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Updated: September 3, 2025
She did not know, nobody could know, how steadily, how silently all this artificial life was draining the veins and blanching the cheek of her daughter Bathsheba, one of the everyday, air-breathing angels without nimbus or aureole who belong to every story which lets us into a few households, as much as the stars and the flowers belong to everybody's verses.
In the cases in which we know of no intermediate or transitional states, we should be extremely cautious in concluding that none can have existed, for the metamorphoses of many organs show what wonderful changes in function are at least possible. For instance, a swim-bladder has apparently been converted into an air-breathing lung.
Secondly, that water possesses oxygene also as a part of its composition, and contains air likewise in its pores; whence the blood of fish receives oxygene from the water, or from the air it contains, by means of their gills, in the same manner as the blood is oxygenated in the lungs of air-breathing animals; it changes its colour at the same time from a dark to a light red in the vessels of their gills, which constitute a pulmonary organ adapted to the medium in which they live.
They were of the Labyrinthodont family, and their association with the fish of the coal, of which so large a proportion are ganoids, reminds us that the living perennibranchiate amphibia of America frequent the same rivers as the ganoid Lepidostei or bony pikes. Slab of sandstone from the coal-measures of Pennsylvania, with footprints of air-breathing reptile and casts of cracks.
To put it more precisely, the lung of the quadrupeds retains this hereditary growth-process of the fishes; for the hydrostatic floating bladder of the latter is the air-filled organ from which the air-breathing organ of the former has been evolved. There is an interesting cenogenetic phenomenon in the formation of the heart of the higher Vertebrates that deserves special notice.
As Huxley long ago remarked, "When Nature undertook to build the skull of a land animal she was too lazy to start on new lines, and simply took the old fish-skull and made it over, for air-breathing purposes." And a clumsy job she made of it!
The ground freezes their tunnels into tubes of iron, the ice seals the surface, past all gnawing out; and yet, amid the quietly flowing water, where snow and wind never penetrate, these warm-blooded, air-breathing muskrats live the winter through, with only the trout and eels for company.
In many it has been converted into the swim-bladder, and in this form serves a useful purpose, but varies very greatly in shape and size. Finally, in a few instances, it retains some measure of its probably original function of air-breathing.
It is evident, therefore, that the Mexican Axolotls, although they have been perennibranchiate for a great number of generations, have not lost the hereditary tendency to the metamorphosis which changes the larvae of Amblystoma elsewhere into an air-breathing terrestrial animal.
If questioned as to the chief properties of these constituents, he would have replied, with equal facility, that these are among the most important elements; that oxygen might almost be said to be the life-giving principle, inasmuch as no air-breathing creature could get along without it for many moments together; and that nitrogen is equally important to the organism, though in a different way, inasmuch as it is not taken up through the lungs.
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