73 o THE POPULAR SCIENCE MONTHLY. 



or influence, by action and reaction with the environment, and between 

 the different parts of the organism ; the second presiding over the ex- 

 change of matter with the environment, by absorption and elimination ; 

 the third presiding over exchanges of matter between different parts 

 of the organism. The first system of functions and organs may be 

 compared to a system of telegraphy, foreign and domestic ; the second 

 to foreign commerce ; the third to an internal carrying-trade. Fol- 

 lowing out any one of these groups in higher animals, say the nervous 

 system, it quickly differentiates again into two sub-systems, viz., cere- 

 bro-spinal and ganglionic, each having its own distinctive functions, 

 which we can not stop to explain. Then the cerebro-spinal again dif- 

 ferentiates into voluntary and reflex systems. All of these have mean- 

 while separated into sensory and motor centers and fibers. Then, 

 taking only the sensory fibers, these again are differentiated into five 

 special senses, each having a wholly different function. Then, finally, 

 taking any one of these, say the sense of touch or feeling, this again 

 is differentiated into many kinds of fibers, each responding to a differ- 

 ent impression, some to heat, others to cold, still others to pressure, etc. 

 "We have taken the nervous system ; but the same differentiation and 

 redifferentiation takes place in all other systems, and is carried to 

 higher and higher points according to the position in the scale of the 

 animal which is to be formed. 



Or, to vary the mode of presentation a little, the cells of the origi- 

 nal aggregate, commencing all alike, immediately begin to take on 

 different forms in order to perform different functions. Some cells 

 take on a certain form and aggregate themselves to form a peculiar 

 tissue which we call muscle, and which does nothing else, can do noth- 

 ing else, than contract under stimulus. Another group of cells take 

 on another peculiar form and aggregate themselves to form another 

 and very different tissue, viz., nervous tissue, which does nothing and 

 can do nothing but carry influence back and forth between the great 

 external world and the little world of consciousness within. Still 

 another group of cells take still another form and aggregate to form 

 still another tissue, viz., the epithelial, whose only function is to absorb 

 nutritive and eliminate waste matters. Thus, by differentiation of 

 form and limitation of function, or division of labor, the different 

 parts of the organism are bound more and more closely together by 

 mutual dependence, and the whole becomes more and more distinctly 

 individuated, and separation of parts becomes more and more a mutila- 

 tion, and finally becomes impossible without death. This process, as 

 already said, reaches its highest point only in the later stages of devel- 

 opment of the highest animals. 



The late of progress is, of course, admitted to be a law of ontog- 

 eny ; but observe here, also, it is true only of the whole and not neces- 

 sarily of all the parts, except from the point of view of the whole. 

 Thus, for example, starting all from a common form or generalized 



