The best exercise for the young should thus be more directed to develop the basal powers old to the race than those peculiar to the individual, and it should enforce those psycho-neural and muscular forms which race habit has banded down rather than insist upon those arbitrarily designed to develop our ideas of symmetry regardless of heredity. The best guide to the former is _interest_, zest, and spontaneity. Hereditary moment, really determine, too, the order in which nerve centers come into function. The oldest, racial parts come first, and those which are higher and represent volition come in much later.[2] As Hughlings Jackson has well shown, speech uses most of the same organs as does eating, but those concerned with the former are controlled from a higher level of nerve-cells. By right mastication, deglutition, etc., we are thus developing speech organs. Thus not only the kind but the time of forms and degrees of exercise is best prescribed by heredity. All growth is more or less rhythmic. There are seasons of rapid increment followed by rest and then perhaps succeeded by a period of augmentation, and this may occur several times. Roberts's fifth parliamentary report shows that systematic gymnastics, which, if applied at the right age, produce such immediate and often surprising development of lung capacity, utterly fail with boys of twelve, because this nascent period has not yet come. Donaldson showed that if the eyelid of a young kitten be forced open prematurely at birth and stimulated with light, medullation was premature and imperfect; so, too, if proper exercise is deferred too long, we know that little result is achieved. The sequence in which the maturation of levels, nerve areas, and bundles of fibers develop may be, as Flechsig thinks, causal; or, according to Cajal, energy, originally employed in growth by cell division, later passes to fiber extension and the development of latent cells; or as in young children, the nascent period of finger movements may stimulate that of the thumb which comes later, and the independent movement of the two eyes, their subsequent coördination, and so on to perhaps a third and yet higher level. Thus exercise ought to develop nature's first intention and fulfil the law of nascent periods, or else not only no good but great harm may be done. Hence every determination of these periods is of great practical as well as scientific importance. The following are the chief attempts yet made to fix them, which show the significance of adolescence.