4 ORGANIC AND INORGANIC STRUCTURES. 



case, there is simply a laying of particle after particle on 

 the outside, with no change in the interior ; in the other, 

 the increase occurs at every part of the growing structure, 

 the nutrition is interstitial. And wherever, in living 

 forms, the growth is superficial only and there are nu- 

 merous examples of this the growing part performs no 

 vital functions ; and although it is the product of life, it is 

 not itself living. Thus the greater part of the stem of a 

 large tree is to all intents and purposes dead; and its 

 increase, as might be anticipated, takes place after the 

 manner of the increase of dead things in general, the 

 nutrition is superficial. The lignine which is laid down in 

 the interior of the cells, although produced by the agency 

 of life, is not Ijving matter ; and the fashion of its growth, 

 therefore, is no exception to the rule. During the growth 

 of lifeless matter, moreover, there is no decay proceeding 

 at the same time. But in living organisms, as before re- 

 marked, destruction is proceeding always side by side with 

 repair, and must of necessity do so, seeing that the per- 

 sistence of living matter depends not on freedom from 

 decomposition and change, but on the maintenance by 

 repair of that which is being ever destroyed. To the 

 living, death is a necessary condition of life. In a crystal 

 there is no life, and therefore no death. 



Again, the growth of a crystal has no definite limit, if 

 it be placed under favourable conditions. But it is far 

 otherwise with things living. The term and amount of 

 growth, as well as the duration of life in each species, are 

 well defined. 



It is true that among the lowest of living forms, life 

 seems to be dependent entirely on such external conditions 

 as heat and moisture, and to be capable of being stayed or 

 even completely stopped for a time on the withdrawal of 

 these, as it can be started or hastened when again placed 

 under their influence. Thus, many infusoria can be dried 

 for a considerable time, so that no vital action can possibly 



