THE RATE OF GROWTH 93 



that given so much substance there should be equal 

 speed of growth, and we should expect, possibly, to 

 find that the speed would be more or less constant. 

 I can perhaps illustrate my meaning more clearly, 

 and briefly render it distinct in your minds, by saying 

 that if the rate of growth, as' I conceive it, should re- 

 main constant, it would take an animal at every age 

 just the same length of time to add ten per cent, to 

 its weight ; it would not be a question whether a baby 

 grew an ounce in a certain length of time, and a boy 

 a pound in the same time, for the pound might not be 

 the same percentage of advance to the boy that the 

 ounce would be to the baby. In reality with an ad- 

 vance of an ounce the baby might be growing faster 

 than the older boy with the addition of the pound. 



To determine the rate I devised the following 

 method. 1 Take the weight at a given age, and the 

 weight at the next older age for which there are 

 observations. From these data calculate the average 

 daily increase in weight for the period between the 

 two determinations of the weight, then express the 

 daily increase as a percentage of the weight at 

 the beginning of the period. From a series of de- 

 terminations the daily percentage increments are 

 readily calculated for successive ages. Subsequently 

 the method was modified for the study of the rate of 

 growth in man by substituting the monthly, or even 

 yearly, percentage increments for the daily. This 

 method is not mathematically exact, since the grow- 



'Z. c. 



