100 THE STOEY OF TEE EARTH AND MAN. 



to conditions of existence? Two explanations, dif- 

 ferent yet mutually connected, may be suggested. 

 One is that armour was especially useful in the 

 Devonian as a means of defence from the larger pre- 

 daceous species, and the gigantic crustaceans of the 

 period. That this was the case may be inferred from 

 the conditions of existence of some modern ganoids. 

 The common bony pike of Canada {Lepidosteus) , fre- 

 quenting shallow and stagnant waters, seems to be 

 especially exposed to injury from its enemies. Con- 

 sequently, while it is rare to find an ordinary fish 

 showing any traces of wounds, a large proportion of 

 the specimens of the bony pike which I have ex- 

 amined have scars on their scales, indicating injuries 

 which they have experienced, and which possibly, to 

 fishes not so well armed, might have proved fatal. 

 Agam, in the modern Amia, or mud-fish, in the bony 

 pike and Polypterus, there is an extremely large air- 

 bladder, amply supplied with blood-vessels, and even 

 divided into cells or chambers, and communicating 

 with the mouth by an " air-duct.^^ This organ is 

 unquestionably in function a lung, and enables the 

 animal to dispense in some degree with the use of its 

 gills, which of course depend for their supply of vital 

 air on the small quantity of oxygen dissolved in the 

 water. Hence, by the power of partially breathing 

 air, these fishes can live in stagnant and badly aerated 

 waters, v/here other fishes would perish. In the case 

 of the Amia, the grunting noises which it utters, 

 its habit of frequenting the muddy creeks of swamps, 



