CHAPTER IV 



AQUATIC ORGANISMS 



IS the testimony of all 

 biology that the water 

 was the original home of 

 life upon the earth. 

 Conditions of living are 

 simpler there than on 

 the land. Food tends 

 to be more uniformly dis- 

 tributed. The perils of 

 evaporation are absent. 

 Water is a denser medi- 

 um than air, and sup- 

 ports the body better, and there is, in the beginning, 

 less need of wood or bone or shell or other supporting 

 structures. Life began in the water, and the simpler 

 forms of both plants and animals are found there still. 

 But not all aquatic forms have remained simple. 

 For when they multiplied and spread and filled all the 

 waters of the earth the struggle for existence wrought 

 diversification and specialization among them, in water 

 as on land. The aquatic population is, therefore, a 

 mixture of forms structurally of high and low degree. 

 All the types of plant and animal organization are 

 represented in it. But they are fitted to conditions so 

 different from those under which terrestrial beings live 

 as to seem like another world of life. 



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