upon the locality and extent of the flora studied, that definitions of close species are not consistent, and that peculiarities of botanical classification will influence any attempt to assess the comparative closeness of species in different genera.

Sends London catalogue of British plants with close species marked.

Charges E. Forbes with fraudulent appropriation of others’ work.

Comments on, and cites possible cases of, CD’s imagined rule that individuals of one or more species in a genus vary in some of those characters by which the species of that genus are distinguished.

Artificiality of orders and genera in botany.

Difficulties in numerical analysis of close species in large and small genera.

HCW has "pretty strong bias towards the view that species are not immutably distinct".

Extracts from MS of vol. 4 of HCW’s Cybele Britannica [1847–59] showing the diversity of views on species among botanists.

Answers CD’s questions about plants common to U. S. and Britain and their distribution in Europe.

Variability of agrarian weeds.

Evidence relevant to E. Forbes’s land-bridge theory.