ORIGIN OF VARIATIONS. 155 



Another case, parallel to the preceding, is found 

 in the thick growth of skin on the palms of the 

 hands and soles of the feet, and sometimes the 

 elbows, of children who play a great deal on the 

 floor. The wear and tear of the skin on these 

 places stimulates the trophic nerves to activity, and 

 by practice, the restoration soon surpasses the de- 

 struction. From these parts which are exposed to 

 blows and friction, we must distinguish those parts, 

 like the toes in a shoe, which are subject to a con- 

 stant pressure. As is well known, the natural growth 

 of such parts may be greatly hindered and changed, 

 even though the pressure be never strong enough to 

 cause pain. 



Let us look again at the supposed case of a trans- 

 formation of a carnivore from a terrestrial to an 

 aquatic animal. While the absence of the terrestrial 

 stimuli have been reducing the length of the limb 

 and the former vigour of development, its constant 

 use in swimming will tend to produce some new 

 changes. As the foot is moved backwards and for- 

 wards in the water, the parts corresponding to the 

 palm and back of the hand will receive alternately a 

 greater and lesser pressure. This alternation of 

 pressure, however, will make the greatest impression 

 on the edges of the paw. The middle part of the 

 paw is supported by bones, but the softer, pliable 



