ORIGIN OF VARIATIONS. 1S7 



On such bodies it is not necessary that the pressure 

 or blows on different sides of it be simultaneous. 

 But every blow will have its effect in the end upon 

 the ultimate shape. Now if the various blows and 

 pressure upon a plastic body, like a limb, be limited 

 in their direction and force, though unlimited in 

 number, they must of necessity repeat themselves, 

 and thus tend to force the limb into a certain defi- 

 nite shape, — just as when we roll a piece of soft clay 

 between the palms of the hands, the equal pressure 

 applied successively to all parts of the lump forms it 

 into a sphere, though the pressure is never applied 

 at more than two points at once. When this pres- 

 sure is no longer applied the ball may at once begin 

 to lose its shape. 



It would be almost impossible to analyse fully the 

 action of new forces on a mechanism so complex as 

 the leg and paw of an animal, but they must always 

 be subject to certain fundamental mechanical laws. 

 Thus, the larger the flat surface of the paw becomes, 

 the more powerful must be the muscle, or the slower 

 must be the swimming stroke. Such laws as this 

 must limit and determine development in a variety 

 of ways. For instance, the slower the swimming 

 stroke became, the less would be the alternation of 

 pressure on the edges of the paw, and thus the 

 resulting stimulus of growth would be lessened. It is 



