[Copy made by CD's amanuensis.] Discusses the rarity of intermediate forms.
April 12. 47.—
My dear Hooker
I think we may pretty securely say that where two marked varieties, quasi-species, or species of plants are apparently connected by some intermediate form, the latter is comparatively scarce. Were this not the case, indeed, the intermediate would have been described as the species, & the two as its extreme forms.
The yellow & purple varieties of Viola lutea, are both frequent in the Highlands; but the party-coloured link is scarce.
Festuca pratensis & Coliacea are readily distinguishable Varieties of a species; the connecting links being apparently very much scarcer than either the completely panicled (pratensis) or completely racemose (Coliacea) form.
The Cowslip & Primrose (as you suggest) are a thousand, a ten thousand times more abundant than the spurious Oxlips which connect them. I say spurious oxlips as distinguished from the true one (P. elatior of Jacq.)
Lysimachia nemorum is frequent in Britain & neighbouring Countries; and
L. Azorica is pretty frequent in the Azores. I found no connecting link in the
latter isles, & the Azoric species appeared so dissimilar to the British one,
that it never even occurred to me that they were
Assuming Alopecurus pratensis & A. alpinus to be only quasi-species, the Siberian intermediate would be another case in point.
Geum urbanum & G. rivale are both frequent in Britain, &c. The intermediates, hybridum & intermedium are scarce.
I am not sure, however, that some exceptional cases cannot be cited. Circæa intermedia appears to be found as often as C. alpina. Connecting links between the more typical examples of Viola tricola & V. arvensis may be as numerous as the more extreme or dissimilar forms, considered to be tricola & arvensis.
Strict evidence can scarcely be obtained to show that a species may continue
to be propagated under two constant forms, the original & the variety,
without reference to the conditions which gave origin to the variety. Such probably is
the fact to a certain extent, altho' proof is next to impossible, because we
cannot show what were the efficient causes of the variation in the first instance. The varieties of our cultivated vegetables, & of various
flowers in gardens, do come up with much constancy, from seed; & yet we usually
may find some “rogues”, as gardeners term them, reverting towards
the type of the species. The offspring of the rogues become some of them, still more
roguish; so that, in a state of nature, the variety would walk
back to the type. I tried this 3 generations with the Scotch Kail, &
some of the 3
Look to the examples of Myosotis Azorica & M. maritima (both from the Azores). They appear much dissimilar. I have had them in cultivation since 1842, raising from seed yearly. They have (or one has) sported into intermediate varieties, which I cannot refer to either satisfactorily.
These varieties, seeding more freely, & bearing our climate better, would lose me the species without care in saving seed from the typical forms. The first year I thought them varieties of Azorica, last year (the third year) some of them reverted very near to maritima. From a trifling peculiarity in the Corolla, I am disposed to refer them all to maritima; & yet, when dried, I am unable to distinguish some of them from Azorica, if the specimens become mingled before they are labelled.— Unfortunately, I cannot say from which species the first plant of the variety originated, but suppose it to have sprung from a seed of maritima.
Hewett C. Watson
- f1 1079.f1Hooker evidently solicited Watson's opinions on varieties at CD's request (see letter to J. D. Hooker, 7 April ) and passed Watson's reply on to CD. The letter was copied at Down House and has corrections and annotations in CD's hand.
- f2 1079.f2The date of the original letter. The copy, from which this transcription is taken, was made after 12 April 1847.
- f3 1079.f3CD used information from Watson on the absence or scarcity of intermediate forms between varieties of the same species in Natural selection chapter six, ‘On natural selection’. He wrote (p. 268): ‘Mr. Watson has given me a list of twelve nearly intermediate varieties found in Britain which are rarer than the forms, which they connect. But both these naturalists [Asa Gray and Watson] have insisted strongly on various sources of doubt in forming any decided judgement on this head.’
- f4 1079.f4Thomas Carew Hunt, consul on the Azores, 1839–48.
- f5 1079.f5A reference to part of Watson's paper, ‘On the theory of ‘progressive development’, applied in explanation of the origin and transmutation of species’, published in the Phytologist (Watson 1845). CD used the information in this letter and in Watson 1845 in Natural selection, p. 126.
- f6 1079.f6The date, salutation, and ‘signature’ are in CD's hand. A few misspelled words and omissions in the copy have been corrected by CD.
- f7 1079.f7CD addressed this question in Natural selection, pp. 268–70, in which he asserted that intermediate forms were never ‘numerous in individuals’ and hence ‘would be apt to be exterminated by fluctuations of seasons, extraordinary increase of enemies &c, and by the inroads of the bordering species, which they link together’ (pp. 269–70). He also discussed the problem of identifying and defining species when there are a number of forms intermediate in character in existence (pp. 100–3, 116). CD had long believed that species and varieties were distinguished only by the number of individuals that each taxonomic group contained (Origin, p. 47) and that large numbers of individuals in a species tempted taxonomists to call it the ‘type’ of a genus (see Correspondence vol. 2, letter to G. R. Waterhouse, [26 July 1843], and Correspondence vol. 3, letter to J. D. Hooker, 31 March ).
- f8 1079.f8CD's annotations were apparently made at different times during his work on Natural selection and the Origin, in which Watson is frequently cited. The reference to ‘Ch. 2’ is to Natural selection. Though the manuscript of this chapter has subsequently been lost, CD recorded that the degeneration of cabbages was on page ten (Natural selection, p. 25). Watson's information on the rarity of intermediates was used both in chapter six of Natural selection (see n. 3, above) and in chapter six of the Origin (p. 176). The reference to ‘5’ is to chapter five of the Origin, ‘Laws of Variation’.