FURTHER CLASSIFICATION. 129 



Do not fail to learn that these groupings are but the 

 result of an attempt to represent systematically the like- 

 nesses and differences of animals. The boundaries of the 

 groups were but roughly laid out by the early systematic 

 zoologists. They have had to be changed in the past as 

 the structural and developmental relationships of animals 

 have become better known. They are changing now, and 

 are liable to further change with increasing knowledge in 

 the future. Any proposed system of classification, to be 

 acceptable, must be natural; i.e., founded on existing 

 important relationships. 



NEED. ZOOL. 9 



