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Darwin Correspondence Project

From Samuel Pickworth Woodward   2 May 1856

40. U. Park Stt. Barnsbury

May 2 1856.

Dear Sir

Your question respecting the relative value of species in times past & present, can only, I fear, be answered hypothetically—1

1. The proportion of species to genera was only 7 to 1 in the Silurian Period; it rose to 14 in the Jurassic, & 16 in the Eocene Tertiary. At the present day there are 40 sp. to each genus of shells, on the average, according to my estimate (p. 417)2 Now as shells are very simple contrivances presenting few points of comparison, (for in this inquiry we must set aside colour) the difficulty of determining species must increase very rapidly with the number in each genus. There are 3 or 400 sp. of Conus & 4 or 500 Pleurotomas—so that any new species possessing the essential characters of either genus will probably run very close to some sorts already known— The number of possible variations of these types must be well-nigh exhausted.

It does not follow from this that the species are bad—ie varieties of a common stock.

On the other hand, fossil shells present difficulties, on account of their condition, which usually more than make up for their comparative fewness.

When suites of fossils can be obtained in good condition—such as the Bayeux fossils & the U. Greensand sp. of Mans, which are like recent shells—they present just the same degrees of specific variation as their living representatives

In Deshayes Fossils of the Paris Basin about half the species are good, according to my notion of species.3 In his catalogues of recent shells (Veneridæ, Cycladidæ, Tellinidæ) perhaps one fourth are good for something.4

The accurate Dr Louis Pfeiffer seems to have grown weary, or disgusted with the task of discriminating Helicidæ after describing 3000 species, & tossed in another odd 900 species sans discretion—perhaps to please Mr Cuming.5

Mr Gray says only 110 of the reported species of shells are really distinct;6 & you know the rest of the joke. Henceforth the definition of “Species” will be—“a little group of Genera”.— “Genus; the smallest term in Zoology; something less than a variety”.

2. Your question may be answered by appealing to Geographical Distribution. Dr Hooker says “the species in isolated islands are generally well-defined; this is in part the consequence of a law that genera in islands bear a large proportion to the species” (Antarctic Botany.)7 I believe this to be true, & that the Island faunas are older (generally) than those of Continents. Still it cannot be denied that the Helicidæ of Madeira exhibit a high degree of specific variation— Even in the more ancient St. Helena have we not Bulimus auris-vulpinus & B. Darwini? And if you appeal to the Galapagos Ids I must almost admit the converse of the above proposition!

3. Lastly we may compare the relative value of species in Genera of different degrees of antiquity. The worst genus of all—Clausilia is at least of Eocene date—its 280 species are restricted to the Lusitanian Province, save a few which range through the Germanic to China.8

The Terebratulæ of the Oolites are not more variable than the Terebratellæ of the present seas— Mr Buckman’s attempt to demolish some of them was founded on a comparison of young & undeveloped specimens—9 Some of these have internal characters with which he was unacquainted. In every case the period of maximum development of a genus may be taken as the time at which the diagnosis of its species is most difficult—& the entire fauna of each period presents the same aspect of a combination of groups, some commencing, others culminating, the rest declining. It is no use pursuing this further, as I cannot give you a matter-of-fact answer. The truth is that the species of every group, country & time, cannot be distinguished empirically, but require judgment & experience—& this will be no new truth to you—

Pray when you can, let me know what points in my book I ought to re-consider!

Yrs sincerely | S. P. Woodward

CD annotations

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CD’s reply (letter to S. P. Woodward, 27 May 1856) makes it clear that CD had asked Woodward whether the variability of species within a particular genus was different at different periods.
Woodward 1851–6, 3: 417, a table illustrating the ‘Development of Families, Genera, and Species, in Time’. It is possible that Woodward enclosed an advance copy of the third part of his book on shells with this letter: the volume was received by CD before 15 May (see letter to S. P. Woodward, 15 May [1856]). CD also recorded having read the entire work on 5 June 1856 (Correspondence vol. 4, Appendix IV, 128: 18). In any case, Woodward could be confident that CD would have access to this reference in the very near future: the third volume was published in May (see letter to S. P. Woodward, 15 May [1856]).
Ludwig Georg Karl Pfeiffer compiled catalogues of various Mollusca; the Helices are described in Pfeiffer 1848. Hugh Cuming had made one of the most extensive collections of shells of the time, which he gave to the British Museum. Pfeiffer and Gérard Paul Deshayes had both prepared catalogues for the British Museum of various families from Cuming’s collection.
John Edward Gray himself had a reputation for needlessly coining new specific and generic names (DNB). See also letter from Charles Lyell, 1–2 May 1856.
Woodward divided the globe into twenty-seven land provinces and eighteen marine provinces. As he stated in Woodward 1851–6, 3: 382: ‘The Land Provinces represented on the map are the principal Botanical Regions of Prof. Schouw, as given in the Physical Atlas of Berghaus’. He refers to Joachim Frederik Schouw’s map of botanical distribution in Heinrich Karl Wilhelm Berghaus’s Physikalischer Atlas (Berghaus 1845–8). For the marine provinces, Woodward followed Edward Forbes’s work (E. Forbes 1846 and 1856).
James Buckman studied the palaeontology and stratigraphy of the Jurassic series of the Cotswolds.
The number of CD’s portfolio of notes on variation.


Berghaus, Heinrich Karl Wilhelm. 1845–8. Physikalischer Atlas. 2 vols. Gotha.

Correspondence: The correspondence of Charles Darwin. Edited by Frederick Burkhardt et al. 27 vols to date. Cambridge: Cambridge University Press. 1985–.

Deshayes, Gérard Paul. 1824–37. Description des coquilles fossiles des environs de Paris. 2 vols. and atlas. Paris.

Deshayes, Gérard Paul. 1853–4. Catalogus Concharum Bivalvium, quæ in Musæo Britannico asservantur. Pt 1: Veneridæ; pt 2: Petricoladæ (concluded); Corbiculadæ. [Edited by J. E. Gray.] London.

DNB: Dictionary of national biography. Edited by Leslie Stephen and Sidney Lee. 63 vols. and 2 supplements (6 vols.). London: Smith, Elder & Co. 1885–1912. Dictionary of national biography 1912–90. Edited by H. W. C. Davis et al. 9 vols. London: Oxford University Press. 1927–96.

Forbes, Edward. 1846. On the connexion between the distribution of the existing fauna and flora of the British Isles, and the geological changes which have affected their area, especially during the epoch of the Northern Drift. Memoirs of the Geological Survey of Great Britain, and of the Museum of Economic Geology in London 1: 336–432.

Hooker, Joseph Dalton. 1844–7. Flora Antarctica. 1 vol. and 1 vol. of plates. Pt 1 of The botany of the Antarctic voyage of HM discovery ships Erebus and Terror in the years 1839–1843, under the command of Captain Sir James Clark Ross. London: Reeve Brothers.

Pfeiffer, Ludwig George Karl. 1848. Monographia heliceorum viventium. Sistens descriptiones systematicas et criticas omnium huiuas familiae generum et specierum hodie cognitarum. 2 vols. Supplement, 4 vols. in 6. 1853–77. Leipzig.

Woodward, Samuel Pickworth. 1851–6. A manual of the Mollusca; or, a rudimentary treatise of recent and fossil shells. 3 pts. London. [Vols. 6,8,9]


Proportion of molluscan species to genera in various periods. The difficulty of determining species increases with the number of species per genus. Identifying species within a genus is most difficult in that period in which the genus shows its greatest development.

Letter details

Letter no.
Samuel Pickworth Woodward
Charles Robert Darwin
Sent from
Source of text
DAR 181: 153
Physical description
4pp †

Please cite as

Darwin Correspondence Project, “Letter no. 1864,” accessed on 21 January 2022,

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