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Letter 2381

Huxley, T. H. to Darwin, C. R.

17 Dec 1858

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

Transcription

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— 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) 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”

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

    Footnotes Add

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    f1 2381.f1
    Baer 1835.
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    f2 2381.f2
    Carl Vogt's Embryologie des Saumones forms the second part of Agassiz and Vogt 1839–42.
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    f3 2381.f3
    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.
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    f4 2381.f4
    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’.
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