156 DEVELOPMENT AND HEREDITY. 



edges will be berit first one way and then the other 

 by the resisting water as the paw is moved forward 

 and backward. No matter how small a part of the 

 edge of the paw be thus affected, there will be in 

 that part of the paw a greater wear and waste of 

 tissue, and a correspondingly increasing restoration, 

 which would cause a growth on the edges of the 

 paw, and thus increase the area of its flat surface. 

 It will perhaps be objected to this that the edges of 

 the paws of land animals are also continually sub- 

 jected to the alternations of pressure, and pulling 

 and pushing in all directions,- according to the rough- 

 ness of the ground on which they tread, and yet, 

 nevertheless, the paw does not increase beyond a 

 certain small surface. But notice the difference 

 between the great variety of stresses brought to 

 bear on a land animal in walking, running, jumping, 

 and in striking against various obstacles, and then 

 contrast the simple alternation caused by swimming. 

 We must believe that the shape of the paw of the 

 land animal is the result of all the forces acting upon 

 its development, and in the same way the different 

 shape of the aquatic animal's paw is the result of 

 the different forces acting upon it ; and one cause 

 of difference we may recognise, I think, in the con- 

 ditions I have here mentioned. Any plastic viscous 

 body may be moulded by separate repeated blows. 



