ORGANIC RESPONSE 279 



has a flat seed, covered with stiff hooks which cling 

 tenaciously. Another familiar example is the sand- 

 bur (Cenchrus), with its pod covered with spines. 

 The usefulness of bright-colored and succulent 

 fruits to the plant which produces them is supposed 

 to consist in the fact that birds which eat them 

 transport the hard seeds within the fruit to a distance 

 from the parent tree. 



Associations of Animals. Among the most prim- 

 itive animals the individual lives its own life without 

 reference to any other individual, although favorable 

 conditions or abundance of food often brings numbers 

 of the same species together in a given -locality. In 

 many species, however, we frequently find animals 

 associating themselves together in groups, apparently 

 from mere love of company or gregariousness. But 

 the mutual advantages of self-protection afforded 

 by such a relation are not inconsiderable. Many 

 fishes swim in " schools " of thousands of individuals. 

 Deer and antelope feed in herds, and jnany kinds 

 of birds in flocks. Under such circumstances it is 

 very difficult for an enemy to approach, without 

 some member of the company giving the alarm. 

 The same sort of association also serves an aggres- 

 sive purpose. Wolves and dogs hunt in packs, 

 and it is related that the pelicans of the Gulf of 

 Mexico form in a circle, narrowing toward the 

 shore, and by splashing the water, drive in the fish 

 to shallow water, where they are easily captured. 

 The social instincts of such insects as the ants and 



