dog, or even the flea, is beyond its reach.  The secret of biology, that which makes its laws and processes differ so widely from those of geology or astronomy, is a profound mystery.  Science can take living tissue and make it grow outside of the body from which it came, but it will only repeat endlessly the first step of life—­that of cell-multiplication; it is like a fire that will burn as long as fuel is given it and the ashes are removed; but it is entirely purposeless; it will not build up the organ of which it once formed a part, much less the whole organized body.

The difference between one man and another does not reside in his anatomy or physiology, or in the elements of which the brains and bodies are composed, but in something entirely beyond the reach of experimental science to disclose.  The difference is psychological, or, we may say, philosophical, and science is none the wiser for it.  The mechanics and the chemistry of a machine are quite sufficient to account for it, plus the man behind it.  To the physics and chemistry of a living body, we are compelled to add some intangible, unknowable principle or tendency that physics and chemistry cannot disclose or define.  One hesitates to make such a statement lest he do violence to that oneness, that sameness, that pervades the universe.

All trees go to the same soil for their ponderable elements, their ashes, and to the air and the light for their imponderable,—­their carbon and their energy,—­but what makes the tree, and makes one tree differ from another?  Has the career of life upon this globe, the unfolding of the evolutionary process, been accounted for when you have named all the physical and material elements and processes which it involves?  We take refuge in the phrase “the nature of things,” but the nature of things evidently embraces something not dreamed of in our science.

VII

It is reported that a French scientist has discovered the secret of the glow-worm’s light.  Of course it is a chemical reaction,—­what else could it be?—­but it is a chemical reaction in a vital process.  Our mental and spiritual life—­our emotions of art, poetry, religion—­are inseparable from physical processes in the brain and the nervous system; but is that their final explanation?  The sunlight has little effect on a withered leaf, but see what effect it has upon the green leaf upon the tree!  The sunlight is the same, but it falls upon a new force or potency in the chlorophyll of the leaf,—­a bit of chemistry there inspired by life,—­and the heat of the sun is stored up in the carbon or woody tissues of the plant or tree, to be given out again in our stoves or fireplaces.  And behold how much more of the solar heat is stored up in one kind of a tree than in certain other kinds,—­how much in the hickory, oak, maple, and how little comparatively in the pine, spruce, linden,—­all through the magic of something in the leaf, or shall we say