below it slowly rises up and takes the place of the lost leader.  Here is an action not prompted by the environment, but by the morphological needs of the tree, and it illustrates how different is its unity from the unity of a mere machine.  I am only aiming to point out that in all living things the material forces behave in a purposive way to a degree that cannot be affirmed of them in non-living, and that, therefore, they imply intelligence.

Evidently the cells in the body do not all have the same degree of life,—­that is, the same degree of irritability.  The bone cells and the hair cells, for instance, can hardly be so much alive—­or so irritable—­as the muscle cells; nor these as intensely alive as the nerve and brain cells.  Does not a bird possess a higher degree of life than a mollusk, or a turtle?  Is not a brook trout more alive than a mud-sucker?  You can freeze the latter as stiff as an icicle and resuscitate it, but not the former.  There is a scale of degrees in life as clearly as there is a scale of degrees in temperature.  There is an endless gradation of sensibilities of the living cells, dependent probably upon the degree of differentiation of function.  Anaesthetics dull or suspend this irritability.  The more highly developed and complex the nervous system, the higher the degree of life, till we pass from mere physical life to psychic life.  Science might trace this difference to cell structure, but what brings about the change in the character of the cell, or starts the cells to building a complex nervous system, is a question unanswerable to science.  The biologist imagines this and that about the invisible or hypothetical molecular structure; he assigns different functions to the atoms; some are for endosmosis, others for contraction, others for conduction of stimuli.  Intramolecular oxygen plays a part.  Other names are given to the mystery—­the micellar strings of Naegeli, the biophores of Weismann, the plastidules of Haeckel; they all presuppose millions of molecules peculiarly arranged in the protoplasm.

On purely mechanical and chemical principles Tyndall accounts for the growth from the germ of a tree.  The germ would be quiet, but the solar light and heat disturb its dreams, break up its atomic equilibrium.  The germ makes an “effort” to restore it (why does it make an effort?), which effort is necessarily defeated and incessantly renewed, and in the turmoil or “scrapping” between the germ and the solar forces, matter is gathered from the soil and from the air and built into the special form of a tree.  Why not in the form of a cabbage, or a donkey, or a clam?  If the forces are purely automatic, why not?  Why should matter be gathered in at all in a mechanical struggle between inorganic elements?  But these are not all inorganic; the seed is organic.  Ah! that makes the difference!  That accounts for the “effort.”  So we have to have the organic to start with, then the rest is easy.  No doubt the molecules of the seed would remain in a quiescent state, if they were not disturbed by external influences, chemical and mechanical.  But there is something latent or potential in that seed that is the opposite of the mechanical, namely, the vital, and in what that consists, and where it came from, is the mystery.