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In the cases in which we know of no intermediate or transitional states, we should be very cautious in concluding that none could have existed, for the homologies of many organs and their intermediate states show that wonderful metamorphoses in function are at least possible. For instance, a swim-bladder has apparently been converted into an air-breathing lung.
It is a fact of much significance, that those fishes without a swim-bladder do not seem to be at any disadvantage from its absence, but are able to make their way vertically through the water quite as well as those which possess this organ.
The fish has neither swim-bladder nor sound; and as, moreover, its fins are of the feeblest kind, it is probably on this account that it has been gifted with the power of adhering to other floating bodies, by way of compensation for the above-named deficiencies.
The illustration of the swim-bladder in fishes is a good one, because it shows us clearly the highly important fact that an organ originally constructed for one purpose, namely flotation, may be converted into one for a widely different purpose, namely respiration. The swim-bladder has, also, been worked in as an accessory to the auditory organs of certain fishes.
The Pleuronectidae, while very young and still symmetrical, with their eyes standing on opposite sides of the head, cannot long retain a vertical position, owing to the excessive depth of their bodies, the small size of their lateral fins, and to their being destitute of a swim-bladder. Hence, soon growing tired, they fall to the bottom on one side.
The part which, I think, will have most influence is when he gives whole series of cases, like that of whalebone, in which we cannot explain the gradational steps; but such cases have no weight on my mind if a few fish were extinct, who on earth would have ventured even to conjecture that lung had originated in swim-bladder?
It is known that the fish can greatly lighten the specific gravity of its body by the inflation of its "swim-bladder," which, when perfectly extended, occupies nearly the entire cavity of its abdomen. In addition to this, there is a membrane in the mouth which can be inflated through the gills.
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:
Many interesting stories of fishes can be told or read to the children, and among other things they can learn about the swim-bladder, the large, strong air-sac, which can be compressed or distended at pleasure, making the fish lighter or heavier and enabling it to rise to the surface of the water or sink to the bottom.
A highly interesting example of this exists in the swim-bladder of the fish, which there is good reason to believe is a survival of an ancient structure used for quite a different purpose. It was originally developed, in the opinion of the writer, as an air-breathing organ, in a very ancient semi-amphibious class of fishes, from which the existing bony fishes have descended.
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