For purely mechanical things we can find equivalents.  Arrest a purely mechanical process, and the machine only rests or rusts; arrest a vital process, and the machine evaporates, disintegrates, myriads of other machines reduce it to its original mineral and gaseous elements.  In the organic world we strike a principle that is incalculable in its operation and incommensurable in its results.  The physico-chemical forces we can bring to book; we know their orbits, their attractions and repulsions, and just what they will and will not do; we can forecast their movements and foresee their effects.  But the vital forces transcend all our mathematics; we cannot anticipate their behavior.  Start inert matter in motion and we know pretty nearly what will happen to it; mix the chemical elements together and we can foresee the results; but start processes or reactions we call life, and who can foresee the end?  We know the sap will mount in the tree and the tree will be true to its type, but what do we or can we know of what it is that determines its kind and size?  We know that in certain plants the leaves will always be opposite each other on the stalk, and that in other plants the leaves will alternate; that certain plants will have conspicuous and others inconspicuous flowers; but how can we know what it is in the cells of the plants that determines these things?  We can graft the scion of a sour apple tree upon a sweet, and vice versa, and the fruit of the scion will be true to its kind, but no analysis of the scion or of the stock will reveal the secret, as it would in the case of chemical compounds.  In inorganic nature we meet with concretions, but not secretions; with crystallization, but not with assimilation and growth from within.  Chemistry tells us that the composition of animal bodies is identical with that of vegetable; that there is nothing in one that is not in the other; and yet, behold the difference! a difference beyond the reach of chemistry to explain.  Biology can tell us all about these differences and many other things, but it cannot tell us the secret we are looking for,—­what it is that fashions from the same elements two bodies so unlike as a tree and a man.

Decay and disintegration in the inorganic world often lead to the production of beautiful forms.  In life the reverse is true; the vital forces build up varied and picturesque forms which when pulled down are shapeless and displeasing.  The immense layers of sandstone and limestone out of which the wonderful forms that fill the Grand Canon of the Colorado are carved were laid down in wide uniform sheets; if the waters had deposited their material in the forms which we now see, it would have been a miracle.  We marvel and admire as we gaze upon them now; we do more, we have to speculate as to how it was all done by the blind, unintelligent forces.  Giant stairways, enormous alcoves, dizzy, highly wrought balustrades, massive vertical walls standing four-square like huge