Chap. I. SERVICEABLE ASSOCIATED HABITS. 41 



of the body; and it is, I believe, always accompanied 

 by a sudden and forcible inspiration, which is the 

 natural preparation for any violent effort. But when 

 a man or horse starts, his heart beats wildly against 

 his ribs, and here it may be truly said we have an organ 

 which has never been under the control of the will, 

 partaking in the general reflex movements of the body. 

 To this point, however, I shall return in a future 

 chapter. 



The contraction of the iris, when the retina is stimu- 

 lated by a bright light, is another instance of a move- 

 ment, which it appears cannot possibly have been at 

 first voluntarily performed and then fixed by habit; 

 for the iris is not known to be under the conscious 

 control of the will in any animal. In such cases some 

 explanation, quite distinct from habit, will have to be 

 discovered. The radiation of nerve-force from strongly- 

 excited nerve-cells to other connected cells, as in the 

 case of a bright light on the retina causing a sneeze, may 

 perhaps aid us in understanding how some reflex actions 

 originated. A radiation of nerve-force of this kind, if 

 it caused a movement tending to lessen the primary irri- 

 tation, as in the case of the contraction of the iris pre- 

 venting too much light from falling on the retina, might 

 afterwards have been taken advantage of and modified 

 for this special purpose. 



It further deserves notice that reflex actions are in 

 all probability liable to slight variations, as are all 

 corporeal structures and instincts; and any variations 

 which were beneficial and of sufficient importance, would 

 tend to be preserved and inherited. Thus reflex actions, 

 when once gained for one purpose, might afterwards 

 be modified independently of the will or habit, so as to 

 serve for some distinct purpose. Such cases would be 

 parallel with those which, as we have every reason to 

 4 



