GROWTH AND THE AGING PROCESS 



953 



10 15 20 25 30 35 40 

 DAYS 



Fig. I. Three dimensional diagram to show 

 the changing survival curve properties in an 

 "old" orthoclone. The Fj data show signifi- 

 cant changes and the Fj is essentially non- 

 viable. (Redrawn from Lansing, 1952). 



organism does stand out — this is the inherent limitation of growth of the organism. 

 There is no such characteristic for crystals, ahhough their growth can be Hmited by 

 external factors. 



This definite limitation of the growth process makes us think of another defi- 

 nitely limited characteristic of an organism — the duration of life. The life of man is as 

 subject to this limitation as that of other living beings. "The days of our years are 

 threescore and ten", says the psalmist, "and if by reason of strength they be four- 

 score, still is that strength labor and sorrow, for it is soon cut off and we fly away". 



Just as the amount of growth and the time for growth, at least under given 

 environmental conditions, is limited in each species, so is the life span of a fixed 

 duration. Here, then, we have a. possible clue to the riddle of the life-history. It is 

 almost as though the period of growth and the duration of life could be considered 

 as two factors in an algebraic equation. A third factor is needed to complete the 

 equation. Is this the factor o^ senescence? 



Almost immediately those who have concentrated on the study of the human 

 organism may raise the objection that here growth ceases long before the proces- 



Fig. 2. Comparison of the growth curves of man (dotted line continued into solid line) 

 with curve made from a composite of a number of species of animals (solid line) . The 

 growth curves of many warm-blooded animals investigated are so alike that they can be 

 made to coincide. Man differs from other species in having a long juvenile period (age 3 

 to 13 years). In other species the end of weaning is almost coincidental with the attainment 

 of sexual maturity, so that they have little or no juvenile period. 

 (Redrawn after Brody, 1945). 



Literature p. gyi 



