CHAPTER XIII. 

 THE PHYSIOLOGICAL UNIT. 



LET us call up the physiological unit, and examine 

 it as keenly as we can ; for it yields the explana- 

 tion the only rational explanation which evolution 

 offers of organic structure, and of the multiplied 

 forms of life. 



1. We, first of all, notice that it is extremely small 

 so small that we must add another lens to our 

 microscope, the lens called imagination, to bring it 

 within the range of vision. Extremely minute though 

 it he, it may still contain within it the secret of organic 

 life and of sensation ; for these mysteries are not ques- 

 tions of bulk, but of kind. Let us then study closely 

 this minute exponent of the invisible, who holds the 

 mystic scroll, whereon is written the solution of every 

 problem in the range of organized nature. 



2. We find that it is a highly organized body " a 

 definite, coherent heterogeneity." It has parts, and 

 each part is distinctly differenced from every other, 

 and is definitely related to every other. The soaring 

 eagle is not more really a systematized structure. 

 " Molecules, perhaps exceeding in size and complexity 

 those of protein, as those of protein exceed those of 



