BERNARD L. STREHLER 393 



essentially immortal and why should another strain be so highly 

 mortal? In searching around for an answer to this, Dr. Crowell 

 and I, in the absence of clear-cut functional differences between 

 the young and old indi\iduals, settled upon an interpretation which 

 incorporates certain of his earlier studies on the response of the 

 colony to restricted feeding ( 9 ) . He noted, as he has mentioned at 

 this meeting, that the individuals who have precedence in such a 

 colony as Campanularia are those that are at the top of an upright, 

 and that the lateral growth of the stolon and the growth of the 

 apical hydranth are not so readily inhibited. This suggested to us 

 that the colony distributes its feeding individuals on the periphery 

 as a sort of umbrella during periods of poor food supply. They 

 thus are in a position to intercept the greatest number of prey — an 

 economically efficient distribution of a limited supply of protoplasm. 

 Since these animals live under conditions of quite variable food 

 supply, we postulated that their senescence is a built-in clock that 

 forces the colony as a whole to evaluate on a very regular schedule 

 the adequacy of its food supply. If the food supply is not adequate, 

 then regeneration in the lower parts of the uprights does not take 

 place. If there is a large amount of food available, then it is economi- 

 cally feasible to regenerate hydranths all up and down the upright 

 and thus to survi\ e. 



In general terms, a paradox is apparent; namely, only those 

 animals which have devised a means of replacing all of their cells 

 on a regular schedule are able to live as individuals for indefinite 

 periods. Part of this process of replacement involves, of necessity, 

 a destruction of cells in a systematic, ordered way ( 28 ) . If it takes 

 place at the boundary of an animal or in a linear, ordered pro- 

 gression of some sort, then the animal, pro\ided it has a germinal 

 core of cells, is capable of continuing to exist indefinitely in a steady 

 state. If it has no capacity for replacing its cells, or its cell parts, but 

 rather accumulates damage, noxious substances or accidental by- 

 products of metabolism, then it will eventually die. Hydra and the 

 anemones are probably immortal because they have devised and 

 maintained evolutionarily a systematic replacement scheme. We and 

 Campanularia are mortal either because it is advantageous to the 

 species' survival to be mortal or, either directly or as a by-product 

 (3, 33) of some other advantageous genetic character, as appears 



