Only at a very much later date, however, did he conquer the vast Atlantic outside, which, as Prof. Myres points out, has now become a mere inland sea in its turn, when compared with the greater oceans beyond. To this day, however, the part which water transport plays in human life is reflected in the way in which the denser masses of mankind cluster round the shores of the seas and oceans, as any map of the distribution of population will show. In water transport the method of propelling the vessel employed is of great importance. In river navigation it is natural to allow the boat to drift with the current, and the use of a pole to steer with would naturally suggest its use as a paddle or oar in order to move against the current. In North America up till the appearance of the white man, the aborigines had not got past the paddle stage, in spite of the fact that they were very skilful navigators, and had a considerable variety of vessels. The next stage is of course the sail, used by most races who venture on open water, as contrasted with flowing streams and rivers. Although before its virtual replacement by steam, the civilised races had carried the development of the sail to a very high pitch, yet the difficulties associated with it militated against bold navigation, more especially in the early days when there was no science of meteorology. Thus it has been pointed out that the reason why the effective discovery of America was delayed so long was largely due to two meteorological facts. The first of these is that to the north, the place where the crossing is narrowest, the ice which streams down the west coast of Greenland, and even to this day presents difficulties to navigation off the coasts of Newfoundland, formed an effective barrier to early navigators. To the south the great difficulty was the constant north-east trade wind. What ship dare set out towards the unknown blown by a constant wind against which she could not return home again? To Columbus first, says Mr. Mackinder, came the brilliant inspiration that, while travelling outwards with the trade, it would be possible to return with the westerly breezes of more northern latitudes. When, later, the sail was functionally replaced by steam, man became virtually independent of the wind, and only the name of trade wind remains to carry us back to the period when the ocean breezes determined his movements and his commerce.