GROWTH AND SENESCENCE. 1017 



material; other proteins apparently are lacking in it, but maj- 

 yield material adequate for maintenance of the status quo, while 

 still others are inadequate either for maintenance or growth. It 

 will be recalled also that it is claimed that certain specific nitrog- 

 enous substances, the so-called vitamins, must also be furnished 

 in the food in order that normal growth may take place. It Avould 

 seem possible that scientific investigations may eventually enable 

 us to control or even prolong the period of growth through dietary 

 treatment. 



After the period of maturity has been reached the question 

 arises whether the subsequent duration of life can be foretold or 

 formulated in any definite way.. The older naturalists conceived 

 that the duration of mature life might represent a definite multiple 

 of the period of youth. According to Buffon this multiple is 6 to 

 7, according to Flourens it is 5 — that is, the mean duration of life 

 is 5 to 7 times that required for the completion of growth. The 

 data gathered in regard to the average duration of life among 

 different animals have not borne out these suggestions, and Rubner 

 discusses the matter again from the energy standpoint. He 

 estimates the number of calories of food which are required for each 

 kilogram of body weight in the different mammalia from the end 

 of the period of youth to the end of life. For man this jieriod is 

 estimated at sixty years (20 to 80). On this basis he finds that 

 each human kilogram requires 725,770 Calories, while for the other 

 mammalia for which data are accessible an average of only 191,600 

 Calories is required, and the figures in the latter animals are so 

 close as almost to warrant the belief that the same amount is 

 required by each animal in spite of the great variations in the 

 duration of life. It follows from these figures that the human cell 

 is characterized, as compared with that of the other mammalia, 

 by its much gi-eater total capacity for obtaining energy from the 

 foodstuffs. This capacity, the property of assimilation, implies 

 chemical changes and transformations in the living matter, and 

 the fact that eventually this property languishes and expires, that 

 is, the fact that tliere is such a thing as natural or physiological 

 death, means that the somatic protoplasm is capable of effecting 

 only a limited numlier of such transformations. In man a greater 

 number is possible than in the other mammals, and among the 

 latter the number is practically the same, but in the smaller 

 animals, with their more intense metabolism, the series is com- 

 pleted in a shorter time than in the case of the larger animals. 

 Rubner states, moreover, that if a cell, the yeast cell, for example, 

 by artificial means is forced to live without growing and multiply- 

 ing it dies in a very short time. In some way the processes of 

 growth contain the very source of the maintenance of life. The 

 injurious by-products which accompany simple metabolism in the 



