Like any other organism a human being is a bundle of characteristics, physical and psychical. Each person has a definite stature and span, possesses fingers and toes, a head, eyes, ears, hair of a certain color, and so on through a long list of physical traits. Physiological characteristics has he also, such as muscular strength, resistance to fatigue or to disease of many kinds, digestive and assimilative powers, a rate of heart beat, a blood pressure, an habitual gait, posture, a characteristic way of clasping the hands or of twirling the thumbs--and so almost _ad infinitum_. He also possesses certain physiological traits more closely related with the action of the central nervous system--keenness of vision, or hearing, or smell, memory, vivacity, cheerfulness, self-assertiveness, self-consciousness, reasoning power, determination, and the like. There is a period during the existence of each human being when he does not seem to possess these traits or anything resembling them. For at the beginning of his existence as a new and separate creature, every individual, among the groups of higher organisms, has the form of a single organic cell--the germ. This germ may be, as it is in man, of microscopic dimensions, and it always shows a comparatively slight degree of differentiation of structure. Moreover, the parts and organs of the germ bear no actual or visible resemblance at all to the organs and parts of the organism into which the germ rapidly develops. In other words, in the germ of an organism we have a structure, partly material, partly dynamic, the components of which in some way represent the adult characteristics without resembling them. During the period of the development of the individual, that is to say, during its "ontogeny," these characteristics of the germ become expressed in their final or adult form. For our purpose it is not necessary to inquire precisely how it is that the structure of the germ can thus represent or determine the structures growing out of it. It must suffice to see that somehow the characteristics of the germ lead to the formation or development of other characters, and these in turn to still others until at last a period of comparative changelessness is reached, when we say that development is completed. It is important to recognize, however, that this development is fundamentally a process of reaction, the reaction between the germ and its surrounding conditions. The characteristics of the adult organism are _determined_ primarily by the structure of the germ; they _appear_ gradually and successively, as the growing organism reacts to its environing conditions.