66 THE GEOLOGICAL EVIDENCES 



the study of the development of the cameline 

 race, further shows that very young camels have 

 the additional premolar of Pliauchenia, and that 

 this tooth is shed at an early period, very rarely 

 persisting for any length of time. Similarly, the 

 anterior premolar of the normal camel is found 

 in the young llama, but it is dropped long before 

 the animal attains maturity. 



The investigation of the causes which have 

 wrought these remarkable changes in the animal 

 frame constitutes more properly a part of zoolog- 

 ical or physiological inquiry, and I can but briefly 

 refer to the modifications as resulting primarily 

 from the interaction of mechanical forces. The use 

 and disuse of parts must necessarily have a direct 

 bearing upon their ultimate development, and sim- 

 ilarly the manner of use must largely influence the 

 manner of growth of such parts. These are con- 

 ditions known to us in our every day experience 

 but, owing to the very limited time over which our 

 direct observation extends, we are generally able 

 to detect only minor changes, and miss the grander 



