THE NEW EVOLUTION 



All three of these groups include both plant-eating 

 and animal-eating forms. Many vertebrates prey on 

 other vertebrates, as for instance hawks, owls, cats 

 and barracudas, very many vertebrates feed on insects, 

 and many, particularly many fishes, some mammals 

 and some birds, are mollusk feeders. Insects, spiders 

 and crustaceans prey largely on each other; many are 

 parasites on vertebrates, as lice, fleas (fig. i8, p. 33), 

 bot-flies and fish-lice (fig. X4, p. 47), the last being 

 curious crustaceans, and some, both on the land and 

 in the sea, are mollusk feeders. Predacious mollusks 

 prey mostly on each other, as for instance oyster 

 drills and whelks, though some feed on small crusta- 

 ceans and a number on other forms of life. The young 

 of most of the fresh- water clams (fig. 52., p. 97) are 

 for a time parasitic on the fishes. 



In view of this, how can the members of these 

 three great phyla be said to be fundamentally non- 

 competitive? 



The structure of a vertebrate with its internal 

 almost invariably jointed skeleton is such that the 

 smallest possible size permitting of effective function- 

 ing is relatively large. Vertebrates range between 

 about three-quarters of an inch in length for the small- 

 est fish to about no feet for the largest whale. But 

 very few vertebrates are less than two inches in their 

 total length. 



The structure of an arthropod with its external 

 jointed skeleton is such that the maximum size per- 

 mitting of effective functioning is small. Arthropods 

 range from about one one-hundredth of an inch to 



