. Evolution and its relation to religious thought . ; by no effort of the mind can we imagine that theformer could have come out of the latter by modifica-tion. On the contrary, we have positive proof that itdid not so come. But there is an organ in the fish whichis homologous with the mammalian lung, viz., the air-bladder, or swim-bladder. We know it—1. Because wecan trace in the taxonomic series all the gradations fromthe one to the other. In most fishes the air-bladder is SPECIAL PEOOFS. 83 wholly cut off from the gullet, and only very feebly sup-plied with blood. It is used and can be used
. Evolution and its relation to religious thought . ; by no effort of the mind can we imagine that theformer could have come out of the latter by modifica-tion. On the contrary, we have positive proof that itdid not so come. But there is an organ in the fish whichis homologous with the mammalian lung, viz., the air-bladder, or swim-bladder. We know it—1. Because wecan trace in the taxonomic series all the gradations fromthe one to the other. In most fishes the air-bladder is SPECIAL PEOOFS. 83 wholly cut off from the gullet, and only very feebly sup-plied with blood. It is used and can be used only forflotation. In others, as the gar-pike, the swim-bladderis quite yascular and opens by a tube into the this optening air is gulped down from time totime into the bladder, and again from time to time ex-pelled. In other words, this fish supplements its gill-breathing by an imperfect lung-breathing. We have herethe beginning of a lung. In still other fishes, viz., theDipnoi {lepidosireii and ceratodus, Fig. 2), the air-blad-. FiG. 2.—Lepidosiren. der becomes a more perfect lung—i. e., a very vascularsacculated sac ; and there is not only an opening intothe throat, but also from the throat to the snout. Inother words, we have for the first time nostrils. Thesefishes completely combine gill-breathing w^ith lung-breathing. The step from these to the lowest am-i^hibian reptiles is so small, that some have classed thelepidosiren among amphibians instead of fishes. Thesiredon or axolotl of Kew Mexico, the nee turns or meno-branchus of our Northern lakes, and the siren of ourSouthern swamps, have both gills and lungs, and breatheboth air and water; but the lung is very imperfect, beingsonly a sacculated sac, like the air-bladder of the cerato- 84 EVIDENCES OF THE TRUTH OF EYOLUTIOK dus and le23idosiren. ^No one doubts that the air-breath-ing organ of an amphibian is a true lung; yet we havetraced all the gradations between it and the air-bladderof a fish. We
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