5. But if we estimate the southern cap as extending continuously to 75°, what would be the effect of its transference at once to the ice-cap of the north pole? Would it extend it, after assuming its proper glacial slope, below 60°, a point falling within the present subarctic zone? The utmost limit to which Mr. Croll, in his great work on "Climate and Time," conceives it possible that it should extend, in any glacial epoch, is to 55°, or about the northern boundary of England. Now unless the astronomers and physicists are all at sea about the causes of glaciation, the warm temperate zone can never be pushed any further south than the tropical zone, nor the cold temperate any further than the sub-tropical. This would be the extreme limit. Mr. Croll says, in speaking of these glacial periods; "It is, of course, absurd to suppose that an ice-cap could ever actually reach down to the equator. It is probable that the last great ice-cap of the glacial epoch nowhere reached half way to the equator. Our cap (that of Europe) must therefore, terminate at a moderately high latitude." And if the gulf stream flows southward during the glacial period north, as he supposes probable, the cap on this continent would probably terminate at the same moderately high latitude. Assuming that Mr. Croll's estimate is the more probable one, it would only push the cold temperate zone down to the line of the Gulf States; the warm temperate, to the southern line of Mexico; the sub-tropical, to the Central American States, and the tropical to the United States of Columbia, Venezuela, and Guiana. Suppose, then, that some seven hundred thousand years ago, more or less, when the North Pole had fully donned the earth's ice-cap, with all the isothermal and isochimenal changes thereby effected, what must have been the line of march taken by our northern vegetal and animal forms to escape the cataclysm of ice and snow then impending? Manifestly, they would have flocked, first to the Gulf states, then to Mexico, and afterwards to the Central American states; but none of them could ever have been crowded through the Isthmus of Panama, since at the height of the last glaciation, that portion of the continent must have been the tropical barrier to our northern forms, as it is now the equatorial barrier.