312 HAND-BOOK OF PHYSIOLOGY. 



arrangement nearly as simple as those of a crystal, and almost or quite as 

 lifeless. After their deposition, moreover, they undergo no further 

 change than that caused by the addition of fresh matter, and hence they 

 are not instances of that ceaseless waste and repair which have been 

 referred to as so characteristic of the higher forms of living tissue. There 

 is, however, no contradiction here of the axiom, that where there is life 

 there is constant change. Those parts of a vegetable organism in which 

 active life is going on are subject, like the tissues of animals, to con- 

 stant destruction and renewal. But, in the more permanent parts, 

 life ceases with deposition and construction. Addition of fresh matter 

 may occur, and so may decay also of that which is already laid down, but 

 the two processes are not related to each other, and not, as in living parts, 

 inter-dependent. Hence the change is not a vital one. 



The acquirement in growth, moreover, of a definite shape in the case 

 of a tree, is no more admirable or mysterious than the production of a 

 crystal. That chloride of sodium should naturally assume the form of 

 a cube is as inexplicable as that an acorn should grow into an oak, or an 

 ovum into a man. When we learn the cause in the one case we shall 

 probably in the other also. 



There is nothing, therefore, in the products of life's more simple 

 forms that need make us start at the notion of their being the products 

 of only a special transformation of ordinary physical force, and we cannot 

 doubt that the growth and development of animals obey the same general 

 laws that govern the formation of plants. The connecting links between 

 them are too numerous for the acceptance of any other supposition. 

 Both kingdoms alike are expressions of vital force, which is itself but a 

 term for a special transformation of ordinary physical force. The mode 

 of the transformation is, indeed, mysterious, but so is that of heat into 

 light, or of either into mechanical motion or chemical affinity. All 

 forms of life are as absolutely dependent on external physical force as a 

 fire is dependent for its continuance on a supply of fuel; and there is 

 as much reason to be certain that vital force is an expression or represen- 

 tation of the physical forces, especially heat and light, as that these are 

 the correlates of some force or other which has acted or is acting on the 

 substances which, as we say, produce them. 



In the tissues of plants, as just said, there is but little change, except 

 such as is produced by additions of fresh matter. That which is once 

 deposited alters but little; or, if the part be transient and easily perishable, 

 the alteration is only or chiefly one produced by the ordinary process of 

 decay. Little or no force is manifested; or, if it be, it is only the heat 

 of the slow oxidation whereby the structure again returns to inorganic 

 shape. There is no special transformation of force to which the term 

 vital can be applied. With construction the chief end of vegetable exist- 

 ence has been attained, and the tissue formed represents a store of force 



