OLD AGE AND SENESCENCE 515 



Organisms subdivide, and the daughter-cells which thus arise each 

 contain the protoplasm of the parent-cell which is thus perpetuated 

 indefinitely. No slackening of the process of reproduction occurs 

 unless the supply of nutrients fails. Even in those forms such as the 

 Infusoria, in which conjugation of two cells occasionally occurs, this 

 is not generally essential to the maintenance of the indefinite repro- 

 ducibility of the original protoplasm. In the growth of Cancer only 

 the failure of the tissues of the host to support the parasitic tissue 

 sets a term to its existence. If the tissue be transplanted from time 

 to time into a fresh host it is propagated indefinitely. The failure of 

 nutrients is again the only factor which limits indefinite reproduction. 



The mortality of higher organisms is therefore a consequence of 

 their complexity, and a very probable explanation lies in the sub- 

 division and delegation of functions and powers which renders this 

 complexity possible. There is a very noticeable alteration in the 

 relative proportions of the different types of tissue in the body with 

 advancing age. As Metchnikoff has expressed it: "Old age is char- 

 acterized by a conflict between the finer and more complicated ele- 

 ments and the simple or more primitive elements of the organisms, 

 a conflict that ends to the advantage of the latter. The picture is 

 always the same atrophy of the more highly differentiated elements 

 and their replacement by an overgrowth of connective tissue." In 

 other words Sclerous Tissues acquire a dominance over the Parenchy- 

 matous Tissues which are the most important or perhaps exclusive 

 source of the endogenous catalyzers of growth. 



The senescent decay of the body may, in fact, be attributable to 

 the increasing mass of dependent tissues with which nutrients must 

 be shared and for the production and repair of which catalyzers 

 must be provided. So long as the velocity of the forward reaction of 

 growth predominates sufficiently over that of the backward reaction, 

 the impulse to growth secures the continued accretion of tissue. Part 

 of this tissue assists in the production of catalyzers, but part, that 

 part constituted by the tissues of structural rather than functional 

 significance, merely draws away nutrients from the tissues which 

 produce the endogenous catalyzers. This has the effect, so far as the 

 self-maintaining tissues are concerned, of progressive reduction of the 

 Nutrient-le-vel, or diminution of the value of a a" in the autocatalytic 

 equation. The value of a a," however, determines the ultimate or 

 equilibrium-weight of the animal and as it sinks so must the weight 

 of the animal diminish, the parenchymatous tissues being directly 

 and the sclerous tissues only indirectly affected. Hence the proportion 

 of sclerous to parenchymatous tissues is further enhanced and the 

 process of senescence itself partakes of the autocatalytic character. 

 It should be especially noted in this connection that the cost of 

 production of Sclerous Tissues is not to-be estimated merely in terms 

 of their mass. They are "expensive" tissues to manufacture in com- 

 parison with the parenchymatous tissues. Not only are they poorer in 



