Food 91 



autumn, or winter animals. Life apparently originated in the ocean 

 where all materials which are essential for the building of proto- 

 plasm are available in solution. But energy which may be captured 

 and used to activate organisms is much less in submerged marine 

 habitats than in those on land. It is generally true that the most 

 progressive animals have developed where most energy is available. 

 Kennedy (1928) in discussing insects has made it plain that the 

 most specialized and progressive types are active in the habitats and 

 at the times in the daily and annual cycles where and when most 

 radiant energy is available. 



In struggling to leave aquatic habitats some animals have appar- 

 ently been stopped by radiant energy. Direct sunlight kills many 

 marine invertebrates (Huntsmann, 1924) , which therefore must 

 remain submerged or leave their homes in water only at night. On 

 the other hand, some littoral plants and animals which have not 

 been able to leave the salty sea are dependent on the abundant light 

 that is found above the water. For example, the common rdckweed 

 (Fucus) can grow submerged or out of the water but must have 

 a considerable amount of light. "Light is the controlling factor 

 in determining the lower limit of Fucus" (Gail, 1920). 



Food 



The advantages of the land in connection with food manufacture 

 have already been discussed (pp. 56-58) . In this section it is only 

 necessary to mention briefly the limitations that food as an ecological 

 factor may place on aboceanic migrations. 



The food chains, such as that which begins with microscopic 

 floating plants and ends with whales, have been much discussed by 

 oceanographers, and many of them are well known. Most marine 

 animals have a rather stable and ever-present food supply in the 

 water which surrounds them or in the oozy muds which accumulate 

 on the bottom. There are some exceptions to this rule among 

 predaceous marine animals. The black swallower can ingest other 



