eir
typical form of the organs laid down in the first period; and this is
brought about by the exercise of the specific functions of the organs.
This period adds the finishing touches to the finer functional
differentiation of the organs, and so brings to pass the 'finer
functional harmony' of all organs with the whole. The formative activity
displayed during this period depends upon the circumstance that the
functional stimulus, or rather the exercise by the organs of their
specific functions, is accompanied by a subsidiary formative activity,
which acts partly by producing new form and partly by maintaining that
which is already formed.... Between the two periods lies presumably a
transition period, an intermediary stage of varying duration in the
different organs, in which both classes of causes are concerned in the
further building-up of the already formed, those of the first period in
gradually decreasing measure, those of the second in an increasing
degree" (pp. 94-6, 1905).

In the first period the organ forms or determines the function, in the
second period the function forms the organ, or at least completes its
differentiation. It is characteristic that in the first period
functionally adapted structure appears in the complete absence of the
functional stimulus.

The explanation of the difference between the two periods is to be found
in the different evolutionary history of the characters formed during
each. First-period characters are _inherited_ characters, and taken
together constitute the historical basis of the organism's form and
activity; second-period characters are those of later acquirement which
have not yet become incorporated in the racial heritage.

Inherited characters appear in development in the absence of the
stimulus that originally called them forth; acquired characters are
those that have not yet freed themselves from this dependence upon the
functional stimulus. First-period characters were originally, like
second-period characters, entirely dependent for their development upon
the functional stimuli in response to which they arose, and only
gradually in the course of generations did they gain that independence
of the functional stimulus which stamps them as true inherited
characters. Speaking of the formative stimuli which are active in
second-period development, Roux writes:--"These stimuli can also produce
new structure, which if it is constantly formed throughout many
generations