True: 'but there is a great difference between such a climate as that of Scotland, damp and cold, snowy and blowy, and a continental ice-sheet, a mile or two thick, reaching from John o' Groat's House to the Mediterranean. We can see that the oranges of Spain can grow to-day within a comparatively short distance of Edinburgh; but we can not realize that any tropical or semitropical plant could have survived in Africa when a precipice of ice, five thousand feet high, frowned on the coast of Italy; or that any form of life could have survived on earth when the equator in South America was covered with a continental ice-sheet a mile in thickness, or even ten feet in thickness. We can conceive of a glacial age of snow-storms, rains, hail, and wind--a terribly trying and disagreeable climate for man and beast--but we can not believe that the whole world was once in the condition that the dead waste of ice-covered Greenland is in now. {p. 390} Secondly, it may be said-- "The whole world is now agreed that ice produced the Drift; what right, then, has any one man to set up a different theory against the opinions of mankind? " One man, Mohammed said, with God on his side, is a majority; and one man, with the truth on his side, must become a majority. All recognized truths once rested, solitary and alone, in some one brain. Truth is born an acorn, not an oak. The Rev. Sydney Smith once said that there was a kind of men into whom you could not introduce a new idea without a surgical operation. He might have added that, when you had once forced an idea into the head of such a man, you could not deliver him of it without instruments. The conservatism of unthinkingness is one of the potential forces of the world. It lies athwart the progress of mankind like a colossal mountain-chain, chilling the atmosphere on both sides of it for a thousand miles. The Hannibal who would reach the eternal city of Truth on the other side of these Alps must fight his way over ice and hew his way through rocks. The world was once agreed that the Drift was due to the Deluge. It abandoned this theory, and then became equally certain that it came from icebergs. This theory was, in turn, given up, and mankind were then positive that glaciers caused the Drift. But the glaciers were found to be inadequate for the emergency; and so the continents were lifted up fifteen hundred feet, and the ice-sheets were introduced. And now we wait to hear that the immense ice-masses of the Himalayas have forsaken their elevations and are moving bodily over the plains of India, grinding up the rocks into clay and gravel {p. 391} as they go, before we accept a theory which declares that they once marched over the land in this fashion from Hudson's Bay to Cape Horn, from Spitzbergen to Spain.