An adult organism is continually doing certain things--performing certain movements, producing certain secretions, undergoing a great variety of physical and chemical changes. Just what the organism does at any given moment is in reality determined by two groups of factors: first, it depends, obviously, upon the structure of the organism acting, upon the organs it has to act with, and upon the precise 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 of development and a group of hypothetical germinal determiners that tend to be associated within the germ.