82 THE PHYSICAL KINSHIP 



have the habit of coming frequently to the surface 

 and inhaling air. The air-bladder acts as an 

 incipient lung in supplementing respiration by gills. 

 They all live in regions where a dry season regularly 

 converts the watercourses into beds of sand and 

 mud. During the season of drought these strange 

 animals build for themselves a cocoon or nest of 

 mud and leaves. This cocoon is lined with mucus, 

 and provided with a lid through which air is 

 admitted. Here they lie in this capsule through- 

 out the hot southern summer, from August to 

 December, breathing air by means of their lungs 

 and living upon the stored-up fat of their tails, 

 until the return of the wet season, when they 

 again live in the rivers and breathe water in true 

 piscatorial fashion. These capsules have often 

 been carried to Europe, and opened 3,000 miles 

 from their place of construction without harming 

 the life within. 



Here, in these eccentric denizens of the southern 

 world, we find the beginnings of a grand trans- 

 formation — a transformation in both structure and 

 function, a transformation made necessary by the 

 transition from life in the water to life in the air, 

 a transformation which reaches its maturity in the 

 higher air-breathing vertebrates, where the simple 

 air-sac of the fish becomes a pair of lobed and 

 elaborately sacculated lungs, performing almost 

 exclusively the function of respiration, and the 

 gills change into parts of the ears and lower jaw. 



The air-bladder of ordinary fishes, which is used 

 chiefly as a hydrostatic organ to enable the fish 



