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Darwin Correspondence Project

To George Busk   2 September [1871]1

Sept 2d


I am going to beg a great favour of you. I mentioned to you that Nitsche quotes your conclusion that the avicularium is a modified zooïd.2 Now I want to know whether you infer this from analogous position manner of development & other general reasons, or whether, any gradations now exist between the 2 states. It is in relation to gradation that I make these enquiries, & I just want to say either that no gradation is known to exist between an avicularium & a zooid, or to give a few words explaining the nature of any such gradation. Perhaps you have discussed this subject in your paper. Transact of Micro Soc. 1847 to which you refer in your Catalogu..— e3If so can you lend me for short time a copy?—

I remember Some considerable time ago thinking how odd it was that allied Polyzoa, should possess avicularia or vibracula; & I idly speculated whether the movable lower jaw of the former could have been modified into a vibraculum.4 Therefore I was delighted the other day by reading your discussion. (p 104 Catalogue) from which I infer that these organs (which in these extreme states of development differ most widely appearance), graduate into each other so completely that in Lepralia it is almost doubtful which term ought to be used.— I shd. much like to quote you on this head.5 Does the pointed end of the moveable mandible ever project a little beyond the fixed beak; & in such cases does it assume to some extent the shape of a vibraculum; for this wd be the first step towards conversion? again when there does exist what must be called a true vibraculum, is there ever any trace or rudiment at the base of upper beak for this would form a last stages of conversion? Does what I have here said at all express your belief? I shd be greatly obliged for a little information. I am preparing a new Edit of the Origin, in which I shall insert in [answer] of Mivart’s book— a new chapter, chiefly on the subject of gradation, & first appearance of organs,6

Pray forgive me, if you can, pestering letters | of this nature


The year is established by the relationship between this letter and the letter from George Busk, 10 September 1871.
No earlier letter to Busk from CD about Hinrich Nitsche has been found; CD was last in London in mid-July 1871 (letter from G. H. Darwin, [17 July 1871], n. 1). Nitsche cited Busk 1853 in Nitsche 1871, p. 162. CD cites both Busk and Nitsche in Origin 6th ed., p. 193. For more on avicularia, see the letter to Alexander Agassiz, 1 June [1871], n. 3.
Busk referred to his paper in the Transactions of the Microscopical Society of London (Busk 1847) in his Catalogue of marine Polyzoa in the collection of the British Museum (Busk 1852–4, 2: 104).
For more on vibracula, see the letter to Alexander Agassiz, 1 June [1871], n. 3. The former class Polyzoa included the present phyla Ectoprocta (bryozoans) and Endoprocta.
The quotation in Origin 6th ed., p. 194, is from Busk’s catalogue of Cheilostomata (Busk 1852–4, 2: 104). The genus Lepralia is now considered unrecognisable as no type species was given (for more on synonymy, see Philip Bock, Recent and fossil Bryozoa, (accessed 29 October 2010)). For more on the Cheilostomata, see Hayward and Ryland 1998–9). Long, thin mandibles that extend well beyond the palate of the avicularium are now referred to as setiform and are considered as transitional to the bristles of vibracula.
CD inserted a chapter titled ‘Miscellaneous objections to the theory of natural selection’ into Origin 6th ed., responding to, in particular, the criticisms of St George Jackson Mivart in his Genesis of species (Mivart 1871a).


Busk, George. 1847. Observations on the Shepherd’s Purse coralline of Ellis (Notamia bursaria, Fleming). Transactions of the Microscopical Society of London 2 (1849): 110–21.

Busk, George. 1852–4. Catalogue of marine Polyzoa in the collection of the British Museum. 2 vols. London: printed by order of the Trustees.

Busk, George. 1853. Remarks on the structure and function of the avicularian and vibracular organs of the Polyzoa; and on their value as diagnostic characters in the classification of those creatures. [Read 23 November 1853.] Transactions of the Microscopical Society of London n.s. 2 (1854): 26–33.

Nitsche, Hinrich. 1871. On some interesting points concerning the mode of reproduction of the Bryozoa. Quarterly Journal of Microscopical Science n.s. 11: 155–62.

Origin 6th ed.: The origin of species by means of natural selection, or the preservation of favoured races in the struggle for life. 6th edition, with additions and corrections. By Charles Darwin. London: John Murray. 1872.


Is preparing a new edition of Origin [6th ed. (1872)] and asks GB for information on the gradations between the vibracula and avicularia of the Polyzoa and on what he bases his opinion concerning the homology of the avicularium with the zooid.

Letter details

Letter no.
Charles Robert Darwin
George Busk
Sent from
Source of text
DAR 96: 88
Physical description
Adraft 2pp

Please cite as

Darwin Correspondence Project, “Letter no. 7921,” accessed on 20 November 2019,

Also published in The Correspondence of Charles Darwin, vol. 19