MUTUAL AID AND COMMUNAL LIFE 427 



and this gregariousness is undoubtedly advantageous to 

 the individuals of the band. The great herds of reindeer 

 in the North, and of the bison or buffalo which once ranged 

 over the Western American plains are examples of a gre- 

 gariousness in which mutual protection from enemies, as 

 wolves, seems to be the principal advantage gained. The 

 bands of wolves which hunted the buffalo show the advantage 

 of mutual help in aggression as well as in protection. Prairie- 

 dogs live in great villages or communities which spread over 

 many acres. By shrill cries they tell each other of the ap- 

 proach of enemies, and they seem to visit each other and to 

 enjoy each other's society a great deal, although that they 

 are thus afforded much actual active help is not apparent. 

 The beavers furnish a well-known and very interesting 

 example of mutual help; they exhibit a communal life, 

 although a simple one. They live in villages or communities, 

 all helping to build the dam across the stream which is 

 necessary to form the marsh or pool in which the nests or 

 houses are built. 



An interesting series of gradations from a strictly solitary 

 through a gregarious to an elaborately specialized communal 

 life is shown by the bees. Although the bumblebee and 

 the honeybee are so much more famihar to us than other 

 bee kinds that the communal life exemplified by them may 

 have come to seem the usual kind of bee hfe, yet as a matter 

 of fact, there are many more kinds of solitary bees than of so- 

 cial ones. The general character of the domestic economy of 

 the solitary bees is well shown by the interesting little green 

 carpenter bee, Ceratina dupla. Each female of this species 

 bores out the pith from five or six inches of an elder branch 

 or raspberry cane, and divides this space into a few cells 

 by means of transverse partitions. In each cell she lays an 

 egg, and puts with it enough food — flower pollen — to last 

 the grub or larva through its hfe. She then waits in an 

 upper cell of the nest until the young bees issue from their 



