and differentiation of successive layers, gave place to the variously-constructed eyes of insects, mollusks, and vertebrates.  The day for creative work of this sort has probably gone by, as the day for the evolution of annulose segments and vertebrate skeletons has gone by,—­on our planet, at least.  In the line of our own development, all work of this kind stopped long ago, to be replaced by different methods.  As an optical instrument, the eye had well-nigh reached extreme perfection in many a bird and mammal ages before man’s beginnings; and the essential features of the human hand existed already in the hands of Miocene apes.  But different methods came in when human intelligence appeared upon the scene.  Mr. Spencer has somewhere reminded us that the crowbar is but an extra lever added to the levers of which the arm is already composed, and the telescope but adds a new set of lenses to those which already exist in the eye.  This beautiful illustration goes to the kernel of the change that was wrought when natural selection began to confine itself to the psychical modification of our ancestors.  In a very deep sense all human science is but the increment of the power of the eye, and all human art is the increment of the power of the hand.[8] Vision and manipulation,—­these, in their countless indirect and transfigured forms, are the two cooeperating factors in all intellectual progress.  It is not merely that with the telescope we see extinct volcanoes on the moon, or resolve spots of nebulous cloud into clusters of blazing suns; it is that in every scientific theory we frame by indirect methods visual images of things not present to sense.  With our mind’s eye we see atmospheric convulsions on the surfaces of distant worlds, watch the giant ichthyosaurs splashing in Jurassic oceans, follow the varied figures of the rhythmic dance of molecules as chemical elements unite and separate, or examine, with the aid of long-forgotten vocabularies now magically restored, the manners and morals, the laws and superstitions, of peoples that have ceased to be.[9] And so in art the wonderful printing-press, and the engine that moves it, are the lineal descendants through countless stages of complication, of the simple levers of primitive man and the rude stylus wherewith he engraved strange hieroglyphs on the bark of trees.  In such ways, since the human phase of evolution began, has the direct action of muscle and sense been supplemented and superseded by the indirect work of the inquisitive and inventive mind.

VIII.

Growing Predominance of the Psychical Life.