Differentiation. 273 



be conveyed to those brain cells that have been differentiated for seeing 

 only, and the noise would be to them only a flash of light. Any acci- 

 dental blow about the head that gives a violent jar to the optic nerve is 

 falsely reported to the brain cells as flashing lights or stars, as most 

 people know by experience. 



On the other hand, where the differentiations have not gone too far 

 the original habits of the parts may be easily renewed. Thus the dif- 

 ferentiation of the Hydra Polyp from a simple animal has gone far 

 enough to give him a distinct inside and a distinct outside. He con- 

 sists essentially of two skin bags, one inside the other ; the inner one 

 acting as stomach and the outer one being the skin. The differentiation 

 of these two skins is so slight that if the animal be turned wrong side 

 out they will readily exchange offices, the outside skin being well able 

 to digest a dinner if required. This is not only an illustration of the 

 process but is proof of the fact of the differentiation of unlike parts 

 from a homogeneous original. Differentiations proper, as distinguished 

 from diversions, cannot take place except where a stimulus is of a com- 

 pound nature, and where the organ or part to be differentiated is sus- 

 ceptible of division into parts. For example, the function of the di- 

 gestion of food is quite a simple affair with the Hydra. The entrance 

 to his sac-like stomach is surrounded with little arms called tentacles. 

 By the rapid motion of these, when any particle of food strays within 

 their reach, it is whirled into the bag. There it stays till all the nutri- 

 tious matter is absorbed from it, when the remnant is ejected along the 

 road by which it came in. In the Worms great differentiations have 

 taken place. The original bag-like interior has become a tube, and is 

 cut up into several apartments opening from one to another, through 

 which the food is driven backward, comminuted, macerated, digested 

 and absorbed on the way. These several operations are more or less 

 separated, and performed in different parts of the internal tube, and the 

 processes are more completely and perfectly performed. Yet all the 

 functions are but the subdivisions of one function, and all the organs 

 differentiated parts of one organ. 



Differentiation implies division. Division implies previous addition 

 or accretion. Obviously there could be no division of organs in a one- 

 celled animal, but when there are two, one may become unlike the other. 



When the protoplasm of a Moneron has become so differentiated in 

 his descendant of the one- hundred-thousandth generation as to present 

 to us a sense organ, a ganglion and some nerves, we, of course, no 

 .longer call it Moneron. In its original shape the Moneron could not 

 possess these parts, and the differentiations which have supplied them 

 have altered him out of recognition, but it is the same line of life, and 

 he and his descendants are one family, whatever their names or their shapes. 



