434 THE RESULTS OF EVOLUTION 



These lesser specific variations have been variously 

 classified. According to one recent view there are five 

 general categories: "(a) food-securing; (6) self-defense; 

 (c) defense of young; (d) rivalry; (e) adjustment to 

 surroundings." 



Food-securing Adaptations. — Teeth are present in 

 the great majority of the higher animals, and are vari- 

 ously adapted for seizing and kilUng prey or biting off 

 vegetable material, or tearing through the protective 

 husks and shells of nuts as well as for crushing or cutting 

 the food preparatory to swallowing. So close is the form 

 of the teeth related to their function that the paleontolo- 

 gist is frequently able not only to determine from a 

 single tooth the relationships of the species but also to 

 form a fairly clear picture of its food and feeding habits. 

 The same principle applies to an almost equal extent in 

 the case of birds. Certain parrots have a relatively long 

 and curved beak adapted for digging up roots. Other 

 species possess a heavier, shorter bill serving for the 

 cracking of nuts and other thick-shelled seeds. The 

 sparrows and other seed-eating birds have heavy conical 

 bills, while those of the warblers and certain other insect- 

 eaters are more slender. 



In many of the lower animals, such as the insects and 

 crabs and related species, the mouth is unprovided with 

 teeth, and certain of the legs have become adapted to 

 form cutting or piercing organs. The " bill " of the mos- 

 quito and the long proboscis of the butterfly are organs 

 of this type. Finally, there are many attached or station- 

 ary animals, such as sponges, corals, numerous species of 

 worms, barnacles, and various clams, which obviously 

 must subsist upon food brought within reach. In the 

 barnacles, numerous legs create currents in the water 

 and grasp the food as it passes by. In most of the re- 

 maining species mentioned, myriads of cilia on the sur- 

 face of the body drive floating nutritive particles into the 

 mouth. 



