on both factors are essential, the
environment as essential as the organism. The result of this continued
reaction is the development on the part of the organism of certain
physiological processes and structural conditions or characteristics.
The nature of these resulting states, depending upon the two
factors--organism and environment--can be changed by altering either
factor. In general, organisms develop under pretty much the same
conditions as their parents and general ancestry did, and their
germinal substances are directly continuous, and therefore very
similar. Consequently, primary organic structure and environing
conditions of development being alike through successive generations,
the results of their interaction are alike. This alikeness is
heredity--the fact of similarity between parent and offspring. The
usually indefinite question as to the effect of the environment
ordinarily has a real meaning however, and this is, or should be,
whether the alteration of particular elements of the environment, the
presence of special, unusual factors which cannot be said to be
"normally" present--whether these produce any effect upon the organism
which is truly heritable.

This is in reality the old question of the "inheritance of acquired
characteristics," or, in a word, of modifications--a question which
has been debated heatedly and at length. And as in many similar
instances the number of essays and the length and heat of the debate
have been inversely as the number and clearness of the pertinent
facts. The large majority of biologists have long felt that the great
bulk of the evidence was on one side, namely, that acquired traits
were not heritable. At the same time they have recognized the
difficulty of explaining certain apparently demonstrated contradictory
facts. Some recent experimental work has largely cleared away the
theoretical difficulties in this field, and the present status of the
old and really fundamental question may be stated as follows: External
conditions--climate, temperature, moisture, nutritional conditions,
results of unusual activity, and the like--incidences of the
environment, undoubtedly produce effects upon the structure and
behavior of the organism, but these effects must be clearly grouped
into two distinct classes.

In the first place the effect of "external" conditions may be to bring
about a reaction between the _bodily_ parts affected and the
environing conditions. Here the body