But whilst the man is startled by this closer inspection of the laws of matter, his attention is called to the independent action of the mind,--its strange suggestions and laws,--a certain tyranny which springs up in his own thoughts, which have an order, method, and beliefs of their own, very different from the order which this common-sense uses. Suppose there were in the ocean certain strong currents which drove a ship, caught in them, with a force that no skill of sailing with the best wind, and no strength of oars, or sails, or steam, could make any head against, any more than against the current of Niagara: such currents--so tyrannical--exist in thoughts, those finest and subtilest of all waters,--that, as soon as once thought begins, it refuses to remember whose brain it belongs to,--what country, tradition, or religion,--and goes whirling off--swim we merrily--in a direction self-chosen, by law of thought, and not by law of kitchen clock or county committee. It has its own polarity. One of these vortices or self-directions of thought is the impulse to search resemblance, affinity, identity, in all its objects, and hence our science, from its rudest to its most refined theories. The electric word pronounced by John Hunter a hundred years ago,--_arrested and progressive development_,--indicating the way upward from the invisible protoplasm to the highest organisms,--gave the poetic key to Natural Science,--of which the theories of Geoffroy St. Hilaire, of Oken, of Goethe, of Agassiz, and Owen, and Darwin, in zoölogy and botany, are the fruits,--a hint whose power is not yet exhausted, showing unity and perfect order in physics. The hardest chemist, the severest analyzer, scornful of all but dryest fact, is forced to keep the poetic curve of nature, and his result is like a myth of Theocritus. All multiplicity rushes to be resolved into unity. Anatomy, osteology, exhibit arrested or progressive ascent in each kind; the lower pointing to the higher forms, the higher to the highest, from the fluid in an elastic sack, from radiate, mollusk, articulate, vertebrate,--up to man; as if the whole animal world were only a Hunterian museum to exhibit the genesis of mankind. Identity of law, perfect order in physics, perfect parallelism between the laws of Nature and the laws of thought exist. In botany we have the like, the poetic perception of metamorphosis,--that the same vegetable point or eye which is the unit of the plant can be transformed at pleasure into every part, as bract, leaf, petal, stamen, pistil, or seed.