132 INTRODUCTION TO ZOOLOGY. 



case, the remedy is to import a check if that is possible. 

 This has been done in the case of the fluted scale, 

 Icerya purchasi. Certain lady bugs which preyed 

 upon it in its original home, Australia, were brought 

 to the United States, and the fluted scale no longer 

 threatens the welfare of the orange growing industry. 



This tendency of an animal to increase faster than 

 its food can increase is called The Pressure upon 

 Subsistence. Since there cannot be food enough for 

 all, each animal maybe said to struggle to get enough 

 to eat. The struggle, however, is not merely a strug- 

 gle for food. It is a struggle to survive, notwith- 

 standing all the checks upon multiplication of its num- 

 bers. This whole struggle, — struggle for food, struggle 

 to escape from enemies, struggle to live through dis- 

 eases, is called The Struggle for Existence. 



It seems very evident that not all will survive. 

 Those that do survive are likely to be the individuals 

 that in some way are best adapted to overcome the 

 various difficulties they encounter : to escape from 

 their enemies, by flight, light, or concealment; to 

 recover from disease or to escape contagion ; to 

 gather more food or to live upon less. The fact that 

 those who do survive are best adapted to the conditions 

 under which tliey live, supports wliat is known as the 

 theory of The Survival of the Fittest. 



Additional Facts About Vertebrates. 



In all the vertebrates that we have studied we have 

 found an internal, bony skeleton. There are some 

 animals which must be classed with the vertebrates, 

 in which the skeleton is not bony. In such animals 

 the skeleton is cartilaginous, and in the amphioxus, 

 which lias been called the lowest known vertebrate, 

 the only indication of a skeleton is a cartilaginous rod 

 along the back which is called the notochord. The no- 



