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Darwin Correspondence Project

From William Thompson   29 September 1848

Sept. 29, 1848.—I examined a great number of Balani, in reference to the growth made by them during the present season, and found it to average three lines in diameter, and at most four lines. I saw a few minute specimens, only one line in diameter, showing that the species continued to breed until lately: these latter were probably not more than four weeks old. The young of the present year are plainly distinguished from the older ones, by their pure white colour and fresh appearance. Judging from the size of this year’s specimens, and of the older ones on the same stones, I am of opinion that the term of life of the species is two years. Of the older shells, which I examined and found living in the spring, nine tenths are now dead, the walls only remaining, the opercular valves having been washed away.1


Following the extract from Thompson’s letter, CD recorded (Living Cirripedia (1854): 273): Mr. Thompson goes on to say, that the individuals which had, on July 3, a basal diameter of from two and a half to three lines, had attained, by the 30th of September, a diameter of four and a half lines, this being here the maximum size of the species. Earlier, CD quoted additional information supplied by Thompson, perhaps from this or another letter (Living Cirripedia (1854): 272): I am informed by Mr. Thompson, that he has seen specimens [of Balanus balanoides] attached to a spot not covered by water during neap-tides. As a proof of its tenacity of life, Mr. Thompson informs me that he accidentally kept some specimens in a box, in a warm sitting-room, and found them alive seven days afterwards. This same most accurate observer finds, however, that B. balanoides is very susceptible to brackish water; he says, “that having kept some specimens alive for a week in excellent health, the water being changed once in thirty-six hours, they were one day killed instantly by some water, though brought from the same part of the estuary as usual, having been rendered brackish by much rain having lately fallen.”


Living Cirripedia (1854): A monograph of the sub-class Cirripedia, with figures of all the species. The Balanidæ (or sessile cirripedes); the Verrucidæ, etc. By Charles Darwin. London: Ray Society. 1854.


Concerning the measurements of Balanus specimens with respect to growth.

Letter details

Letter no.
William Thompson
Charles Robert Darwin
Source of text
Living Cirripedia (1854): 272–3
Physical description

Please cite as

Darwin Correspondence Project, “Letter no. 1200A,” accessed on 21 September 2021,

Also published in The Correspondence of Charles Darwin, vol. 4