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As the Protoleptus assumed a terrestrial life and needed to walk, the rudimentary feet would tend to elongate, and in consequence need the presence of chitine to harden the integument, until the habit of walking becoming fixed, the necessity of a jointed structure arose.
The aquatic Protoleptus, as we may term the ancestor of Leptus, may have had such tubes as these, which acted like the swimming bladder of fishes for lightening the body, as suggested by Gegenbaur. It is known that the swimming bladder of fishes becomes developed into the lungs of air-breathing vertebrates and man himself. The next inquiry is as to the origin of the wings.
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