AND ITS INHABITANTS 121 



proved the primal function of air breathing. This function 

 is inconceivable as a prime requisite to marine life, and could 

 only have arisen in terrestrial waters beyond the limits of the 

 tidal zone, 4 where, due to increasing aridity of climate, the 

 waters became reduced to stagnant pools which had neither 

 sufficient flowage nor wave action to renew the supply of 

 respirable air exhausted by the contained life. Such conditions 

 prevail today in Australia, where the recurrent droughts cause 

 the rivers to dwindle until only isolated pools are left. Within 

 certain of these rivers, the Mary, Burnett, and Dawson, dwells 

 an ancient relic form, the lung-fish or dipnoan Neoceratodus, 

 which in times of habitat stagnation rises to the surface, gulps 



FIG. 18. Lung-fish, Neoceratodus, breathing. After Dean. 



air (Fig. 18) into its lung, a highly vascular outgrowth or 

 diverticulum of the alimentary canal, and thus aerates the 

 contained blood. Other dipnoans also exist, as the African 

 lung-fish, Protopterus, of the Nile, which, during times of 

 drought, forms for itself a cocoon-like case of slime-hardened 



4 See Barrell, J., Influence of Silurian-Devonian climates on the rise of air- 

 breathing vertebrates. Bull. Geol. Soc. America, vol. 27, 1916, pp. 387-436. 



