The Origin of Life 15 



had few rivals. Developing ' plastic 

 armour " to the utmost capacity the 

 Crustacea were early represented by the 

 Trilobites, creatures formed to plough up 

 the sand, the front of their carapaces 

 shooting downwards and forewards after 

 the manner of a snow plough. Con- 

 temporary with the trilobites were cer- 

 tain huge Crustacea (Gigantostraca) of 

 whale-like proportions that rowed them- 

 selves through life with broad paddle- 

 shaped limbs. They held sway until the 

 sharks appeared and then vanished to 

 be remembered only as " fossils." 



Amongst the higher forms of life, the 

 lung-fish are an interesting link in the 

 chain of evolution. To-day these fish, 

 in which the air bladder has been con- 

 verted into a lung, are confined to 

 Tropical Africa, South America and Aus- 

 tralia, but in geological times they 

 enjoyed a cosmopolitan distribution. 

 They have survived because of their 



