The very fact of a mass of water, 200 fathoms deep and extending over fifteen degrees of latitude, remaining above water of three or four degrees higher temperature shows how little influence difference of temperature has in producing motion. If it had the potency which some attribute to it, one would suppose that this cold stratum should sink down and displace the warm water underneath. If difference of density is sufficient to move the water horizontally, surely it must be more than sufficient to cause it to sink vertically. CHAPTER XIV. THE WIND THEORY OF OCEANIC CIRCULATION IN RELATION TO CHANGE OF CLIMATE. Direction of Currents depends on Direction of the Winds.—Causes which affect the Direction of Currents will affect Climate.—How Change of Eccentricity affects the Mode of Distribution of the Winds.—Mutual Reaction of Cause and Effect.—Displacement of the Great Equatorial Current.—Displacement of the Median Line between the Trades, and its Effect on Currents.—Ocean-currents in Relation to the Distribution of Plants and Animals.—Alternate Cold and Warm Periods in North and South.—Mr. Darwin’s Views quoted.—How Glaciers at the Equator may be accounted for.—Migration across the Equator. _Ocean-currents in Relation to Change of Climate._—In my attempts to prove that oceanic circulation is produced by the winds and not by difference of specific gravity, and that ocean-currents are the great distributors of heat over the globe, my chief aim has been to show the bearing which these points have on the grand question of secular changes of climate during geological epochs, more particularly in reference to that mystery the cause of the glacial epoch. In concluding this discussion regarding oceanic circulation, I may therefore be allowed briefly to recapitulate those points connected with the subject which seem to shed most light on the question of changes of climate. The complete agreement between the systems of ocean-currents and winds not only shows that the winds are the impelling cause of the currents, but it also indicates to what an extent the _directions_ of the currents are determined by the winds, or, more properly, to what an extent their directions are determined by the _direction_ of the winds.