472 PROCESSES INFERRED FROM INDIRECT OBSERVATION 



straight line, nor even a single curvilinear sweep, like the outline of a 

 parabola or of the logarithmic curve which represents the progress of 

 the ordinary type of chemical reaction. On the contrary, our diagram 

 reveals distinct waves or large oscillations in the growth-process and 

 resembles, as a matter of fact, the diagram which may be obtained by 

 superimposing three S-shaped curves upon one another in such a 

 manner that their adjacent extremities merge into one another. 



These waves or oscillations, or "Growth-cycles," as we may term 

 them, are not accidental. They are easily distinguishable from the 

 relatively slight irregularities or fluctuations of growth which every 

 individual child or animal will display more or less frequently during 

 its development. They are distinguishable from such accidental 

 fluctuations because they occur at very nearly the same places in the 

 growth-curve of every normal child, and in the average growth-diagram 

 constructed from the data supplied by a large number of individuals, 

 these large oscillations reveal themselves very distinctly, while the 

 accidental and individual fluctuations cancel out and disappear in the 

 average diagram because, in the long run, if we take a sufficient number 

 and variety of individuals into account, just as many of these accidental 

 fluctuations will be positive (i. e., supernormal in weight) as negative 

 (i. e., subnormal in weight). But the large fluctuations, or Growth- 

 cycles, remain unaffected in magnitude and position, and only appear 

 more definitely in the diagram the greater the number of individuals 

 which we measure or weigh. 



In the Growth of Man there are, in all, three distinguishable growth- 

 cycles which are superimposed upon one another. Each cycle begins 

 with a period of relatively slow growth, followed by a period of very 

 rapid growth, and culminating, with the termination of the cycle, in a 

 period of slackening growth again. In the case of the first two cycles 

 this slackening of growth is followed by the fresh spurt or acceleration 

 due to the succeeding cycle. In the case of the third or adolescent 

 cycle of growth, the period of slackened growth-velocity insensibly 

 merges into the period of relatively stationary development which we 

 recognize as the adult condition. This developmental stasis may be 

 interrupted, however, by the repair incident to the replacement of 

 tissue which has been injured or destroyed, while even in the absence 

 of such Regenerative Growth a vigorous and abnormal growth may 

 occur, the growth, namely, of Malignant Tumors, which we may possibly 

 interpret as constituting the superposition of a fourth, and physio- 

 logically abnormal cycle of growth upon the third and normally final 

 cycle in the development of man. 



Not only the growth of man, but also the growth of every mammal 

 which as yet has been carefully investigated appears to consist of 

 three more or less easily distinguishable cycles of growth. The growth 

 of the Guinea-pig at first appeared to consist of only two difficultly 

 distinguishable cycles, but the investigations of Read have shown that 

 in this mammal the first growth-cycle is actually completed in utero, 



