70 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- 
