262 FOUNDERS OF OCEANOGRAPHY 



Another characteristic result of the difference in tempera- 

 ture is that the secretion of carbonate of lime in the form 

 of shells and skeletons proceeds more rapidly in warm than 

 in cold water. The massive shells of molluscs, the vast 

 deposits of carbonate of lime formed by corals and by 

 calcareous seaweeds, are characteristic of the tropics ; 

 whereas in polar seas, while the animals may be large, they 

 are for the most part soft -bodied and destitute of calcareous 

 secretions. The calcareous pelagic Foraminifera are charac- 

 teristic of tropical and sub-tropical plankton, and few, if 

 any, are found in polar waters. Globigerina ooze, a cal- 

 careous deposit, is abundant in warmer seas, while in the 

 colder Antarctic the characteristic deposit is siHceous 

 Diatom ooze. 



It has been recorded that tropical plankton is especially 

 scanty around coral reefs, and the explanation has been 

 given that the abundant animal life of the reef feeding on 

 the microscopic plants of the plankton keeps the amount 

 visible at any one time very low. It may be a case of rapid 

 production and rapid consumption compared with the 

 slower rates of living and of reproducing in colder seas. 

 And in all plankton investigation and estimation it must be 

 borne in mind that the rate of production of successive 

 generations, of which we know very Httle, is probably quite 

 as important as the quantity of developed organisms present 

 at a given moment. This is a matter I shaU have to return 

 to in a later chapter in connection with the fundamental 

 food supply of the ocean as the basis of man's harvest from 

 the sea. 



The adaptation of many planktonic organisms to the 

 special conditions of their life in the surface waters is 

 interesting, and shows two main tendencies — to render 

 them inconspicuous, and to ensure buoyancy. Many, such 

 as Medusae, are gelatinous and transparent, or, if coloured, 

 are of a bluish tint, so as to tone in with their surroundings. 

 In order to maintain their position at any required level. 



