44 THE ORIGIN OF MAN 



to the gills. Stagnation of the water and a loss 

 of free oxygen would bring the. fishes to the sur- 

 face to gulp down the air, and such pouches if 

 supplied with blood-vessels would serve to aid in 

 aerating the blood. Animals originating and liv- 

 ing in permanent bodies of water and especially 

 in the ocean have no need to breathe the air and 

 therefore an alteration could have taken place 

 only where the water failed the animals. It is 

 thought that under the stimulus of climatic 

 changes the gill-breathing fishes first adapted 

 themselves to burrowing in the sand and thus 

 protected in water and mud holes there was for 

 a time moisture to pass over the gills. After 

 many failures in their efforts to gulp the air into 

 the pharynx, the lung-fishes gradually developed 

 and perfected, their first appearance being in the 

 late Silurian time. 



In Waverlian times (the first division of the 

 Mississippian period) life was most diversified, 

 and the sea was filled with an abundance of crin- 



