THE LIFE CYCLE 37 



characteristic feature of certain other stages of the life cycle. 

 Moreover, there is evidence that water is produced by chemical 

 action in the organism (Babcock, '12), and it is a familiar fact 

 that water is absolutely essential to life. 



But an adequate definition of organic growth must also take 

 account of the fact that it is a process of the living organism. A 

 passive distension of the organism or any part of it by water 

 or other substances, or a passive loss of water, is not properly 

 growth or reduction, because it is not due to the activity of the 

 organism or part. 



If we admit then, first, that organic growth and reduction con- 

 sist essentially in changes in the amount of substance, secondly, 

 that water as well as other substances may be involved in growth, 

 and thirdly, that growth is a process of the living organism, our 

 definitions of growth and reduction must read somewhat as follows: 

 organic growth is an increase, organic reduction a decrease, in the 

 amount of the substance of a living organism or part, resulting 

 directly or indirectly from its specific metabolic activitv^ This 

 definition does not any more than others avoid all difficulties, for 

 sharp lines of distinction do not necessarily exist in natural phe- 

 nomena. Whether we call a certain process growth or not must 

 often depend upon whether we are considering the whole organism 

 or a part; moreover, it is impossible to separate the activity of the 

 organism completely from external factors. 



Although growth in its simplest terms consists in large measure 

 in the synthesis of proteid molecules, it is evident that growth is 

 not always the same chemical process. Under different conditions 

 different proteid molecules may be formed, and very often growth 

 results from the synthesis of various substances other than proteids. 

 Recent investigations seem to indicate that from the point of view 

 of nutrition growth in recovery from starvation is not the same as 

 developmental growth with continuous feeding and that growth in 

 adult life is not the same as growth during youth. 1 Doubtless 

 many other differences will appear as investigation proceeds, but 

 there seems at present to be no adequate reason for limiting the 



1 See the papers by Osborne and Mendel, in the references appended to chap, xi, 

 particularly the recent general discussion of the subject by Mendel ('14). 



