the important fact discovered by Marshall, that the contraction of a muscle is excited by diminishing its normal electrical current,[42] a result which could take place only with a stimulus, says Barker, "closely allied to electricity. Nerve force must therefore be transmuted potential energy." Prof. Huxley says,[43] "the results of recent inquiries into the structure of the nervous system of animals, converge toward the conclusion that the nerve-fibres which we have hitherto regarded as ultimate elements of nervous tissue, are not such, but are simply the visible aggregations of vastly more attenuated filaments, the diameter of which dwindles down to the limits of our present microscopic vision, greatly as these have been extended by modern improvements of the microscope; and that a nerve is, in its essence, nothing but a linear tract of specially modified protoplasm between two points of an organism, one of which is able to affect the other by means of the communication so established. Hence it is conceivable that even the simplest living being may possess a nervous system." Herbert Spencer[44] says all direct and indirect evidence "justifies us in concluding that the nervous system consists of _one_ kind of matter. In the gray tissue this matter exists in masses containing _corpuscles_, which are soft and have granules dispersed through them, and which, besides being thus unstably composed, are placed so as to be liable to disturbances to the greatest degree. In the white tissue this matter is collected together in extremely slender _threads_ that are denser, that are uniform in texture, and that are shielded in an unusual manner from disturbing forces, except at their two extremities." The last consideration is that form of force (thought power) which appears in manifestations called mental. It must be noticed at the outset, that every external manifestation of thought force is a muscular one, as a word spoken or written, a gesture, or an expression of the face always takes place; hence this force must be intimately correlated to nerve force. It is very certain, then, that thought force is capable in external manifestations of converting itself into actual motion. But here the question arises, can it be manifested inwardly without such a transformation of energy? Or is the evolution of thought entirely independent of the matter of the brain?