306 PHYSIOLOGICAL DEVELOPMENT. 



apparatus is the epidermic growth which, under the form of 

 a hair, protrudes from the sac. And that the greatest 

 sensitiveness may be obtained, the external action must be 

 exaggerated or multiplied by the apparatus which conveys it 

 to the recipient nerve ; as in the case of the vibrissa, it is by 

 the development of a hair into an elastic lever, that trans- 

 forms the slight force acting through considerable space on 

 its exposed end, into a greater force acting through a smaller 

 space at its rooted end. Similarly with the organs of the 

 higher senses. In a rudimentary e}^e, we have but a slight 

 peripheral expansion of a nerve to take cognizance of the 

 impression ; and to concentrate the impression upon it, there 

 is nothing beyond a thickening of the epidermis into a lens- 

 shape. But the developed eye shows us a termination of the 

 nerve greatly expanded and divided to receive the external 

 stimulus. It shows us an introverted portion of the integu- 

 ment containing the apparatus by which the external stimulus 

 is conveyed to the recipient nerve. The structure developed 

 in this sac not oaly conveys the stimulus, but also, like its 

 homologue, concentrates it; and in the one case as in the 

 other, the structure which does this is an epidermic growth 

 from the bottom of the sac. Even with the ear it is the same. 

 Again we have an introverted portion of the integument, on 

 the walls of which the nerve is distributed. The otolithes 

 contained in the sac thus formed, are bodies which are set in 

 motion by the vibrations of the surrounding medium, and 

 convey these vibrations in an exaggerated form to the nerves. 

 And though it is not alleged that these otolithes are 

 developed from the epidermic lining of the chamber, yet as, 

 if not so developed, they are concretions from the contents of 

 an epidermic sac, they must still be regarded as epidermic 

 products. 



Whether these differentiations are due wholly to indirect 

 equilibration, or whether direct equilibration has had a share 

 in working them, are questions that must be left open. 

 Possibly a short hair so placed on a mammal's face as to be 



