14 GROWTH. 



Growth, or inherent power of increasing in size, although 

 essential to our idea of life, is not a property of living beings 

 only. A crystal of sugar or of common salt, or of any other 

 substance, if placed under appropriate conditions for obtaining 

 fresh material, will grow in a fashion as definitely character- 

 istic and as easily to be foretold as that of a living creature. 

 It is, therefore, necessary to explain the distinctions which exist 

 in this respect between living and lifeless structures ; for the 

 manner of growth in the two cases is widely different. 



First, the growth of a crystal, to use the same example as 

 before, takes place merely by additions to its outside ; the new 

 matter is laid on particle by particle, and layer by layer, and, 

 when once laid on, it remains unchanged. The growth is here 

 said to be superficial. In a living structure, on the other hand, 

 as, for example, a brain or a muscle, where growth occurs, it 

 is by addition of new matter, not to the surface only, but 

 throughout every part of the mass ; the growth is not super- 

 ficial, but interstitial. In the second place, all living structures 

 are subject to constant decay ; and life consists, not as once 

 supposed, in the power of preventing this never-ceasing decay, 

 but rather in making up for the loss attendant on it by never- 

 ceasing repair. Thus, a man's body is not composed of ex- 

 actly the same particles day after day, although to all intents 

 he remains the same individual. Almost every part is changed 

 by degrees; but the change is so gradual, and the renewal of 

 that which is lost so exact, that no difference may be noticed, 

 except at long intervals of time. A lifeless structure, as a 

 crystal, is subject to no such laws ; neither decay nor repair is 

 a necessary condition of its existence. That which is true of 

 structures which never had to do with life is true also with re- 

 spect to those which, though they are formed by living parts, 

 are not themselves alive. Thus, an oyster-shell is formed by 

 the living animal which it incloses, but it is a lifeless as any 

 other mass of saline matter; and in accordance with this cir- 

 cumstance its growth takes place not inter stitially, but layer by 

 layer, and it is not subject to the constant decay and recon- 

 struction which belong to the living. The hair and nails are 

 examples of the same fact. 



Thirdly. In connection with the growth of lifeless masses 

 there is no alteration in composition or properties of the ma- 

 terial which is taken up and added to the previously existing 

 mass. For example, when a crystal of common salt grows on 

 being placed in a fluid which contains the same material, the 

 properties of the salt are not changed by being taken out of the 

 liquid by the crystal and added to its surface in a solid form. 

 But the case is essentially different from this in living beings, 



