THE EARTH THE ABODE OF LIFE 



muds. This period, when the ocean seems to have 

 passed its best stage for supporting all forms of life, 

 appears to have been that which is geologically known 

 as the Cambrian. After this period there was a wealth 

 of ferns, of animals able to leave hard traces of them- 

 selves in the fossil records. Before this period there 

 was no physiological need for either skin or shell. But 

 once the skin and shell had been developed, primarily 

 as a defence against the sea- water, their great advantages 

 for purposes of the struggle for life among all forms 

 of animals soon made themselves felt, and so they were 

 retained. 



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