Experimental Study of Growth 247 



the proportionate increment would remain the same, but 

 the absolute increments would become steadily larger. In other 

 words, Minot beheves that the rate of growth at any particular 

 time must be measured in terms of the weight of the body at that 

 time. He finds when measured in this way that, "first, the rate 

 of growth diminishes almost uninterruptedly from the time on- 

 ward when the animal recovers from the post-natal loss of 

 weight; second, that diminution is rapid at first but slower 

 afterward." 



Minot shows for guinea pigs that after the post-natal retarda- 

 tion, the increments of growth increase from the second to the 

 fifth day at about 5.5 per cent in terms of body weight. From 

 this time onward the increment decreases very rapidly at first, 

 and then more slowly. Thus from the fortieth to the fiftieth day 

 it is 1.2; from the one hundred and ninetieth to the two hun- 

 dredth day it is .2 ; and after 22 to 24 months about .02. Con- 

 versely, if equivalent amounts of growth are taken and compared 

 with the time required to acquire them, we find that to increase 

 in weight from 200 to 222 grams takes 4.9 days ; to increase from 

 470 to 523 takes 20 days; from 697 to 766 takes 40 days. In 

 each of these cases the weight is increased about 10 per cent. 

 The results show a progressive loss which after a time, as in man, 

 may come practically to a standstill. In one sense, therefore, 

 the animal may be said to begin to grow old almost from the 

 moment that it is born. This, however, is not what is usually 

 meant by growing old, although the phrase has been employed in 

 various ways. Generally we refer to the decline that occurs after 

 growth has come to an end ; but there may be a considerable 

 interval in an' animal's life after it has ceased to grow larger, 

 during which time it has not begun to "grow old." On the 

 other hand, animals that continue to grow, however slowly, as 

 long as they Hve, can scarcely be said ever to grow old, 

 although they may be very old in point of time. This brings 

 us to the question of the length of Hfe of different animals, 

 and whether it can be artificially prolonged by altering the 

 conditions of life. 



