54 



GROWTH 



molecular reaction. According to Robertson, there are at least 

 three such cycles in higher animals and in man, namely, the in- 

 fantile, juvenile, and adolescent cycles. According to Robert- 

 son's theory, each of these cycles, if not complicated by other 

 cycles, is symmetrical about its center as shown in Figure 35. 

 This conception of Robertson's brought forth a large critical 

 literature, the latest of which at the time this book goes to press, 

 is the one by Davenport on "Human Metamorphosis," also the 

 one on the "Human Growth Curve." 



Days| 10 



30 40 50 60 



Figure 36. The change in speed (i.e., in time rate or velocity) of 

 growth of the white rat with advancing age. The circles represent 

 values observed by Donaldson, Dunn, and Watson. 



Figure 36 shows time rates of growth (gains in weight per 

 day) plotted against age for the white rat. The circles repre- 

 sent observed values. Now it is possible, by a process of smooth- 

 ing, to obtain an effect of three, or even four "cycles." As far 

 as fitting Robertson's equation to the cycles is concerned, it can 

 be fitted quite satisfactorily to the adolescent cycle with a maxi- 

 mum gain at about sixty days of age. But the fitting of Robert- 

 son's equation is much more difficult of accomplishment, if at 



