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Darwin Correspondence Project

From H. C. Watson   [28 December 1856]1

The comparative rarity of the subspecies or simple varieties, cut off from alleged true species, might be shown by numerous examples.2 Indeed, this greater rarity would seem to be matter of course; for otherwise the subspecies or variety would have been itself described for the type species in most instances. In the following short list of names, the first of each pair is that of a common British plant, generally received as a true species; while the second name belongs to a subspecies, closely allied to the first, and much more scarce or local:—

Caltha palustris L. C. radicans, Forst.

Ranunculus Flammula, L. R. reptans, Lightf.

Draba verna, L. D. præcox, Reich.

Ulex europæus, L. U. strictus, Mack.

Genista tinctoria, L. G humifusa, Dicks.

Agrimonia Eupatoria, L. A. odorata, Ait.

Apargia autumnalis, L. A. Taraxaci, Sm.

Centaurea nigra, L. C. nigrescens, Aut.

Gentiana germanica, L. G. germanica, W.

Erythræa Centaurium, Pers. E. latifolia, Sm.

Veronica officinalis, L. V. hirsuta, Hopk.

Veronica serpyllifolia, L. V. humifusa, Dicks.

Taxus baccata, L. T. fastigiata, Lindl.

Asplenium Adiantum negriem L— A. acutum, Bory.

The comparative rarity of intermediates is not very readily proved by examples, without the aid of explanatory descriptions that would be tedious. The difficulty lies partly in the fact that they are often without special names; partly also because they are usually assigned as varieties of the species to which they approximate most, and are thus not viewed as intermediates by many botanists. The grounds for designating them intermediates are shortly these. 1. Some of them form a series of varieties passing gradually into both species. 2. In other instances, both species vary, and the varieties approximate. 3. In more numerous instances, perhaps, one of the species varies; its varieties assuming some of the characters of the other species, either positive or negative, but without passing into that other species. Examples of No 1.

Geum urbanum (L). and G. rivale (L). are so connected by a series of intermediate forms, that botanists hesitate to which species the intermediates should be assigned; some aggregating them into a third species (G. intermedium Ehrh.); others pronouncing them hybrid forms.

Saxifraga umbrosa (L.) & S. Geum (L). are also connected by endless intermediates; among which the S. elegans (Mack) and S. hirsuta (L.) are often deemed to be real species.

Primula veris & P. vulgaris (of English authors) are closely connected by a full series of intermediates, always now held varieties or hybrids.

Pyrus Aucuparia (Gaertn.) and P. Aria (Sm.) are imperfectly connected by the P. pinnatifida (Sm.), which seems to be usually called a species because botanists are at a loss to choose its real ally between the other two; its alledged distinguishing characters are variable, & perhaps irregular growths.

Examples of No. 2.

Arenaria rubra (L.) and A. marina (Oed.) both vary into forms that resemble each other more closely than the type species; and probably the Arenaria media (Aut.) may occasionally include varieties of each.

Viola tricolor (L.) and V. lutea (Huds.) have numerous varieties; some of which almost touch or pass into each other, as the V. Curtisii (Forst.) and V. sabulosa (Boreau); the former supposed a variety of V. lutea, the latter a variety of V. tricolor.

Linaria vulgaris (Mill.) and L. repens (Ait.) both seem to vary; the varieties approximating. The most intermediate form is the L. sepium (Allm.) conjectured to be a hybrid.

Circæa Lutetiana (L.) and C. alpina (L.) both vary; and it is occasionally difficult to assign their varieties correctly to the species, so closely do they approximate. The intermediates are more frequent than C. alpina, less so than C. lutetiana.

Examples of No. 3

Erica Tetralix (L.) and E. ciliaris (L.) are very distinct in their typical forms; but a series of varieties occurs, perhaps all of them forms of E. Tetralix, which show a gradual passage from Tetralix towards ciliaris. One of these forms is the E. Mackaii (Hook.) known only in single localities in Ireland and Spain. The other intermediates (Watsoni & Borreri) occur in Cornwall, & only near Truro.

Stachys ambigua (Sm.) is by some authors held a species, by others a variety of S. palustris (L.) In so far as it differs from the latter, it may be said to shade off towards S. sylvatica (L.)

Luzula Borreri (Bromf.) lately described for a species, seems a variety of L. pilosa (W.). Where it differs from the latter, it approaches to L. Forsteri (DC.).

More briefly put it may be said that the following intermediate subspecies &c. are more rare than the species. diag Geum intermedium (Ehrh)

Series of vars. between G. rivale & G. urbanum Saxifraga hirsuta (L) &c.

Subspecies between S. umbrosa and S. Geum. Primula intermedia (var.) &c.

A series of varieties between P. veris and P. vulgaris. Pyrus pinnatifida (Sm.)

A variable subspecies between P. Aucuparia and P. Aria. Linaria sepium (Allm.), &c.

A series between L. vulgaris and L. repens, including varieties of each probably. Arenaria media (Aut.) &c.

Varieties of A. rubra and A. marina, closely approximating. Viola Curtisii (Forst.) & V. sabulosa (Bor, &c.)

Varieties of V. tricolor and V. lutea, closely approximating. Erica Mackaii (Hook.) &c.

Varieties of E. Tetralix passing towards E. ciliaris. Stachys ambigua (Sm.)

A variety, itself rather variable, of S. palustris passing towards S. sylvatica, but not closely so. Potentilla procumbens (Sibth.)

A variety of P. Tormentilla passing rather slightly towards P. reptans. Luzula Borreri (Bromf.)

A subspecies mingling the characters of L. pilosa and L. Forsteri. Juncus diffusus (Hoppe)

A doubtful species between J. effusus and J. glaucus in general characters.ramme

CD annotations

‘Q’added pencil, circled pencil
‘Q’added pencil, circled pencil
‘Q’added pencil, circled pencil
Top of first page: ‘On Rarity of intermediate forms.’pencil; ‘Dec. 28 1856. (8th letter)’ pencil
End of last page: ‘12’pencil

Footnotes

The date is given by CD’s annotation.
CD used information from this memorandum on the rareness of intermediate forms and on the occurrence of varieties connecting distinct species in Natural selection, pp. 102, 268.

Summary

Notes on the comparative rarity of intermediate forms between species, and the varying relationships those forms may have to one or both species between which they are intermediate.

Letter details

Letter no.
DCP-LETT-2023
From
Hewett Cottrell Watson
To
Charles Robert Darwin
Source of text
DAR 98: A15–18
Physical description
Amem 8pp †

Please cite as

Darwin Correspondence Project, “Letter no. 2023,” accessed on 19 March 2019, http://www.darwinproject.ac.uk/DCP-LETT-2023

Also published in The Correspondence of Charles Darwin, vol. 6

letter