416 J. BARBELL- — INFLUENCE OF CLIMATES ON VERTEBRATES 



mm) are almost unique exceptions. The numerous examples of fishes, 

 mostly tropical, which can remain active out of water are almost all 

 fishes of fresh -water origin. Further, it is among the fresh- water fishes 

 that the swim-bladder functions most commonly as a respiratory organ. 

 The rarity of the passage of crustaceous, gasteropods, and vertebrates 

 from a truly marine to a truly terrestrial mode of life through the appar- 

 ently open path of the tidal zone adds to the evidence that an unused 

 food supply could not alone operate as a cause sufficient to induce this 

 change, for so far as this factor is concerned the river faunas would have 

 no clear advantage over those of the tidal zone. 



LURE OF ATMOSPHERIC OXYGEN, AN INEFFICIENT CAUSE 



Viewed from the standpoint of animal activity rather than from that 

 of body building, oxygen, taken into the blood through gills or lungs, is a 

 food as essential as the fluid and solid substances, absorbed through the 

 walls of the intestinal canal. The higher animals, as illustrated by birds 

 and mammals, demonstrate by their warm-bloodedness and sustained 

 activity the great advantages resulting from breathing oxygen directly 

 from the air rather than indirectly abstracting it from solution in water. 

 Further evidence in this direction is supplied by the many instances in 

 which the descendants of land vertebrates have reverted to a life in the 

 Avater. In no group, whether inhabiting fresh Avaters or marine, has there 

 been a reversion to the use of gills or the equivalence of gills after the 

 use of lungs had been definitely acquired. This is in contrast to the 

 rapid reversion to fishlike forms, and in spite of the limitations to mode 

 of life imposed by the necessity of returning to the Avater surface at fre- 

 quent intervals to breathe. 



Still more convincing in this regard is the independent acquisition in 

 numerous instances among fishes of various devices for using air directly 

 as an accessory means of respiration. It is among the fresh-water fishes, 

 and more especially those of the tropics, that such adaptations are fre- 

 quent and conspicuous. Examples may be cited from among teleosts of 

 numerous species, belonging to a number of genera and several families, 

 in which air is SAvallowed and passed along the alimentary canal. In 

 other fishes complex surfaces, highly supplied with blood vessels, exist in 

 chambers above the gills, in some instances as bony outgroAvths from cer- 

 tain gill arches, in others as thickened and puckered surfaces of a super- 

 branchial cavity. The commonest device, however, and that which is 

 developed in the nearest allies to the amphibians, is the use of an air- 

 bladder, Avhich may be single or paired, as an accessory respiratory organ. 



The question, then, naturally arises : Do not fishes tend to become air- 



