The globes above and below in Fig. 17 show the conditions at the June and December solstices. Since the sun's rays are vertical at the Tropic of Cancer at the June solstice, the sun is nearly overhead in the United States, and a given amount of light and heat is concentrated in a relatively small area. In our winter, on the contrary, the gun is vertical at the Tropic of Capri corn, 47° south of the summer position. Therefore, in all parts of the United States the light falls at a considerable slant, a given quantity is spread over a larger area than in 'summer, and the heating effect is less.
The Varying Distance of the Earth from the Sun.— These three causes of the difference between summer and winter are slightly mod ified by the varying distance of the earth from the sun. But this has only a weak effect upon the relative temperature of summer and winter. In January the earth is about 3,000,000 miles nearer the sun than in July. Hence this period is called perihelion, which means " near the sun," while July is called aphelion, or " far from the sun." These conditions make the winters in the northern hemisphere slightly warmer than they would otherwise be, and the summers correspondingly cooler. In the southern hemisphere, on the other hand, the seasons instead of being tempered are made slightly more extreme. In that hemisphere the earth's varying distance from the sun causes the winters to be a little colder and the summers warmer than would otherwise be the case.
How Plants and Animals Respond to the Seasons.—In the whole realm of nature few conditions of environment equal the seasons in their effect upon life. One of the most obvious results is the revival of vegetation in the spring and its return to a barren state of dor mancy in winter. It is hard to realize the marvelous quality of this change until one lives where there is almost complete uniformity at all seasons as in dry deserts, polar regions, high mountains, and damp tropical forests. Equally marvelous and almost more important is the effect of the seasons upon the production of seeds, fruits, tubers, and practically all the devices by which plants store up either food or water. Without these storage organs man and many animals rang ing from the bear and the squirrel to birds, insects, and worms would be unable to find food.
All sorts of grains, nuts, root crops, and fruits owe their origin primarily to the necessity of storing up food during one season so that the plant may have something upon which to make a start when a period of unfavorable weather is ended and the season of growth has come. In the parts of the torrid zone where there is plenty of moisture at all seasons, seeds and fruits are rare, and man is corre spondingly handicapped in his search for food. In such places the plants grow so fast that many of them can easily reproduce themselves by mere spores such as those of the fern or by the vegetative growth of shoots as in the banyan, banana, and mangrove. In the oceans,
where the contrast between one season and another is reduced to very slight proportions, no seed plants have ever been evolved. What few there are have come back to the water from the land. It is enough for the water plant to send out spores—mere unclothed cells. They do not have to endure the rigors of a long cold or dry season. It is not necessary that they make haste to grow as fast as possible in order to make the most of tilt time when the weather is favorable. So no little plant has to be packed away with its main organs already developed, and no store of food is needed to insure it a good start. Hence the plant life of the ocean has remained at a low level, while the stimulus of variety and especially of the seasons has caused the vegetation of the lands to be highly varied and progressive.
Among animals the effect of the seasons is as marked as among plants. The hibernation of bears, rodents, and insects, the migra tions of birds and fish, the growth and shedding of winter hair or fur, and the putting on of fat at the approach of winter are all responses to the change of seasons. These and other similar changes have much importance for man. Wool, fur, lard, and bacon fat are articles which the animals produce seasonally in order to protect themselves from the winter. In warm countries sheep's wool becomes hair and the hogs are all lean.
The fact that warm-blooded animals, that is, the birds and mammals, are found almost wholly on the lands and are air breathers, even when in the water, appears to be partly due to the seasons. When animals first came out of the water and lived on the land mil lions of years ago, a great advantage was supposely reaped by those. able to warm themselves a little and thus continue their activities in cold weather. Warm blood put a great premium on intelligence and on the development of the higher qualities such as parental care and love for offspring. The cold-blooded animals practically never care for their eggs or young. They do not need to. Among warm-blooded animals, however, if there are cold seasons the eggs must be kept warm and the young must be protected from bad weather. This was apparently one of the primary reasons why the parents took an interest in their young. Little by little the swing of the seasons selected for preservation the types of animals that had these new and higher instincts. This gave a peculiarly good chance for natural selection to preserve those whose brains were most highly developed. Thus along with the parental instinct the development of intelligence was fostered by the seasons.