314 HAND-BOOK OF PHYSIOLOGY. 



vital activity can take place. Ths renewal must be something more than 

 replacement, however, as the decay must be more than simple mechanical 

 loss. The idea of life must include both storing up of force, and its 

 transformation in the expenditure. 



Hence we must be careful not to confound the mere preservation of 

 individual form under the circumstances of concurrent waste and repair, 

 with the essential nature of vitality. 



Life, in its simplest form, has been happily expressed by Savory as a 

 state of dynamical equilibrium, since one of its most characteristic fea- 

 tures is continual decay, yet with maintenance for the individual by equally 

 constant repair. Since, then, in the preservation of the equilibrium there 

 is ceaseless change, it is not static equilibrium but dynamical. 



Care must be taken, however, not to accept the term in too strict a 

 sense, and not to confound that which is but a necessary attendant on 

 life with life itself. For, indeed, strictly, there is no preservation of 

 equilibrium during life. Each vital act is an advance toward death. 

 We are accustomed to make use of the terms growth and development in 

 the sense of progress in one direction, and the words decline and decay 

 with an opposite signification, as if, like the ebb of the tide, there were 

 after maturity a reversal of life's current. But, to use an equally old 

 comparison, life is really a journey always in one direction. It is an 

 ascent, more and more gradual as the summit is approached, so gradual 

 that it is impossible to say when development ends and decline begins. 

 But the descent is on the other side. There is no perfect equilibrium, 

 no halting, no turning back. 



The term, therefore, must be used with only a limited signification. 

 There is preservation of the individual, yet, although it may seem a para- 

 dox, not of the same individual. A man at one period of his life may 

 retain not a particle of the matter of which formerly he was composed. 

 The preservation of a living being during growth and development is more 

 comparable, indeed, to that of a nation, than of an individual as the term 

 is popularly understood. The elements of which it is made up fulfil a 

 certain work the traditions of which were handed down from their pre- 

 decessors, and then pass away, leaving the same legacy to those that fol- 

 low them. The individuality is preserved, but, like all things handed 

 down by tradition, its fashion changes, until at last, perhaps, scarce any 

 likeness to the original can be discovered. Or, as it sometimes happens, 

 the alterations by time are so small that we wonder, not at the change, 

 but the. want of it. Yet, in both cases alike, the individuality is pre- 

 served, not by the same individual elements throughout, but by a succes- 

 sion of them. 



Again, concurrent waste and repair do not imply of necessity the exist- 

 ence of life. It is true that living beings are the chief instances of the 

 simultaneous occurrence of these things. But this happens only because 



