Mr. Herbert Spencer, in his "Principles," thinks he has mastered the necessary psychological, if not mechanical, engineering for the successful construction of this bridge. In that branch of his work entitled the "Principles of Psychology," he so far abandons the exact scientific method as to take up psychical phenomena, and deal with them genetically, as he would with the phenomenal manifestations of organic life, in the continuous chain of ideas every where presented as consecutive thoughts in the universe. He finds, or claims to find, in these psychical manifestations, a constant tendency towards differentiation--towards advanced and continuously advancing differences, varieties, and new modes of thought--the same as, or similar to, those taking place in living organisms. He accordingly assumes, for the science of mind, as complete a foundation on which to base the doctrine of "evolution," as in the case of either physical or physiological science. But he is no less troubled, in this psychological realm, with divergent varieties, and exceptional variations and changes, than when he plants himself on the more solid substratum of life in the abounding realm of nature. His psychological differentiations present too many and constantly-shifting divergencies and re-divergences--exceptional branchings in one direction, and still more exceptional in another--to admit of any sufficiently potentiated potentiality for bridge timber. The arch to such a bridge would have to abut, according to Professor Tyndall, on a vital foundation at one end, and spring from undifferentiated sky-mist at the other. The bridge will never be built. Chapter II. Life--Its True Genesis.