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 cooerdination, 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.

The doll curve reaches its point of highest intensity between eight
and nine,[3] and it is nearly ended at fifteen, although it may