Differentiation. 277 



a mass already formed, will be in the direction of the currents. This 

 direction of least resistance is along the most habitual route, because 

 along that route the molecules by way of which the current travels, are 

 kept by it in proper position as to the direction of their poles, their 

 axes remaining in line and not getting crosswise. Such habitual tracks 

 of polar currents finally become differentiated into nerves in which the 

 molecules are always better pathways than are furnished by undifferen- 

 tiated tissue. The selection of the termini of such route governs the 

 location of the line. Those points on the outside edges of the body of 

 the organism, which come most into contact with the environment and 

 are therefore the seats of activity or stimulation, become the termini. 

 In an Amoeba such points are not fixed, but are temporarily established 

 each time a contact is formed. When some point happens to enjoy a 

 greater amount of stimulation than others, the route leading from it 

 across the body of the organism becomes a track of less resistance ; 

 and in an advanced descendant becomes the seat of a nerve fibre, while 

 the terminal point develops a sense organ. When the Amoeba becomes 

 differentiated with organs, he is no longer an Amoeba. The first sense 

 organ is an organ of touch and its location is upon the part exposed and 

 stimulated the most, and which therefore becomes the elementary limb. 

 This sense usually remains in connection with a projecting organ as a 

 flagellum, a tentacle, a ray, as in the star fishes, a tongue, a finger. 

 Locomotion being an advantage to an animal destitute of chlorophyl, 

 it becomes a subject of natural selection and constant improvement. 

 Natural selection, too, would emphasize the facility with which the 

 terminal sense organ would receive the stimulations of contact, and so 

 constantly improve it. An organism is thus built up and made loco- 

 motive and susceptible to stimulation by polar energy. Its constitu- 

 tion is not very well settled at first, and its locomotion is vague and in- 

 definite. But, in the course of generations, the constant influence of 

 this energy finally brings comparative order out of this chaos; most 

 particles in the body become way stations on some line of communica- 

 tion, and have their definite and constant relationships with their neigh- 

 boring particles. If the polar energy were itself uniform and unvary- 

 ing, perhaps perfectly symmetrical organisms would result from its op- 

 erations. But while it is to be regarded as one, yet, like a great river, 

 it is full of eddies and whirlpools, rapids, cataracts, and stretches of 

 placid and smooth-flowing current. An organism, subjected to the 

 formative influence of the great agent, is considerably knocked about 

 before it is finished or, rather, it never is finished, for it continues to 

 be harried, overhauled, and operated upon in various ways, till at last it 

 becomes so clogged up with unassimilable foreign substances as to die 

 of old age, or is sooner done for by some accident. But the general 



