THE STRUGGLE FOR EXISTENCE 289 



cushion scales and kept them in check. Some of these 

 lady-birds ( Vedalia cardinalis) were brought to California 

 and released in a scale-infested orchard. The lady-birds, 

 having plenty of food, thrived and produced many young. 

 Soon the lady-birds were in such numbers that numbers of 

 them could be distributed to other orchards. In two or 

 three years the Vedalias had become so numerous and 

 widely distributed that the cottony cushion scales began to 

 diminish perceptibly, and soon the pest was nearly wiped 

 out. But with the disappearance of the scales came also a 

 disappearance of the lady-birds, and it was then discovered 

 that the Vedalias fed only on cottony cushion scales and 

 could not live where the scales were not. So now, in order 

 to have a stock of Vedalias on hand in California it is neces- 

 sary to keep protected some colonies of the cottony cushion 

 scale to serve as food. Of course, with the disappearance 

 of the predaceous lady-birds the scale began to increase 

 again in various parts of the State, but with the sending of 

 Vedalias to these localities the scale was again crushed. 

 How close is the interdependence of these two species ! 



Similar relations can be traced in every group of ani- 

 mals. When the salmon cease to run in the Sacramento 

 River in California the otter which feeds on them takes, it 

 is said, to robbing the poultry-yards ; and the bear, which 

 also feeds on fish, strikes out for other game, taking fruit 

 or chickens or bee-hives, whatever he may find. 



