5 8 GROWTH 



available is only S3 per cent per day. This is a much lower value 

 than one might infer from reading such works as the book en- 

 titled The Problem of Age Growth and Death (New York, 

 1908), by C. S. Minot. 



By way of summary it may be said that the self-accelerating 

 phase of growth is made up of several segments. Each segment 

 represents a period of growth at constant percentage rate which 

 may be represented by the equation 



W = Ae kt (4) 



in which W is the weight of the animal at the age t, e is the base 

 of natural logarithms, and k is the relative (or when multiplied 

 by 100 the percentage) rate of growth. The segments of con- 

 stant growth rate are separated by relatively abrupt breaks, the 

 abruptness of the breaks being of the order of metamorphosis in 

 cold-blooded animals. 



V. A Set of Physico -Chemical Postulates on the Dynamics 

 and Kinetics of Growth 



Having considered the detailed features of the growth curve 

 there remains the desire to contemplate the process of growth 

 as a whole, and to formulate the results in a series of postulates 

 or propositions. These propositions cannot be demonstrated, but 

 they seem to be reasonable. 



1. Why does growth occur? Because there is an inherent tend- 

 ency, or force, which causes cells to divide. The detailed mecha- 

 nism of this force is not clear j no more clear, for example, than 

 the mechanism concerned in the force of gravitation which 

 causes a pendulum to vibrate. 



2. Why does growth cease? Because of the finite nature of 

 the universe in which the reproducing cells find themselves. 

 Growth is inhibited by the products of growth, the concentra- 

 tions of which on account of the finite nature of the universe 

 increase with age. Thus bacteria which turn milk sugar into 

 lactic acid, thereby souring the milk, sooner or later cease grow- 



