But this does not quite suffice to explain matters, for experience shows that asymmetrical frequency-curves of variations also occur, even in species with sexual reproduction. As De Vries has recently shown, there are also 'half-Galton curves,' that is, curves which suddenly break off at their highest point. We must conclude from this that the frequency of the different variations depends not only on their degree, but also on the greater or less facility with which they arise from the constitution of the species. This consideration can be readily elucidated with the help of Ammon's exposition, and especially of his graphic representation of the 'playground of variations.' If we think of the indifferent variations occurring in any character of a species as arranged in a series ascending from the smallest to the largest, this line may be regarded as the abscissal-axis, and from it ordinates may be drawn which express the frequency of the variation in question by the differences in their length. If the tips of these ordinates be united, we have the curve of frequency (Fig. 120, _A_), which according to Galton ought to be symmetrical, and in most cases really is so. Ammon calls the space between the smallest and the largest variations the 'variation-playground,' that is, the playground within which all variations are equally advantageous to the species. This is not co-extensive with the variation-area, for there may be more marked deviations below the beginning or above the upper end of the variation-playground, but these, being disadvantageous, fall under the shears of personal selection. The variation-playground may also be called the area of indulgence of variation, because the variations falling within it are spared from the eliminating activity of selection, or the variation-area of survivors, because on an average only those survive whose variations do not overstep the limits of this area. [Illustration: FIG. 120. _A_, symmetrical, and _B_, asymmetrical curve of frequency; after Ammon. _U_, minimal, _O_, maximal limit of individual variation. _U-O_, the 'variation-playground.' _M_, the mean of variation. _H_, the greatest frequency or mode of variation.]