All this meant that the young animals became more and more dependent upon the mothers. Hence when types that placed the young in pouches were developed, they had an advantage in the struggle for existence because the young could be protected not only from enemies but from the inclement weather. The last step was the evolution of true mammals whose helpless young are born alive. Their evolution, so far as we can tell, took place chiefly in the great continental interiors where the contrasts of the seasons are greatest, and where the rigors of winter are among the most powerful factors in eliminating many types and preserving those whose intelligence is relatively high.
In the oceans nothing of this sort has taken place, for there the almost complete uniformity from season to season has not favored the evolution of the higher types. When the higher types go back to the monotony of the oceans, as the whale has done, the lack of seasonal stimulus joins with the uniformity of the environment in other respects in causing them to lose their higher capacities. Thus the seasons have much to do with the fact that the oceans are the home of low, cold-blooded forms of animals as well as of low, spore-bearing types of plants, while the lands and especially those parts with strong seasonal contrasts, are the home of the highly developed mammals, birds, and seed-bearing plants.
How Man Responds to the Seasons.—Among men the influence of the seasons is no less than among plants and animals. There is scarcely an occupation which does not vary according to the seasons. With farming this is preeminently true. A farmer who has few live stock,—and there are millions of such,—has practically nothing to do during the winter. If snow lies on the ground or the soil is frozen, time often hangs heavy on his hands. In the summer, on the other hand, in spite of the long days he is busy every moment and his work often piles up ahead of him. With students and most people who are engaged in literary and scientific pursuits quite the opposite is true. In winter, when daylight is short, they often injure their eyes by poring over books from morning till midnight. In summer
when the long days are best for study so far as light is concerned, although not necessarily otherwise, they frequently spend weeks or months with little or no study. Between the farmers and the students are people upon whom the seasons have almost every degree of effect. The railroad man, the manufacturer, the banker, the carpenter, and the hardware merchant all have busy seasons and slack seasons at regular times of the year. Moreover, the character of their work varies from season to season. Health and recreation vary similarly, for people generally have the best health in the autumn, while such games as hockey and football are rarely played except at certain seasons. The difficulty is not to find examples of seasonal varia tions, but to find occupations or activities upon which the seasons have no effect. And all these seasonal activities depend directly or indirectly upon the differences in weather arising from the inclina tion of the earth's axis.
How the Seasons have Helped to Civilize Mankind.—Without the seasons mankind might perhaps never have become civilized. When early man began to rely on his mind instead of on physical strength, one of his first important ideas was to store up food for seasons of scarcity. So long as he lived by hunting this was relatively unimportant, but as soon as he began to practice farming he could not live unless he stored up food in summer to last him through the winter. In temperate regions with strong seasonal changes this was far more necessary than in warm regions with no real winter. Moreover, the strong contrast between the seasons stimulates him to be active and energetic not only in storing up food, but in making new inventions. In every stage of life those people are most success ful who plan intelligently for the future which lies months or even years ahead of them. The inclination of the earth's axis and the resultant seasons have been one of the chief incentives to this kind of foresight.