From Hewett Cottrell Watson to J. D. Hooker1 12 April 18472
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.3 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 vars of the same species, while I was in the Azores.
But Mr Hunt4 has sent examples from St. Michael’s, which so assume the characters of L. nemorum that I cannot now separate the Azoric & British by any clear character. As yet, this link is known from one isle only.—
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 3rd generation came very close to the form as now established about old castle walls, &c and called “indiginous”. I made some remarks on this point in Phytol. Vol. II p. 226–8. 5
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
[Copy made by CD’s amanuensis.] Discusses the rarity of intermediate forms.
Please cite as
Darwin Correspondence Project, “Letter no. 1079,” accessed on 28 July 2016, http://www.darwinproject.ac.uk/DCP-LETT-1079