seems to be one great totality whose parts are in reciprocal relation and mutual interaction, like the organs of a living organism. I would like to consider the unity of science under the figure of a tree of life which grows upward from the earth from which one part takes its power and nutriment and in which it finds its support. The parts of this tree—roots, stem, branches, and whatever they may all be called—appear to us externally different, but within they belong together; they stand among each other continually in helpful interaction. As the organs, so are the tissues adjusted to each other, and not one of the millions of cells in a tree is without purpose, and if each cell does not stand in fast relation to all others, how also need not each single scientific question be related to all others? This can as little destroy the unity of science as the unity of organic structure of a tree can be destroyed by the fact that each cell does not stand in mutual relation with every other cell.
Wonderingly we see this tree of science develop and broaden out; but for this provision is made, namely, that this tree shall not grow even into the heavens.
After thousands of years of seeking and groping, mankind has finally discovered how he may reach high aims of knowledge in spite of the limitations of his mind, by the often slow and heavily progressing inductive method, and the principle of the division of labor, which first leads to division, but after a rich harvest binds all together. It becomes even clearer that the synthetical mental work, flowing out of the principle of division of labor, must lead to even greater conceptions, and that the number of men must be even greater who, raising themselves above the level of specialists, will be investigators in the best sense of the word.
Held in bounds by the exact nature of its work, science strides forward, ever attaining more and more of what is seemingly unattainable to the human mind, and likewise ever more clearly recognizing the unattainable as unattainable. Indeed, more and more we come face to face with the limits of our knowledge. To the Grecian thinkers it seemed a play that allowed the living to spring out of the lifeless, plants or animals from slime or damp earth. But the inductive method has led us thus far to know that, so far as observation can go, the living can arise only from the living. Even the smallest known living beings, the bacteria, do not come into being parentless, as not long since the last notes of retreat of the defense of spontaneous generation declared. In the organism itself, all that is living proceeds only out of the living—the cell from a cell, the nucleus from a nucleus—and the smallest plastid lying on the very border of microscopic observation proceeds from its like. The possibility enlarged upon by many naturalists, that in the organism living constituents can arise spontaneously, is only a reaction of the old doctrine of spontaneous generation; for, so far as investigation shows, there can rise within the organism organized substance
