VERTEBRATES — CLARK 285 



part of the vegetation of the sea is microscopic, too small for any ver- 

 tebrates to consume directly. The coastal algae and marine flowering 

 plants such as eelgrass are eaten by manatees, dugongs, some sea 

 turtles such as the green turtle, and a large lizard, but these are quite 

 insignificant when considered in relation to the marine vertebrates 

 as a whole. 



On land the green plants are large — large enough to be consumed 

 directly by vertebrates of any size without the intervention of inter- 

 mediaries. The earliest vertebrates appear to have been almost 

 wholly, perhaps entirely, carnivorous, parallel with the present verte- 

 brates of the sea, and the fresh-water fishes have remained almost 

 wholly carnivorous. In the strictly terrestrial classes, the reptiles, 

 birds, and mammals, plant-feeding types soon appeared. These re- 

 mained relatively few in the reptiles, although some of the vegetarians 

 attained gigantic size, but soon became numerous in the warm-blooded 

 birds and especially the mammals so that today the dominant, and also 

 the largest, terrestrial mammals are plant feeders, and very many of 

 the birds, including all the largest, are vegetarians, at least as adults, 

 though many are throughout their lives. 



As important as the amount and type of available food are the 

 physical conditions that must be met in order to make use of it. For 

 animals as a whole the maintenance of an adequate supply of water 

 within the body is the chief physical requirement. For aquatic ani- 

 mals there is no difficulty here, though certain physiological adjust- 

 ments must be made for the varying amounts of salts in solution. 

 Amphibious animals living either in water or in a humid atmosphere 

 are almost as independent of the necessity for water control as the 

 aquatic. Strictly terrestrial animals, especially those living in dry 

 regions, must develop special mechanisms for the conservation of 

 water within their bodies, replenishing the water usually by inter- 

 mittent drinking. 



The next most important factor affecting vertebrate life is illumi- 

 nation, necessary for certain physiological processes, heat, and vision. 

 In the completely dark depths of the sea the fishes either possess lumi- 

 nous organs or feed on luminous creatures. It is possible that the 

 widespread luminosity among all types of marine animals may be 

 related to the fulfillment of certain light requirements in the absence 

 of light from the sun. 



The oxygen requirements of vertebrates appear to be of minor 

 significance in their effect on evolution. Oxygen occurs abundantly 

 in water as well as in the air and is taken up by means of gills or in- 

 ternal lungs, or through the skin. 



All terrestrial vertebrates must be capable of active locomotion in a 

 medium much lighter than their bodies. This is usually effected by 



