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Darwin Correspondence Project

From T. H. Huxley   17 December 1858

Der. 17th. 1858

My dear Darwin

Von Bärs work is called “Untersuchungen über die Entwickelungsgeschichte der Fische” & was published as a separate brochure in 1835— Appended to it is a special “Anhang” “Ueber die Schwimmblase der Fische” in which Von Bar particularly considers the homologies of the air bladder—1 He considers that the air bladder is developed as a diverticulum of the wall of the anterior part of the alimentary canal— The connecting duct remains open in some Fishes but is obliterated in others— He thinks however that the anterior division of the air bladder, in such Fishes as the Carp, arises independently of the posterior division (or true air bladder) in connexion with the auditory organs, and only opens afterwards into the posterior air bladder

Vogt (“Embryologie des Saumones” in Agassiz. Poissons d’Eau douce)2 gives an account of the development of the air bladder which agrees in essentials with that of Von Baer. Both agree in stating that the young fish fills its air bladder by swallowing air—at the surface of the water and Von Bar even thinks that for a time the young fish respires in this way—

Von Bär winds up thus

“The air bladders of Fishes which form parts of a pneumatic apparatus are of at least two kinds; the one is analogous to the Tympanum & Eustachian tube of the higher animals; the other is indeed an outgrowth of the alimentary canal but has only a general analogy with the lungs of the higher animals; it is rather a body sinus (Rumpf-sinus) whose principal function must be to render the body of the fish specifically lighter—though some influence upon the composition of the blood may be exerted at the same time”3

Structurally the air bladder of Polypterus is altogether a batrachian lung—but it differs from a true lung in its blood not being returned to the heart

Ever yours truly | T H Huxley

CD annotations

1.8 posterior division] ‘posterior’ added pencil
1.10 posterior air bladder] ‘posterior’ added pencil
4.4 the lungs] ‘the’ added pencil
4.4 it is rather] ‘it’ added pencil
Top of first page: ‘Ch. 8’4 brown crayon


Carl Vogt’s Embryologie des Saumones forms the second part of Agassiz and Vogt 1839–42.
CD used the example of the swim-bladder of fishes as an illustration of how, through natural selection, an organ serving one function could be transformed to serve another: in this case an organ of flotation became one of respiration. See Origin, pp. 190 and 452. He did not, however, specifically cite either Karl Ernst von Baer’s or Vogt’s works.
CD’s annotation refers to chapter 8 of Natural selection, ‘Difficulties on the theory of natural selection in relation to passages from form to form’. In Origin this material was incorporated into chapter 6, ‘Difficulties on theory’.


Baer, Karl Ernst von. 1835. Untersuchungen über die Entwickelungsgeschichte der Fische; nebst einem Anhange über die Schwimmblase. Leipzig.

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.

Origin: On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life. By Charles Darwin. London: John Murray. 1859.


K. E. von Baer’s view of the air bladder of fishes.

Letter details

Letter no.
Thomas Henry Huxley
Charles Robert Darwin
Sent from
Source of text
DAR 166: 289
Physical description
4pp †

Please cite as

Darwin Correspondence Project, “Letter no. 2381,” accessed on 21 October 2021,

Also published in The Correspondence of Charles Darwin, vol. 7