DARWIN, AND THE ORIGIN OF SPECIES 203 



the lives of all who depend upon him for food must be staked 

 upon his ability to creep silently and rapidly to the side of his 

 unsuspecting enemy, or upon his power to elude his pursuers by 

 stealth, or upon his skill in stalking his prey without warning it 

 by any movements which may be detected by its delicate and 

 alert senses, — no one who bears all this in mind can doubt that 

 the ability to arrest the descending foot before it treads upon a 

 thorn or cracks a dry twig, has selective value. But, says Ro- 

 manes, even if we admit that the sole of the foot has selective 

 value, the savage is able to interpret its w5.rnings and to adjust 

 his footsteps intelligently; and while the reflex mechanism acts 

 a little more promptly, the saving of time is too small to have 

 selective value. I am not aware that any one has measured the 

 time required to drive into the foot a thorn which has scratched 

 it, or the time required for cracking a dry twig which the foot 

 has touched ; but Romanes tells us in an other place (" Mind and 

 Motion and Monism," p. 9) that while a nerve-centre requires 

 only about one-twentieth of a second to perform its part in a 

 reflex action, where no thought or consciousness is involved, the 

 operations which are comprised in perceiving a simple sensation, 

 and the volitional act of signalling the perception, cannot be per- 

 formed in less than one-twelfth of a second, which is nearly twice 

 as long as the time required by the lower nerve-centres for the 

 performance of the reflex action. 



It seems probable that, in less than a thirtieth of a second, a 

 scratch from a thorn might have become a disabling injury, 

 which would place a warrior at the mercy of his pursuer; or that 

 the prey which might have preserved the hunter and his family 

 from starvation might be alarmed by the crackling of a twig in 

 as short a time; although the mere saving of time is, no doubt, 

 less valuable than the freedom from care about his footsteps 

 which permits the warrior or the huntsman to fix every sense 

 and every faculty on the matter in hand, and to trust to this 

 reflex mechanism for prompt warning by the mechanical arrest 

 of a dangerous step long enough for conscious intelligence to 

 seek a place to finish it. 



But Romanes says he does not see how we are to explain 

 either the origin or the development of a reflex mechanism by 



