THE RELATION OF LIFE TO OTHER FORCES. V19 



specially concerned, resembles somewhat the increase of unorganized 

 matter. The accidental difference of its being in one case superficial 

 and in the other interstitial, is but little marked in the process as it oc- 

 curs in the more permanent parts of vegetable tissues. The layers of 

 lignine are in their 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 re- 

 pair which have been referred to as so characteristic of the higher forms 

 living tissue. There is, however, no contradiction here of the axiom, 

 that where there is life there is constant change. Those parts of a vege- 

 table organism in which active life is going on are subject, like the tis- 

 sues of animals, to constant destruction and renewal. But, in the more 

 permanent parts, life ceases with deposition and construction. Addi- 

 tion 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 can- 

 not 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 supposi- 

 iion. Both kingdoms alike are expressions of vital force, which is itself 

 ftut 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 representa- 

 tion 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 sub- 

 .stances which, as we say, produce them. 



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

 cept 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 



