272 Dynamic Theory. 



sound. I am not claiming that the simple animal possesses the faculty of 

 hearing in the sense that a mammal does, but that the aerial vibrations that 

 cause a responsive vibration in the fibres of the mammal ear also cause a 

 molecular vibration through the cell of the simple animal. This vibration 

 may be called by any name we like a jar, a shock, a shudder, a thrill, 

 a stimulus, or a sense. The essential fact is that in the simple undif- 

 ferentiated animal this jar or shock may affect any or all its molecules 

 alike. But the first differentiation of this stimulus is that by which one 

 part of the animal receives the brunt of the shock and the rest of the 

 animal is not shocked at all, or is shocked at second hand by a propa- 

 gation of the stimulus through the intermediate molecules of the animal. 

 Such differentiation as this would begin only in an animal having al- 

 ready a considerable development, and just as soon as the habits of the 

 animal caused one part of the body to be constantly exposed and the 

 rest to be constantly protected from the direct impact of the stimulus. 

 And by the laws of habit the effect would be to increase the sensitive- 

 ness of the exposed part to the direct impact, to decrease the sensitive- 

 ness of the non exposed parts to the direct impact, but to make 

 such parts more sensitive to a vibration conveyed through the inter- 

 mediate molecules, and finally to cause these intermediate molecules to 

 become more supple and pliant in the reception and conveyance of the 

 vibratory stimulus. This process of differentiation would result in an 

 auditory sense organ, an auditory nerve, and a ganglion. 



But more than this. The stimulating vibration could not stop at the 

 ganglion, but would necessarily pass on to the extremities of the body, 

 through the medium of another set of intermediate molecules. Thus 

 the muscles of the extremities would receive their stimulus at third 

 hand, and the path of such stimulus from the ganglion to the extremity 

 would, by habit, become differentiated into an efferent nerve. Now, in 

 each of these cases of supposed differentiation, all the parts are re- 

 moved from the direct effect of the original stimulus or shock except 

 the one sense organ. The sensibility of that is constantly increased, 

 but the sensibility of the other parts for the external stimulus is ren- 

 dered entirely latent and ineffectual, or is perverted into a sensibility 

 for the stimulus at second and third hand. Stated in general terms, 

 the development of a new function in any organ or part has a tendency 

 to obscure and render abortive any old function, and any differentiation 

 by which one part acquires greater efficiency in any function, entails at 

 the same time an equivalent loss of such efficiency on the other parts, 

 by causing in them a discontinuance of the habit of performing such 

 function. The ear of a vertebrate cannot be stimulated by a flash of 

 light, nor the eye by a wave of sound. If the optic nerve could be con- 

 nected with the tympanum of the ear, every sonorous vibration would 



