284 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1952 



proceeding rapidly in Central and South America and parts of Africa 

 as a result of human activities. 



Provided that they still remain extensive enough to support ver- 

 tebrate life, land waters — lakes and rivers — are far less variable in the 

 conditions necessary for that life than the dry land, especially in 

 tropical and temperate regions. The temperature range is less, sea- 

 sonal variation is much smaller, and diurnal variation negligible. 

 Humidity is constant, although there may be considerable diversity 

 in the amount and character of the dissolved salts and gases. 



Of the biological factors influencing the development and evolu- 

 tion of the vertebrates the most important is food. The food of all 

 terrestrial vertebrates, including those of the land waters, is both ani- 

 mal and vegetable. The vegetable food consists of plants, predomi- 

 nantly the leaves, seeds, fruits, and twigs of green plants, though all 

 other parts of green plants and all other plants are eaten, and in ad- 

 dition plant detritus with its accompanying saprophytes, algae or 

 fungi. 



Of terrestrial animals both on land and in shallow fresh water by 

 far the most abundant both in species and in numbers are the insects, 

 which as a group are mainly feeders on plants or on detritus, though 

 many are predaceous or parasitic. The insects are supplemented by 

 the chiefly carnivorous spiders and their allies and, especially in wa- 

 ter, by the mollusks, which are largely plant or detritus feeders. The 

 other invertebrate groups play a relatively unimportant part on land. 

 Plants, insects, spiders, and mollusks occur wherever there is an 

 adequate supply of water, permanent, temporary, or even irregular, 

 and under all conditions of temperature and humidity except perma- 

 nent frost. All are known since the earliest evidences of a terrestrial 

 fauna. Although the plants and insects have varied greatly from 

 time to time, and at present vary greatly from place to place, all 

 plants and all insects in the present discussion may be considered as 

 single units from the point of view of their availability as food for 

 vertebrates. 



In the sea the extreme importance of diatoms and some allied 

 plants and of the planktonic animals that feed upon them, especially 

 such crustaceans as copepods and mysids and such mollusks as ptero- 

 pods, has long been recognized, as has the importance of detritus, 

 especially in the abysses. But it has not been generally recognized 

 that the vertebrate food chain on land is similarly based upon green 

 plants and arthropods supplemented by mollusks and to a lesser de- 

 gree by other invertebrates, with detritus playing an important, 

 though here a relatively minor, part. 



The essential parallelism between the food chain in the sea and the 

 food chain on land is obscured by the fact that by far the greatest 



