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Updated: June 3, 2025
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.
Now I will tell you of a true mammal, warm-blooded, just as you are, and air-breathing, but which never comes on land. This is the Manatee or Sea Cow. It lives in the warm waters of the Sunny South, coming up from the sea in the big rivers. It is a very large animal, sometimes growing as big as a medium-sized Walrus. The head is round, somewhat like that of a Seal.
In fact, there could have been no life-breathing atmosphere until the earlier land vegetation had whipped out its more destructive elements, and paved the way, in necessary conditions, for the appearance of air-breathing animals.
However, the human lungs, and those of all air-breathing Vertebrates, develop from the same simple vesicular appendage of the head-gut that becomes the floating bladder in the fishes. At first this bladder has no respiratory function, but merely acts as hydrostatic apparatus for the purpose of increasing or lessening the specific gravity of the body.
But it is only when we come to air-breathing creatures that we find the vital characteristic of self-mobility manifested in the highest degree. Flying insects, mammals, birds, travel with velocities far exceeding those attained by any of the lower classes of animals; and so are more strongly contrasted with their inert environments.
The moist climate and low lands that caused the swamps to grow so rapidly have ceased to be, and in their place we appear to have warm, dry air, and higher lands. On these lands of the Triassic time the air-breathing life made very rapid advances. The plants are seen to undergo considerable changes.
It is likely indeed, that, for a short period, while his gills were decrepit, and his lungs infantile, he might have breathed air and water alternately and at will. Now, however, his gills were, for all practical purposes, useless; his lungs, ready but unpractised. The necessity of air-breathing was forced on him at a moment's notice. Small wonder that he commenced by overdoing matters.
Nor can I see any insuperable difficulty in further believing it possible that the membrane-connected fingers and forearm of the galeopithecus might be greatly lengthened by natural selection, and this, as far as the organs of flight are concerned, would convert it into a bat. p. 181. For instance, a swim-bladder has apparently been converted into an air-breathing lung. p. 181. And again:
But the crew of the ship did not give themselves much concern about the ice itself; for it was soon discovered that the floes were covered in many places with seals. "Now you must understand that seals are not fish, but are air-breathing, warm-blooded animals, like horses and cows, and therefore they must always have their heads, or at least their noses, out of water when they breathe.
I shall add a few words, of necessity almost epigrammatic, upon his work and character. He dealt with life, and life with him was not merely this particular air-breathing phase of being, but the spiritual existence which included it like a parenthesis between the two infinities.
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