306 HABIT AND INTELLIGENCE. [cHAP. 



constant exercise of the eye in the act of seeing have any- 

 such effect. The exercise of the eye, within the limits 

 of what is healthful, does no doubt tend to increase the 

 sensitiveness of the retina ; and I do not say it is im- 

 possible, though I do not admit it as probable, that the 

 muscular arrangements to which the mobility of the ej'-e- 

 balls and eyelids is due may have been produced by the 

 effort to move them, continued through successive genera- 

 tions; and that the expansion of nerves over the retina 

 may have been produced by the constant stimulation of 

 No physi- the nerves themselves. But no such merely physical theory 

 will'^"^^^ will account for the origin of the special complexities of 

 account the visual apparatus. Neither the action of light on the 

 origin of eye, nor the actions of the eye itself, can have the slightest 

 ^^A^h tendency to produce the wondrously complex histological 

 ear ; structure of the retina ; nor to form the transparent 



humours of the eye into lenses ; nor to produce the deposit 

 of black pigment that absorbs the stray rays which would 

 otherwise hinder clear vision ; nor to produce the iris, and 

 endow it with its power of partly closing under a strong 

 light so as to protect the retina, and expanding again when 

 the light is withdrawn ; nor to give the iris its two nervous 

 connexions, of which one has its root in the sympathetic 

 ganglia, and causes expansion, while the other has its root 

 in the brain, and causes contraction.^ 



I have spoken first of the organs of special sense, because 

 it is in their case that the impossibility of formation by 

 any physical action is most obviously evident. But there 

 are other cases where the impossibility is equally demon- 

 strable. Wlien any structure is formed or modified either 

 by the action of external forces, or by the action of the 

 organism itself, this is a case of self-adaptation — not of 

 adaptation only, but of self-adaptation. But, as we have 

 seen, the complexities of the eye and of the ear cannot 

 be so produced; and, quite independently of any special 

 complexity, there are many structures in which the rela- 

 tion of structure to function is such as to make any phy- 

 sical explanation of their formation as totally impossible 

 as in the case of the eye and the ear, though for an entirely 



1 Carpenter's HumRn Physiology, p. fi39. 



