THE VITAL AND PHYSICAL FORCES. 51 



Chemical Affinity, and Mechanical Motion ; and that it may in 

 turn excite all these forces. Thus it appears that we have a 

 right to say that Nerve-force is as completely correlated to 

 Electricity and the other physical forces as they are to each 

 other \ its peculiarity being that it is only manifested through a 

 certain peculiar material structure. But this is not sufficient to 

 separate it, though it keeps it distinct. Until lately we thought 

 that Magnetism could only be manifested by iron ; and it is 

 only through a peculiar combination of metals that Heat can 

 be made to generate Electricity. So it is only through nervous 

 structure that Electricity, etc., can generate Nerve-force. 



From Nerve-force I was led to consider other Vital forces, as 

 those of growth and development, muscular force, the chemical 

 transformations peculiar to living bodies and others ; and was 

 able to reduce them all to one general expression, that of cell- 

 force ; all of these forces being, in my apprehension, but varied 

 expressions of that which is manifested in its simplest form in 

 the development of a cell, the elementary form of all organized 

 structure ; as is indicated by the circumstance that when a cell 

 has taken on one mode or action, it seems incapable of 

 performing any other, each peculiar vital endowment being 

 manifested by a set of cells appropriated to it. 



The question next arises whether these vital forces have any 

 relation to the physical ; and guided by the connection between 

 one of them (Nerve-force) and Electricity, I was led to look at 

 well-known facts in a new point of view, and to conclude that 

 the Vital forces manifested by plants are really the Heat and 

 Light which they receive, transformed, by acting through organic 

 germs, into cell-forces ; so that the Vital forces of plants are 

 correlated to Heat and Light, as is further shown by the fact that 

 certain plants can generate these by their own vital powers. 

 The proof is of the same kind in regard to animals ; but their 

 dependence on external Heat is not so obvious, in consequence 

 of a provision for the internal production of heat existing among 

 many of them. But it is as true of them as of plants, that the 

 activity of their vital operations is in direct proportion to the 

 measure of heat which their bodies receive in one mode or 

 the other. Thus a frog will live slow or fast, just according as 

 the temperature of the air or water is near 32° or 80°. 



