HOW PLANTS AND ANIMALS GROW. 515 



organs of the same individual? We can not attribute them to 

 organization or structure, as so far as we know these are the result 

 of vital activities, and can not, therefore, be the cause. As forci- 

 bly stated by Prof. Huxley in his paper on the cell theory (1852), 

 cells " are no more the producers of the vital phenomena than the 

 shells scattered in orderly lines along the sea-beach are the in- 

 struments by which the gravitative force of the moon acts upon 

 the ocean. Like these, the cells mark only where the vital tides 

 have been and how they have acted." 



It has been suggested that the various factors in nutrition, in- 

 cluding even " structure " and " composition," must be looked upon 

 as modes of motion in accordance with the concepts of modern 

 physics, and from this point of view the body of a man has been 

 mipared to a fountain. " As the figure of the fountain remains 

 the same, though fresh water is continuously rising and falling, so 

 the body seems the same, though fresh food is always replacing 

 the old man, which in turn is always falling back to dust. And 

 the conception which we are urging now is one which carries an 

 analogous idea into the study of all molecular phenomena of the 

 body." 



The pertinence and significance of these physical considera- 

 tions should not, however, lead us to assume that life is but a 

 form of energy. We can not doubt that energy is the motive 

 power in living beings, and that its transformations and activities 

 which are evident in all organic processes are properly considered 

 as modes of motion, but we must discriminate between the motive 

 power that does the work and the directing force which guides it 

 in the lines along which it acts and determines the results pro- 

 duced. We are unable to detect any difference in the potential 

 energy of living and of dead protoplasm, but we recognize an im- 

 mense difference in their significant properties a difference so 

 wide that life can not be defined as a form of energy. 



The manifestations of energy in organic processes are readily 

 perceived, and there are definite standards with which to measure 

 them, but our most delicate means of research throw no light on 

 the purely vital endowments of protoplasm, which not only direct 

 and control its activities, but are transmitted in well-defined char- 

 acters from parent to offspring. There is no life without pre- 

 existing life from which it is derived, and the physical basis 

 through which it acts, or is made manifest, furnishes no satisfac- 

 tory explanation as to its real essence and constitution. 



In discussing the economies of foods and diets, if we keep in 

 mind the significant facts that vital activities direct and deter- 

 mine the transformations of energy and the collocations of matter 

 in plants and animals, in accordance with the nutritive require- 

 ments of every organ and tissue, and that in the higher animals 



