Now, the distance to which the sun travels north or south of you, if you live on the equator—I use ordinary terms instead of astronomical ones for simplicity's sake—is so comparatively small that within the tropics themselves you never notice much difference as to the amount of heat between one period of the year and another. In equatorial countries the day and night temperature is much the same all the year round: if the country be plain, it is always hot; if mountainous, like the district about Bogotá, it is "a perpetual spring"; one day is always much the same as the one that went before and the one that comes after it. Even on the actual tropics, again, the difference is too slight to make any marked change in the temperature; people living on the northern tropic (Cancer), for example, have the sun vertical to them on June 21st, and some forty-three degrees south of them on December 21st. Nevertheless, the sun is still as near them and as powerful as he is at Milan or Venice in the height of summer; and the consequence is that, as a matter of fact, the thermometer within the tropics and at sea level seldom descends below 75° or 80°, even at midnight in the relative winters. For the heating power of the sun depends, of course, upon the directness of his rays, and lessens with their obliquity; in Venice and Milan they are strong enough to make the ground very hot in July and August, though it has been cooled before by a northern winter; much more than in Jamaica or Madagascar, which have never been cooled, does the accumulated heat keep everything warm even when the sun is most oblique—and he never reaches the same obliquity as in an English summer. The ground is hot, the houses are hot, wood and stone are hot, and they have all been hot from time immemorial.
Yet tropical and equatorial trees and plants have their definite seasons to flower and fruit, just the same as elsewhere. This seems surprising at first when one visits the tropics. You can not see why everything should not flower and fruit the whole year round. And yet, at one time pineapples are "in," at another mangoes. And these seasons differ in the northern and southern hemispheres; what is mango winter in the one being mango summer in the other. I do not say the seasons anywhere in the tropics differ markedly; still, they do differ; the tropical year is divided into times and months for agriculture just as much as any other. Thus there are regular dates in each hemisphere for planting, tending, and cutting the sugar cane. Now, what is the reason of these changes in vegetation, when temperature remains so constant? Why do not trees and shrubs of each kind flower up and down throughout the year irregularly—now one individual and now another? Why are there seasons for things at all in the tropics?
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.
Indeed, we may say that in half the world the seasons, organically speaking—I mean, the seasons of plant and animal life—depend upon heat and cold, summer and winter, snow or sunshine; but in the other half they depend almost entirely upon drought and rainfall. Even as near home and as far north as Algeria, the summer is far too dry and dusty for agriculture; the autumn rains set in about October or November; they are immediately followed by the plowing; and the winter becomes for most purposes the practical summer. Fruits and vegetables are at their best in January and February; the fields are full of flowers up to March or April; in June, July, and August the country is an arid and weary desert. But the seasons for dates are almost reversed; they ripen in autumn. In Egypt again, where everything depends upon the inundation, the seasons are still more complicated; the inundation begins to subside in October; in Upper Egypt the winter season which follows is far the most important for agriculture, and crops sown as the water subsides are reaped from four to seven months after. But in the Delta, rice, cotton, and indigo are sown in the spring (March or April) and harvested in October, November, and December. Here, irrigation and temperature come in as disturbing elements, for the Delta feels something of the cold of winter.
I could give many other instances, but these will suffice. As a general rule, we may say that in the temperate and frigid zones the seasons for plants and animals are ruled by heat and cold, but that in tropical and even in subtropical climates, rainfall and drought, themselves largely due to the same circumstances, are the ruling factors.
Again, everybody knows that winter and summer, and the other phenomena which simulate or accompany them, such as wet and dry seasons, depend upon the fact that the earth's axis is not perpendicular to the plane in which the earth moves round the sun, but slightly inclined to it. Now, a year in itself, viewed as a measure of time, is merely the period which it takes the earth to perform one such complete revolution. During one half of each such revolution the north pole is turned at a considerable angle toward the sun, and during the other half, the south pole. When the north pole is so turned we call it summer in the northern hemisphere; when the south pole is being favored, and the north is receiving less light and heat, we call it winter. Let us suppose for a moment that the earth had not got this twist or kink in its axis; that the equator was always presented exactly toward the sun; what then would happen? Obviously, there would be no change of seasons. The day and night would have fixed lengths which never varied; climate would in each place be uniform and, barring accidents of elevation or distribution of land and water, the climate of each place would also depend entirely the whole year round on its distance from the equator. Roughly speaking, the temperature of a district would be the temperature it now possesses in March and September, only not quite so cold as March nor so warm as September, owing to the absence of accumulated heat from summer or of reserves of ice and snow from winter. In one word, under such conditions there would have been climates—marked belts of climate; but there would not have been seasons.
Seasons, however, depend in great part, as Mr. Alfred Russel Wallace has ingeniously shown, on a great many things besides this mere inclination of one end or the other of the earth toward the sun in June and January. Much must be laid to the count of accumulated stores of heat or cold; and though accumulated cold is physically a misnomer, still for all practical purposes we may apply the words fairly enough to the ice caps of the pole and the glaciers of mountain systems. And here we come face to face with the very core of our problem: for the odd part of it is that seasons (at least as we know them) seem to be quite a recent and exceptional phenomenon in the history of our planet. So far as we can judge, geologically speaking, the earth during all its earlier life enjoyed, over all its surface, what we should now consider tropical or subtropical conditions. England—or rather the land that occupied the part of the earth's crust where England now stands—had a vegetation of huge tree ferns and palms and cycads during the Primary period; as late even as the middle Tertiaries it had a vegetation like that of South Carolina or Upper India. Greenland itself, in quite recent times, flourished like a green bay tree, and did not belie its odd modern name. The world as a whole enjoyed perpetual summer. In one word, except in something like the equatorial sense, there were practically no seasons. The sun went north and south, no doubt, as now, but the temperature, even in the relative winter, seems to have remained perennially mild and genial.
It is true, occasional slight traces of glacial epochs, earlier than the great and well-known Glacial epoch, break here and there the almost continuous geological record of palmy and balmy world-wide summers; yet, taking the geological monuments as a whole, they show us few or no signs of anything worth calling a serious winter till quite recent periods. Large-leaved evergreens are still, in the day-before-yesterday of geology, the order of the day; magnolias and liquidambars, cinnamons and holly oaks, vines and rotang palms formed the forests even of Miocene Britain. The animals during all the Tertiary period were of what we now regard as tropical or subtropical types—lions, rhinoceroses, hippopotamuses, monkeys, or more antique races, equally southern in aspect. There could have been little change of winter and summer during this long warm spell; the variations can have been scarcely more than those of dry and rainy seasons. The trees never lost their leaves; the fruits and flowers never ceased to follow one another; no interruption of the food supply drove insects to hibernate in their silken cocoons, or squirrels and bears to lay by stores of food or fat for the cold and hungry winter.
Nevertheless, taking the world round as it stands, we must believe that the distinction of seasons grew up, both for plants and animals, and for man or his ancestors, during this age of relatively unmarked summers and winters. For the tropics more than anywhere else preserve for us to-day the general features and aspect of this earlier time; they have never had the continuity of their stream of life rudely interrupted by the enormous changes of the Glacial epoch. Yet, even in the tropics, things, as we saw, have seasons. There are annuals and perennials there, as elsewhere. Each kind has its month for sprouting, for flowering, for fruiting, for shedding its seed; and men in the tropics, some of them long isolated in oceanic islands, or in great insulated regions like Australia or New Guinea, from the rest of their kind in the temperate regions, nevertheless know and observe the year, and perform all their functions, agricultural or religious, by yearly cycles. For example, there is among them all an annual feast for the dead, and widows mourn their husbands for one year from their burial. Observation of the year, therefore, both automatically by organisms at large and consciously by man, antedates and is independent of observation of the existence of summer or winter.
I do not think, however, that man would have noted the merely astronomical year—the year of the sun's position—at least till a relatively late stage in culture, if he had not first noticed the organic year—the regular recurrence of plant and animal seasons. So many yams—that is to say, so many yam harvests—in other words, so many years, is a common savage way of reckoning times and ages. But they call it "yams," not summers or winters. And when I say yams, I give that merely as a single instance, for elsewhere the "seedtime and harvest" are reckoned indifferently in maize or millet, rice or barley, according to the agriculture of the particular people. Even hunting races know that at certain times of year certain foods abound; and this is true of equatorial savages and equatorial plants or animals, as well as of others.