OLD AGE AND SENESCENCE 513 



that the process of senescence is in reality continuous, and, moreover, 

 that it proceeds with a regularly increasing velocity which depends 

 upon the age rather than upon the weight of the animals. The same 

 characteristics are displayed by the curves in Fig. 41 on p. 506. 



There is no particular reason, implied in the nature of an auto- 

 catalytic process, why the mass of its product should diminish. In 

 fact, the station of Equilibrium in a purely autocatalytic process, un- 

 complicated by side reaction, is asymptotically approached and never 

 actually attained, so that the total mass of product, so far from decreas- 

 ing at the apparent close of the reactions, is actuallly increasing at 

 an infinitesimal rate. The process of growth, however, although it is 

 autocatalyzed, does not conform to this particular characteristic of 

 autocatalytic reactions and, a maximum yield of product having been 

 attained, the tissues slowly disintegrate, even gathering speed as time 

 proceeds, until, if no other factor intrudes to terminate life, Senile Atrophy 

 of the tissues leads to irreparable weakening of some essential organ. 



A variety of hypotheses have been advanced to account for the 

 phenomena of senescence which, even if all other dangers of life could 

 be surmounted, would set an inevitable term to existence. A very 

 natural supposition is that proposed by Biitschli, that death is due to 

 the exhaustion of a certain substance the "life ferment" which is 

 gradually used up during life. We cannot disassociate senescent 

 atrophy from senescent death, however, since the death of aged indi- 

 viduals is obviously determined by the progressive atrophy or degener- 

 ation of essential tissues. Now senescent atrophy is attributable to the 

 inability of the tissues to maintain their weight and we must therefore, 

 in the terms of Biitschli's hypothesis, suppose that the gradual con- 

 sumption of an essential substance which was originally contained in 

 the germ-cells and can be manufactured only by them, has deprived 

 the tissues of the power to form new protoplasm. Now this is not the 

 case, for even in old age, injury, or removal of the Products of Growth, 

 will institute vigorous Regeneration and repair. The capacity to grow 

 is not lost or even impaired by age. Thus Osborne and Mendel have 

 maintained rats in an infantile stage of development by depriving them 

 of the single amino-acid Lysine. But upon readmission of lysine to 

 the diet, even at an age exceeding the average normal duration of life 

 (700 days), growth is immediately inaugurated, at the same speed 

 that it would, in the normal course of events, have taken place in a 

 normally fed animal of similar weight and stage of development. 

 The retardation of growth by the accumulation of the products of 

 growth is therefore one of the important factors in determining the 

 inability of the adult tissues to maintain their weight in aged animals. 

 It is not the only factor, however, because in that case, as we have 

 seen, indefinitely prolonged equilibrium and not decline would be the 

 resultant. 



A modification of Butschli's hypothesis is that proposed by Rubner, 

 namely, that the protoplasm of an animal is able to sustain a limited 

 number of molecular transformations and no more. Thus he points 

 33 



