Differentiation. 271 



gan badly adapted for its performance, in order that there may be a dif- 

 ferentiation of the organ in favor of such function. 



Applying this deduction to the case of the Moneron, it follows that in 

 this animal every function is performed in some sort of waj T , good, bad or 

 indifferent, that is performed by its immediate progeny or descendants. 

 The same can be said of these descendants in reference to their descendants. 

 And the same is to be said of every organ which becomes in one gen- 

 eration different from its predecessor of the generation before, or which 

 undergoes modification or diversion during its own individual existence. 

 But differentiation is not only the modification of organs from an adapt- 

 ability for one function to an adaptability for another. All sorts of 

 stimuli that influence organisms are of a compound or composite nature. 

 Under the term pressure, we include pressures of many degrees of in- 

 tensity ; and touch is used to express the contact of many different sorts 

 of objects. The impact of aerial vibrations that gives us the sensation 

 of sound, may be infinitely varied in force and quality. The ethereal 

 vibrations that represent the radiant energy or sunlight are likewise in 

 great variety of length and amplitude. So the process of alimentation is 

 a compound one involving a number of steps. Now a differentiation by 

 which one part of an animal is habitually subjected to one sort of stim- 

 ulus, and thus erected into an organ, implies that before such differen- 

 tation. the part out of which the organ was constructed had been under 

 the influence of that stimulus, and had responded or reacted against such 

 stimulus in the performance of a function. Thus the first organ of 

 hearing must have arisen on a part of the body which had previously 

 become more sensitive to the vibrations of air than any other part, pre- 

 sumably a part more exposed to the impact of such vibrations, as the 

 foot in the case of some of the Mollusks that expose no other part of 

 the body, but generally the head, which in most animals is the most ex- 

 posed part. The organ thus differentiated from an already sensitive 

 part of the body would be itself subject to still further differentiation, 

 since each tone of sound that strikes it would produce a different effect 

 upon it. Some vibrations having a more acute effect on some fibres of 

 the organ than on others, such fibres would gradually become the pro- 

 perty of such vibrations, and finally be sensitive to such tones only. 

 This as we shall see is actually the case in the vertebrate ear ; the fibres 

 of corti, to the number of some thousands, forming a progressive series 

 of strings, each capable of being stimulated only by its sympathetic 

 note. On the other hand, it could not be assumed that any one part of 

 the body of a simple animal could in the first instance become respon- 

 sive to the impact of sonorous waves, more than any other part. Origi- 

 nally all parts must have been alike, therefore all must have been sensitive 

 to those vibrations which in the higher organisms create the sensation of 



