et we may
illustrate their reproductive possibilities and results by comparing
them with those of a lower order.

Animal structures are mainly composed of four elements: oxygen,
hydrogen, nitrogen and carbon. Other constituents, such as phosphorus,
sulphur, potassium, sodium, calcium, magnesium, and iron, enter into
their composition, but are found in much smaller quantities. From these
elements is fabricated an organism which manifests peculiar properties
and marvelous functions. If the proportion of these chemical elements be
varied, the organic compound will be changed, or, the proportions
remaining the same, if the _grouping_ of the elements be altered,
different compounds will be produced, showing that the properties of
organized substances depend upon the _molecular_ constitution of matter.

Rising in the scale of organization, we observe that every variation of
the physical and chemical processes implies a corresponding modification
of the vital. This is verified by the peculiarities of the several races
of mankind. Individual differences are likewise modifications of these
processes. Dynamical or vital differentiation depends upon these
modifications for the display of vital energy, and is always associated
with molecular changes. But it should be borne in mind that an effect
may not resemble its cause in _properties_, and the _qualities_ of a
chemical compound may be quite different from those of its individual
constituents. Organic matter, although more complex, may exhibit
properties, both like and unlike its constituent elements. Within
certain boundaries, the elements seek to satisfy their affinities. We
discover that there are limits between the genera of animals, as well as
the races of mankind. Not less really, though perhaps not as absolutely,
are there individual precincts within the sphere of the human
temperaments, which cannot be passed.

If we cannot satisfactorily explain, we can at least discover a reason
for temperamental limitation. It is not designed to circumscribe
healthful reproduction, but to serve as an effectual hindrance to
abnormal deviations. We may state our belief in more positive terms:
that the temperamental variations are essential to _genesis_ and
_fertility_, and indispensable to _health_ and _normal development_.

Every individual is susceptible to impressions which dispose to action.
Impressions which excite or increase this disposition, are called
_stimuli_. Vital change