2 GROWTH. 



to any particular kind of living being, may be taken to 

 mean, separation from a parent, with a greater or less 

 power of independent existence as a living being. 



Taken thus, the term, although not defining any par- 

 ticular stage in development, serves well enough for the 

 expression of the fact, to which no exception has yet been 

 proved to exist, that the capacity for life in all living beings 

 is got by inheritance. 



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 characteristic 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 superficial, 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 pre- 

 venting 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 exactly 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 



