THE RELATION OF LIFE TO OTHER FORCES. 319 



position, to which there is no close likeness in any artificial apparatus, 

 even the most complicated. This is the real distinction, as regards com- 

 position, between a living tissue and an inorganic machine; namely, the 

 difference between the structural arrangement by which force is trans- 

 formed and manifested anew. The fact that one agent for transforming 

 force is made of albumen or the like, and another of zinc or iron, is a 

 great distinction, but not so essential or fundamental a one as the differ- 

 ence in mechanical structure and arrangement. 



In proceeding to consider the difference between what may be called 

 the transformation-products of living tissue, and of an artificial machine,, 

 it will be well to take one of the simple cases first the production of 

 mechanical motion; and especially because it is so common in both. 



In one we can trace the transformation. We know, as a fact, that heat 

 produces expansion (steam), and by constructing an apparatus which pro- 

 vides for the application of the expansive power in opposite directions 

 alternately, or by alternating contraction with expansion, we are able to 

 produce motion so as to subserve an infinite variety of purposes. For 

 the continuance of the motion there must be a constant supply of heat, 

 and therefore of fuel. 



In the production of mechanical motion by the alternate contractions 

 of muscular fibres we cannot trace the transformation of force at all. We 

 know that the constant supply of force is as necessary in this instance as 

 in the other; and that the food which an animal absorbs is as necessary as 

 the fuel in the former case, and is analogous with it in function. In 

 what exact relation, however, the latent force in the food stands to the 

 movement in the fibre, we are at present quite ignorant. That in some 

 way or other, however, the transformation occurs, we may feel quite certain. 

 There is another distinction between the two exhibitions of force which 

 must be noticed. It has been universally believed, almost up to the pres- 

 ent time, that in the production of living force the result is obtained by 

 an exactly corresponding waste of the tissue which produces it; that, for 

 instance, the power of each contraction of a muscle is the exact equiv- 

 alent of the force produced by the more or less complete descent of so 

 much muscular substance to inorganic, or less complex organic shape; in 

 other words, that the immediate fuel which an animal requires for the 

 production of force is derived from its own substance; and that the food 

 taken must first be appropriated by, and enter into, the very formation 

 of living tissue before its latent force can be transformed and manifested 

 as vital power. And here, it might be said, is a great distinction between 

 a living structure and a simply mechanical arrangement such as that 

 which has been used for comparison; the fuel which is analogous to the 

 food of a plant or animal does not, as in the case of the latter,, first form 

 part of the machine which transforms its latent energy into another 

 variety of power. 



