56 Causes of Emigrations 



compounds without light, but these are exceptional, and most of 

 the foods consumed by animals result from photosynthesis. As light 

 is absorbed rapidly when passing through water, it is apparent that 

 more energy is available on land for food building by plants than 

 in water. In the ocean and in deep lakes photosynthesis is limited to 

 a rather narrow stratum near the surface. Many streams are so 

 turbid that little light can penetrate them. 



Of course a plant requires more than light for the building of 

 organic compounds. Water and chemical compounds which furnish 

 carbon, nitrogen, phosphorus, and other essential elements in avail- 

 able forms are also necessary. Sea water probably contains all 

 chemical elements, but some are present in minute quantities or un- 

 available forms. In the ocean there is more organic food near land. 

 In the open sea there is often a scarcity of available carbon, phos- 

 phorus, and nitrogen compounds, and such materials are continually 

 added to the sea from the land (Johnstone, 1908) . On the land 

 there is plenty of light everywhere, but the distribution of available 

 water and favorable temperatures is often variable or discontinuous, 

 and some essential elements, such as calcium or iodine, may be lack- 

 ing. Most marine animals usually live in their food or have it 

 brought to them in the water that surrounds them. Land animals 

 to a greater extent are obliged to seek for foods which remain in 

 particular places (Clark, 1925) . 



In the past the development of land animals was asscociated with 

 terrestrial plants, which preceded them and furnished basic foods. 

 The spermatophytes are largely confined to land habitats. Only 

 about thirty species of them live even along the margins of the 

 ocean (Buxton, 1926) . Their tissues, and especially their seeds and 

 fruits, furnish water and highly nutritious organic foods in con- 

 centrated forms. They have been an important factor in making 

 life for land animals possible (Berry, 1920) . 



Along the shores of oceans the chief sources of food for marine 



