64 THE THEORY OF NATURAL SELECTION 



hind legs of such an animal as a cat are admirably 

 adapted for the purpose of making a spring. In order 

 to arrive at such a structure by the modification of 

 limbs previously adapted only for running, changes 

 must occur in almost all the bones, muscles, and 

 ligaments of the limbs, and these changes must keep 

 pace with one another so that one part may not grow 

 out of proportion with the rest. It is quite impossible 

 to suppose that this can be effected by the natural 

 selection of minute fortuitous variations of the various 

 parts, each occurring independently. But simultane- 

 ously with these changes the fore legs have become 

 modified in a totally opposite direction. They have 

 become straight, firm, and pillar-like for receiving the 

 weight of the body in the downward leap. Compare, 

 says Herbert Spencer, the silence of a cat's leap up 

 on to a table with the thud made by the fore legs as it 

 jumps down upon the floor. 



Modification of the fore legs and of the hind must 

 thus have proceeded in almost exactly opposite direc- 

 tions in the two cases, and in each a great number of 

 parts are separately co-ordinated. For natural selec- 

 tion to have had any effect, all the co-ordinated parts 

 of one pair of legs must have varied in one direction, 

 whilst similar parts in the other pair of legs varied 

 simultaneously in another direction. It is out of the 

 question to suppose that this could have happened 

 simply by chance. 



' What, then, is the only defensible interpretation ? 

 If such modifications of structure produced by modifi- 

 cations of function as we see take place in each indi- 



