EVALUATION OF CAUSES IN RISE OF AMPHIBIANS 417 



breathing merely from the advantages in oxidation, even if living per- 

 manently in the water, and would not such amphibious types tend in 

 turn to abandon the use of gills and become solely lung breathers? Al- 

 though such progress may appear natural, nevertheless a negative answer 

 is indicated by the small direct use made of air for respiration by the 

 marine pelagic fishes, even where these, notably as in flying fishes, live at 

 the surface, are of active habit and come in frequent contact with the air. 

 It is especially in fresh-water fishes that accessory respiratory organs are 

 employed and their use is directly related to the varying impurity of the 

 waters in which they live. It is thus an adaptation which has been 

 forced repeatedly to a greater or less degree on fishes by the recurrence 

 of an unfavorable environment rather than one assumed within a con- 

 stant environment because of inherent advantages. The same conclusion 

 is reached by an examination of the other phyla of marine organisms. 

 Although many of them live at the surface of the water, or on bottoms 

 so shallow that they can readily reach the surface, they have not acquired 

 supplemental or substitute breathing organs which enable them to live 

 permanently on the air. Many crustaceans can utilize air if their gills are 

 kept moist with sea-water, but the use of air has not materially modified 

 their respiratory apparatus or enabled them to readily abandon the sea. 

 It is probably true that in the Lower Paleozoic insects arose from 

 trilobites and arachnids from merostomes. These advances for land life 

 and atmospheric respiration are, however, like the development of lungs 

 in vertebrates, almost unique in the phylum. The marine forms have 

 not tended toward atmospheric respiration again and again in the way 

 that they have tended to become modified to many forms adapted to life 

 in various Avays. The development of spiracles in insects involved as 

 profound an organic transformation as that of the development of lungs 

 and a four-chambered heart in the rise of air-breathing vertebrates. 

 Among arachnids the organic change is less profound. Both classes may 

 have attained the power of air-breathing from the habitat of the land 

 waters. Their evolutionary histories, so far as known, are not at variance, 

 therefore, with the argument derived from the vertebrates. The rise to 

 air-breathing was not the result of the lure of atmospheric oxygen, but 

 appears to have been an advance by compulsion, owing, it would seem, to 

 oscillations in environment as measured by the varying content of dis- 

 solved oxygen in the land waters. 



THE SWIM-BLADDER ORIGINALLY A RESPIRATORY ORGAN 



The swim-bladder, or air-bladder, as it is variously called, appears to 

 be now chiefly a hydrostatic or equilibrating organ. In some fishes, 



