These equalising currents, however, are almost entirely due to the form and position of the continents, and especially to the fact that they are so situated as to leave vast expanses of ocean along the equatorial zone, and extending north and south to the arctic and antarctic regions. If with the same amount of land the continents had been so grouped as to occupy a considerable portion of the equatorial oceans--such as would have been the case had Africa been turned so as to join South America, and Asia been brought to the south-east so as to take the place of part of the equatorial Pacific, then the great ocean-currents could have been but feeble or have hardly existed. Without these currents much of the north and south temperate lands would have been buried in ice, while the largest portion of the continents would have been so intensely heated as perhaps to be unsuited for the development of the higher forms of animal life, since we have shown (in chapters X. and XI.) how delicate is the balance and how narrow the limits of temperature which are required. There seems to be no reason whatever why some such distribution of the sea and land should not have existed, had it not been for the admittedly exceptional conditions which led to the production of our satellite, thus necessarily forming vast chasms along the region of the equator where centrifugal force as well as the internal solar tides were most powerful, and where the thin crust was thus compelled to give way. And as the highest authorities declare that there are no indications of such an origin of satellites in the case of any other planet, the whole series of conditions favourable to life on the earth become all the more remarkable. CLOUDS, THEIR IMPORTANCE AND THEIR CAUSES Few persons have any adequate conception of the real nature of clouds and of the important part they take in rendering our world a habitable and an enjoyable one.