so 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.

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 pre