ADAPTATIONS 



129 



FIG. 62. Section of bark of live oak tree with acorns placed in it by the Californian 

 woodpecker (Jfelanerpes formicirorus bairdii). From photograph, Stanford 

 University, California. 



tures useless in procuring food but often of great effective- 

 ness as weapons of defense. To the category of structures 

 useful for self-defense belong the many peculiarities of col- 

 oration known as "recognition marks." These are marks, 

 10 



