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Darwin Correspondence Project

From Searles Valentine Wood   5 June 1846

28 Fortess Terrace | Kentish Town

My dear Sir

Accept my thanks for your kind communication which is so far satisfactory that the experiment has been tried altho with a result contrary to what I shod. have expected. a casual deviation from the normal form is easily understood which may arise from an accidental interruption of Natures Laws but for that cause to be continually in operation is not so explicable

In regard to your question respecting variations among the Mollusca1 I can only say that as far as my observations have gone I think their range is greater among fossils than in recent species & the most extraordinary varieties are from the Mam: Crag2 among estuary shells these are in some instances of extraordinary proportions removing the extremes of variation far beyond what is generally considered the limit of a species these variations (or perhaps more properly distortions) consist principally in the elongation or shortening of the spiral cone beyond its general proportions without affecting in any way the form of the aperture consequently there need be little or no alteration in the animal inhabitant & this I have always considered as depending upon an external cause & more particularly with regard to the Crag shells whose location was in an estuary where a sudden alteration or reduction of temperature may have partially paralized or injured the natural powers of the animal. the shells of the present day from similar localities do not appear to be similarly affected which I wod. attribute to a more uniform or rather a less variable temperature

I have been hunting largely among the Freshwater & Estuary deposits of the Eocene Period in Hampshire but I have not met with the like variations among their inhabitants which I wod attribute to the same cause & that which conduces to a healthy condition of the animal whatever may be its locality whether on land or in the water will be favourable to its uniformity3 the Helix aspersa for example where it is in great profusion & flourishes well thousands may be examined without perceiving the slightest deviation in form

I do not therefore at all agree with Mr Forbes if he says the cause is an internal one which I cannot well understand the alteration in the shell is of course caused by the animal but the alteration in the animal is I believe produced by a cause exterior to itself either from food or temperature

I must apologize for this apparent delay in answering your note but I only obtained it a short time since for my visits to London are few & far between & your note I was told had been long at Somerset House.4

Believe me Dear Sir | Yours truly | Searles Wood June 5. 1846.

CD annotations

crossed pencil
scored pencil
4.2 understand the] altered in pencil to ‘understand. The’
Top of first page: ‘Mr. W. told me [over illeg] that common Periwinkle is more variable in Crag than in living state.’ ink


CD’s query to Wood has not been located; however, CD appears to have asked about information received from Edward Forbes regarding variation in molluscs (Natural selection, p. 106). Wood was an expert on the fossils of the East Anglian Crag.
Mammaliferous crag of Norfolk. This observation by Wood was later cited by CD in Natural selection, p. 106.
Noted in Natural selection, p. 107.
The address of the Geological Society.


Natural selection: Charles Darwin’s Natural selection: being the second part of his big species book written from 1856 to 1858. Edited by R. C. Stauffer. Cambridge: Cambridge University Press. 1975.


Variation in Mollusca.

Letter details

Letter no.
Searles Valentine Wood
Charles Robert Darwin
Sent from
Kentish Town
Source of text
DAR 181: 143
Physical description
ALS 5pp †

Please cite as

Darwin Correspondence Project, “Letter no. 983,” accessed on 20 May 2022,

Also published in The Correspondence of Charles Darwin, vol. 3