The answer is, because the same causes which produce summer and winter in temperate climate produce other changes of other sorts in the tropical region. The temperature, it is true, remains the same, or approximately the same; but the meteorological conditions vary. Even with ourselves, summer is not only hotter but also drier than winter; winter is marked by rain and snow as well as by lowered temperature. In the tropics, on the other hand, it is rather the summer or summers that are wet, for there is a certain moving zone of equatorial calms in which it practically keeps on raining always. But this zone is not fixed; it flits with the sun. When the sun goes northward for the northern summer the rainy zone goes with him; when he turns southward again the zone shifts after him. Thus places on or near the two tropics have one rainy season a year, while places on the equator have usually two. The intervening dry seasons are often very dry and parched, indeed; and where this is markedly the case, the rainy season acts just as spring does in the north, or as the inundation does in Egypt; it is the beginning of vegetation. The plants that were dry and dormant during the arid months wake up into fresh life; the branches put forth new leaves; the brown seeds germinate; the flowers appear; and in due time the fruit ripens. Everything in these cases depends upon the recurrence of the rainy season, just as everything in India depends upon the bursting of the monsoons, and everything in Egypt on the rising of the Nile. I have seen a dry plain in Jamaica bare and brown one day, and covered six or eight inches high with fresh green waving guinea-grass the day but one after. The rains had come meanwhile, and Nature had awaked with more than springlike awakening. In those hot climates everything grows by magic as soon as it gets the needed water.