ADAPTATIONS 129 
Fia. 62.—Section of bark of live oak tree with acorns placed in it by the Californian 
woodpecker (Melanerpes formicivorus 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 
