448 SENESCENCE AND REJUVENESCENCE 



If growth is a process of this kind, the rate of growth must 

 increase up to a certain maximum as growth proceeds and then, 

 after maintaining this maximum for a longer or shorter time, must 

 decrease. Both Robertson and Ostwald present a great variety of 

 data from various sources to support their conclusions, and many 

 of Ostwald's curves are very characteristic curves of autocatalysis. 

 Robertson has attempted to show further that in any growth-cycle 

 of an organism, tissue, or organ, the maximum increase in volume or 

 weight in a unit of time occurs when the total growth of the cycle is 

 half completed. From this point of view senescence consists merely 

 in the retardation during the later stages of a growth-cycle of the 

 rate of reaction by the accumulation of the products of reaction. 

 Senile atrophy and death are not a feature of the reaction and must 

 be due to special conditions not directly connected with growth. 

 Rejuvenescence, so far as it occurs, must consist of a reversal of the 

 reaction and consequently a removal of the accumulated products 

 which were responsible for the retardation. 



The foundation upon which this conception of growth rests 

 consists of the observational, statistical data of the increments of 

 growth or of certain growth-components, such as weight, length, 

 water-content, etc., in various organisms. Ostwald has shown that 

 the absolute increments of growth or growth-components show very 

 generally an increase during the earher and a decrease during the 

 later portion of the growth-cycle under consideration and so when 

 graphically presented appear as an S-shaped curve like the curve 

 of autocatalysis. Robertson's conclusions rest on the same basis 

 as Ostwald's. Stated in general terms these results mean simply 

 that up to a certain point, the larger, or heavier, or longer the 

 organism becomes, the greater its absolute increase in a given time, 

 but beyond that point the absolute increase in a given time becomes 

 smaller, although the total size, or weight, or length is still in- 

 creasing. 



The same statistical data may be handled in another way. 

 From the absolute increments we may determine the relative 

 increments of weight, length, etc., that is, the increase in a given 

 period of time in proportion to the weight or length at the beginning 

 of that time. This relative increment may be expressed as a per- 



