When the averages of 7-13 years for elite girls of Mexico City recorded by Ramón (1976) are given as age on the X axis and height for girls (HG) in m on the Y axis, the straight line HG = 0.77 + 0.0560 Age results with DF Adj (degrees of freedom adjusted) r² = 0.9965, Fit SE 0.0068 and 2162 F ratio. This is the necessary and sufficient proof that some adolescent girls pass through a similar growth rate. The tangent b is near 0.0560. These two equations are extremely close and lawlike, although from different systems like longitudinal vs cross-sectional. Figure 4 has HG in m = 0.75 + 0.0584.