PEOTOZOA, CCELENTEKA, AXD ECHTNODERMA 103 



of the chalk should resemble so closely the living 

 urchins of the abyss. 



But before leaving the Echinoids reference must 

 be made to two more points that have been made by 

 the illustrious American naturalist. Agassiz points 

 out that all those genera that have the greatest 

 bathymetrical range, extending from the littoral to 

 the abysmal region, are at the same time genera which 

 date back to the Cretaceous period, while those 

 having a somewhat more limited range go back to the 

 tertiaries, and those that extend only slightly beyond 

 the littoral area go back only to the later tertiaries. 



This interesting generalisation brings home to 

 our minds the enormous length of time that it must 

 have taken these animals to migrate from the shallow 

 to the deep sea. In the struggles for existence 

 between marine animals it must always have been 

 the last resort of those unable to compete with the 

 younger generations in shallow water to migrate 

 into the- deeps, 



The scarcity of food, the darkness, and the pressure 

 of these regions can never be so favourable for the 

 support of animals as the conditions of the shores. 

 We can well imagine that a species would take every 

 opportunity that is afforded to return from such 



