310 HAND-BOOK OF PHYSIOLOGY. 



the loss, and so there is growth; the reconstructed part is better than it 

 was before, and so there is development. In the decline of life, on the 

 contrary, the renewal is less than the destruction, and instead of devel- 

 opment there is degeneration. But at no time is there perfect rest or 

 stability. 



It must not be supposed, therefore, that life consists in the capability 

 of resisting decay. Formerly, when but little or nothing was known 

 about the laws which regulate the existence of living beings, it was rea- 

 sonable enough to entertain such an idea; and, indeed, life was thought 

 to be, essentially, a mysterious power counteracting that tendency to 

 decay which is so evident when life has departed. Now, we know that 

 so far from life preventing decomposition, it is absolutely dependent upon 

 it for all its manifestations. 



The reason of this is very evident. Apart from the doctrine of corre- 

 lation of force, it is of course plain that tissues which do work must sooner 

 or later wear out if not constantly supplied with nourishment; and the 

 need of a continual supply of food, on the one hand, and, on the other, 

 the constant excretion of matter which, having evidently discharged 

 what was required of it, was fit only to be cast out, taught this fact very 

 plainly. But although, to a certain extent, the dependence of vital 

 power on supplies of matter from without was recognized and appreciated, 

 the true relation between the demand and supply was not until recently 

 thoroughly grasped. The doctrine of the correlation of vital with other 

 forces was not understood. 



To make this more plain, it will be well to take an instance of trans- 

 formation of force more commonly known and appreciated. In the 

 steam-engine a certain amount of force is exhibited as motion, and the 

 immediate agent in the production of this is steam, which again is the 

 result of a certain expenditure of heat. Thus, heat is in this instance 

 said to be transformed into motion, or, in other language, one molecular 

 mode of motion, heat, is made to express itself by another mechanical 

 mode, ordinary movement. But the heat which produced the vapor is 

 itself the product of the combustion of fuel, or, in other words, it is the 

 correlated expression of another force chemical, namely, that affinity of 

 carbon and hydrogen for oxygen which is satisfied in the act of combus- 

 tion. Again, the production of light .and heat by the burning of coal and 

 wood is only the giving out again of that heat and light of the sun which 

 were used in their production. For, as it need scarcely be said, it is only 

 by means of these solar forces that the leaves of plants can decompose 

 carbonic acid, etc., and thereby provide material for the construction of 

 woody tissue. Thus, coal and wood being products of the expenditure 

 of force, must be taken to represent a certain amount of power; and, 

 according to the law of the correlation of forces, must be capable of yield- 

 ing, in some shape or other, just so much as was exercised in their forma- 



