Int.] 



ON NORMAL GROWTH 



289 



the case of certain selected, typical organisms. This may be most 

 quickly done by the use of curves whose abscissae represent 

 time intervals and whose ordinates represent size or weight. 

 Figures 80, 81, and 82 are such curves. In all cases excepting 

 that of guinea pig (in which the curve represents the growth 

 of only a comparatively late developmental period, namely, 

 from birth onward) the curves exhibit one characteristic shape. 



The absolute increments are not, however, shown directly by 

 these curves. To obtain them one must transform the curves 

 into others in which the 

 successive ordinates shall 

 represent the absolute incre- 

 ment of weight over the 

 last preceding. Under these 

 circumstances the absolute 

 increments rapidly reach a 

 maximum from which they 

 decline to zero.* 



Why does the growth de- 

 cline to zero ? The theorj^ 

 has been suggested | that 

 there is a " certain impulse 

 given at the time of im- 

 pregnation which gradually 

 fades out, so that from 

 the beginning of the new 

 growth there occurs a 

 diminution in the rate of 

 growth." The facts of 



g.* 





5 10 M0&. 



Fig. 81. — The continuous line (a) represents 

 tlie weights in fractions of a kilogramme 

 attained by guinea pigs from birth until 

 12 months old. The broken line (V) rep- 

 resents the daily percentage increments 

 (%'s at the right) of the same guinea pigs 

 up to 7 months. After Minot ('91). 



* Another method of representing curves of growth has been proposed by 

 Professor Minot ('91, p. 148), who argues that for a given period the rate of 

 growth should be expressed as the fraction of weight added during that period ; 

 for, he says, "the increase in weight depends on two factors: first, upon the 

 amount of body substance, or, in other words, of growing material present at a 

 given time ; second, upon the rapidity with which that amount increases itself." 

 Such a curve of percentage daily increments is given for the guinea pig in Fig. 

 81. Since, however, the greater part of the "body substance " at its period of 

 greatest growth is not " growing material," as assumed, but water, the peculiar 

 value of the curve of percentage increments is doubtful. 



t By Minot, '91, p. 151. 



