Experimental Study of Growth 247 
the proportionate increment would remain the same, but 
the absolute increments would become steadily larger. In other 
words, Minot believes 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 .o2. 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 4o days. In 
each of these cases the weight is increased about ro 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 live, 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 life of different animals, 
and whether it can be artificially prolonged by altering the 
conditions of life. 
