THE EVOLUTION OF MAN 



primeval fish breathing through gills to the first 

 primeval animal breathing through lungs in 

 other words, to that theoretical mixed group con- 

 taining the principal characteristics of amphib- 

 ians, reptiles and mammals. The fossil remains 

 of those primitive relatives of Ceratodus are con- 

 sidered as parts of creatures belonging to this 

 transition group. /At all events, this Australian* 



)t Ceratodus shows very clearly what the conditions 

 are in which a lung may develop. This is simply 

 the outcome of lack of water, or lack of air in 



/the water. / 



Some might ask how it happened that a new 

 organ could develop just when it was needed 

 most, very much like a fairy table which is set 

 whenever the wish is expressed. The witchery 

 of nature can never come out of the unknown; 

 it has always some logical connection. Indeed, 

 the lungs of Ceratodus on closer study reveal the 

 fact that they are merely a transformation of an 

 organ which all genuine fish carry with them 

 the so-called swimming bladder. This swimming 

 bladder forms a sort of balloon filled with air in 

 the body of the fish, and it serves in the first place 

 as a means of overcoming the weight of the fish 

 in the water. This organ fulfills a useful purpose 

 in rising and sinking, and to this end it was pro- 



102 



