But this is a selected case, and such regular examples of the expected periodicity are rarely found. Often one or more of the various steps are lacking, or even leaves with smaller numbers may be interspersed among those with larger numbers of leaflets. But while the regularity of the periodicity is in some degree diminished by such occurrences, yet the rule always holds good, when taken broadly. It may be expressed by stating that the bases and apices have on the average fewer leaflets on each leaf than the middle parts of the stem and branches, and that the number of leaflets gradually increases from the base toward a maximum, which is reached in organs on the middle or upper part of the axis, and then diminishes from this toward the apex. This periodicity is not limited to the stems and branches, considered singly, but also holds good in a comparison made between the branches of a single stem, in regard to their relative places on that stem. So it is also for the whole plant. The first stems, produced by the subterranean axis, ordinarily show only a low maximum deviation: the next succeeding being [364] more divergent and the last ones returning to less differentiated forms. It is evident that on a given stem the group of deviating leaves will be extended upward and downward, with the increase of the number of these organs. This shows that a stem, or even a plant, promises a higher degree of differentiation if it commences with its aberration earlier. Hence it becomes possible to discern the most promising individuals in early youth, and this conclusion leads to a very easy and reliable method of selection, which may be expressed simply as follows: the seedlings which commence earliest with the production of four- and five-foliolate leaves are the best and should be selected for the continuance of the race. And it is easily seen that this rule agrees with that given above, and which was followed in my pedigree-culture. Furthermore it is seen that there is a complete agreement between the law of periodicity and the responses of the deviations to nourishment and other conditions of life. Weak plants only produce low degrees of deviation, the stronger the individual becomes, the higher it reaches in the scale of differentiation, and the more often it develops leaves with five or more blades. Whether weakness or strength are derived from outer causes, or from the internal [365] succession of the periods of life, is evidently of no consequence, and in this way the law of periodicity may be regarded as a special instance of the more general law of response to external conditions.