1914] Lafayette B. Mendel 163 



Quite aside from the question as to what initiates growth, the 

 capacity to grow — the " Wachstums fähigkeit," " Wachstumsmög- 

 Hchkeit" as it has variously beert termed — is commonly made a 

 property of the cells of the organism. Whatever may be the ulti- 

 mate cause of growth, the capacity to grow is currently associated 

 with a youthful character of the cells involved. From this stand- 

 point age is an important factor in the possibility of growth. • An 

 embryonic condition of the cells is accordingly most favorable to 

 growth. Minot's third law of age, dealing with the growth func- 

 tion reads : "The rate of growth depends on the degree of senes- 

 cence." From a purely theoretical Standpoint it is quite conceiv- 

 able that ordinary cessation of growth may be due to a natural in- 

 hibitory factor which develops in the course of time rather than 

 because the capacity to grow is lost. Nevertheless the idea that the 

 growth power inevitably declines and is lost with age has found a 

 firm foothold in physiological literature. The current notions may 

 best be set forth by a few quotations. Donaldson has written that 

 "the capacity for undergoing expansive change is transient, and 

 that those cells which fail to react during the proper growing period 

 of an animal have lost their opportunity for ever." This has like- 

 wise repeatedly been urged by Rubner. We do not know in truth, 

 he says, whether Nature demands an absolutely uniform daily 

 growth, or whether remissions in growth are permitted or even 

 advantageous. This alone is certain, that interference with the 

 growth impulse should not last during the entire period for growth ; 

 otherwise the size of the individual will suffer permanent detriment. 

 " Verlorene Körpergrösse in der Jugendzeit kann nach Vollendung 

 der Wachstumsperiode nimmermehr abgeglichen werden." 



In his entertaining book on The Problem of Old Age, Growth 

 and Death, Minot has hinted that some factors other than nutrition 

 and age may enter into the question here discussed. "When the 

 cell is in the young State, it can grow rapidly ; it can multiply f reely ; 

 when it is in the old State it loses those capacities, and its growth 

 and multiplication are correspondingly impeded, and if the Organi- 

 zation is carried to an extreme, the growth and the multiplication 

 of the cell cease altogether. We find, however, that there is some- 

 thing a little more complicated yet to be considered, for it is not 

 merely a question of the capacity of the cells, bat also of the exer- 



