And thus our conclusion attains almost certainty, when we remember that it stands the crucial test of experiment—­that we need only decompose the blood in order to find there what we contend to be an essential ingredient of it.

I must now say a few words in explanation of the term protoplasm.  Protoplasm is a soft, gelatinous substance, transparent and homogeneous, easily seen in large plant-cells; it may be compared to the white of an egg.  When at rest all sorts of vibratory, quivering and trembling movements can be observed within its mass.  It forms the living material in all vegetable and animal cells; in fact, it is that component of the body which really does the vital work.  It is the formative agent of all living tissues.  Vital activity, in the broadest sense of the term, manifests itself in the development of the germ into the complete organism, repeating the type of its parents, and in the subsequent maintenance of that organism in its integrity and both these functions are exclusively carried on by the protoplasm.  Of course, there is a good deal of chemical and mechanical work done in the organism, but protoplasm is the formative agent of all the tissues and structures.

Of tissues and structures already formed, we may fairly say that they have passed out of the realms of vitality, as they are destined to gradual disintegration and decay in the course of life; it is they that are on the way of being cast out of the organism, when they have once run through the scale of retrograde metamorphosis; and it is they that give rise to what we have called the smell of the animal.  What lives in them is the protoplasm.

In the shape of food the outer world supplies the organism with all the materials necessary for the building up of the constantly wasting organic structures; and, in the shape of heat, there comes from the outer world that other element necessary for structural changes, development and growth—­the element of force.  But the task of directing all the outward materials to the development and maintenance of the organism—­in other words, the task of the director-general of the organic economy falls to the protoplasm.

Now this wonderful substance, chemically and physically the same in the highest animal and in the lowest plant, has been all along the puzzle of the biologist.  How is it that in man protoplasm works out human structure; in fowl, fowl structure, &c. &c., while the protoplasm itself appears to be everywhere the same?  To Professor Yaeger belongs the great merit of having shown us that the protoplasms of the various species of plants and animals are not the same; that each of them contains, moreover, imbedded in its molecules, odorant substances peculiar to the one species and not to the other.