THE RELATION OF LIFE TO OTHER FORCES. 717 



there is no need of death the one being a condition consequent on the 

 other. During the whole life of a living being, on the other hand, 

 there is unceasing change. At different periods of existence the relation 

 between waste and repair is of course different. In early life the addi- 

 tion is greater than 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 development there is degeneration. But at no time is 

 there perfect rest or stability. 



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

 ity 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 de- 

 cay 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 cor- 

 relation 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 dis- 

 charged 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 ap- 

 preciated, the true relation between the demand and supply was not un- 

 til 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 molecu- 

 lar mode of motion, heat, is made to express itself by another me- 

 chanical 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 combustion. 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 



