470 THE MECHANICAL FUNCTIONS. 



the mode of executing some other function. 

 An express provision for giving buoyancy has 

 been made in the construction of the shell of a 

 species of tortoise inhabiting the coasts of the 

 Scychelle Islands. The under surface of the 

 shell, instead of being gently concave, as in land 

 tortoises, has a deep circular concavity in the 

 centre, above four inches in depth, which, when 

 the animal goes into the water, retains a large 

 volume of air, buoying up the whole mass while 

 it remains in that element.* The greater size of 

 turtles, when compared with tortoises, is a far- 

 ther instance of the superior facility with which 

 organic growth proceeds in aquatic than in land 

 animals formed on the same model of construc- 

 tion. 



* Home's Lectures, vi. 37. 



