This leads us to consider some of the elements, conditions and forces which give character to the organization. External circumstances supply necessary conditions to inward activity, for without air, food, or sunlight all living animals would perish. Everywhere, life is dependent upon conditions and circumstances; it is _not_ self-generating. But the conditions of reproduction are very complex. External forces are transformed, and, in turn, become vital or formative powers. Development is a transmutation of physical and chemical forces into vital energy. Although unable to compute the ultimate factors of life, yet 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_.