cise
condition of these organs and of the whole individual; and second, it
depends upon the nature of those conditions outside of and affecting
the organism which lead it to act at all. Either group of factors
taken alone will not lead to any activity; activity of an organism
must be a reaction between organismal structure and environing
conditions--an irritable substance and stimuli to activity. And the
character or quality of an act is affected by circumstances within
either set of factors.

In much the same way the germ acts, and its action is similarly a
reaction between the structure of the germ and its environing
conditions. The germ reacts by producing certain parts,
differentiating certain structures, in short, by developing. The
normal activities or reactions of the adult organism we call in
general its "behavior." The normal activities or reactions of the germ
and embryo we call "development"; the normal behavior of the germ is
development. And in the latter, as well as in the former, changes in
either set of factors lead to changes in the nature of the result of
their interaction, i. e., to changes in the characteristics actually
appearing as the result of development.

In their fully developed state some of the traits or characteristics
of organisms are single, simple, fundamental characters, not
analyzable into more elementary factors. Such are the number of
fingers, or of joints in the fingers, absence of pigments of several
kinds from the eyes or hair, presence of cataract, _et cetera_. These
so-called "unit characters" are roughly analogous to the chemical
elements which may, as units, be combined and recombined in diverse
ways, but which always maintain their integrity as elements although
different combinations produce wholes that are unlike. Each unit
character in the adult is the result of a series of reactions between
the environing conditions of development and a germinal structural
unit, as yet hypothetical and provisionally called the "determiner,"
which in some way not yet understood represents this adult trait.

On the other hand, there are many of these things which we call
characteristics which seem to be composite, capable of being analyzed
or factored into a group of simpler components or unit characters.
Such apparently are stature, span, resistance to fatigue, and probably
most psychic traits. Each of these complexes results apparently from a
series of reactions between the conditions