THE LIFE CYCLE 35 



only adding new material, but is at the same time breaking down 

 and eliminating material previously accumulated. The total 

 result as regards size or bulk is simply the difference between the 

 two processes. Some of the substances accumulated within the 

 organism break down less rapidly than others, but even such sub- 

 stances may be more or less completely removed. In the more 

 complex organisms also some of the substances of the substratum 

 are apparently more stable, i.e., inactive chemically, under physio- 

 logical conditions, and the processes of breakdown are therefore 

 less conspicuous as a factor in the total result than in the simpler 

 forms. Under conditions where the breakdown of material over- 

 balances the increment, as for example in starvation, the higher 

 orgaiusms soon die with a considerable portion of their substance 

 intact, but in many of the simpler forms the material previously 

 accumulated serves to a large extent as a source of energy and the 

 organism remains alive and active, but undergoes reduction until 

 it represents only a minute fraction of its original size. Various 

 species of the flatworm Planaria may undergo reduction from 

 a length of twenty-five or thirty millimeters (Fig. i) to a length 

 of three or four milhmeters (Fig. 2) with a corresponding change 

 in other proportions before they die, and many others among 

 the simpler organisms are capable of undergoing great reduc- 

 tion without death. Since the addition of material and 

 increase in size play a much more conspicuous part in the hfe of 

 organisms in nature, and particularly in the higher organisms, 

 than do the reductional processes, it has come about that the term 

 growth has usually been appHed to the incremental, or productive, 

 factors, and the significance of reduction in the life cycle has 

 scarcely been considered. 



Various authors have laid stress upon the permanency of the 

 changes involved in growth. As a matter of fact, these changes 

 are not necessarily permanent, although they are more stable in 

 the higher than in the lower organisms. To say that growth con- 

 sists in permanent increase in volume or change of form is to ignore 

 entirely the phenomena of reduction which are, it is true, most 

 striking in the lower organisms, but which may occur to some 

 extent in all. 



