Page:Popular Science Monthly Volume 43.djvu/233

Rh prevalence on our river banks of oaks, hickories, and maples is also very noticeable. Again, ocean currents are of great importance in distributing plants. The cocoanut, buoyed by its loose husk and protected by an impenetrable shell, floats in the sea until it is brought often to some coral island where it may grow. Many small seeds are also conveyed by ocean currents, and it is very probable that they retain their vitality, for Mr. Darwin has recorded some interesting experiments showing that a good proportion of seeds can withstand injury from salt water for a considerable length of time. The action of freezing water, as manifested in frost, has the well-known effect of freeing nuts from their protecting envelopes; and frozen water, in the shape of glaciers and icebergs, is of a little importance in transporting seeds. It is possible that during the Glacial period seeds were conveyed from place to place incased in ice.

Of all devices for dispersion the most remarkable are those by which the aid of animals is secured, and this aid is so valuable that plants spare no expense to obtain it. Usually animals are well paid for their services, but many plants, however, do not hesitate to deceive their benefactors by all sorts of trickery. This latter class, though, has not been nearly as successful as the others in the struggle for existence.

It is now well known that what are popularly called "fruits" exist for the mutual benefit of plants and the lower animals—not for man. And it is generally believed that these fruits have developed their attractive qualities through natural selection. The results reached by man in selecting and propagating the best varieties of fruits are the strongest grounds for thinking that these fruits were once evolved from very crude conditions through similar selection by the lower animals, particularly birds. Such fruits, for instance, as by natural variation became at all agreeable to birds would be sought out by them, to the exclusion of less attractive fruits. In consequence, the favored fruits would stand better chances of setting seeds than would their less favored companions. Variations being transmitted from parent to off-spring, it is reasonable to suppose that favorable variations would become still more favorable by further selection, until, by the accumulation of even slight variations through geologic ages, there would result fruits highly attractive to certain animals by their color, perfume, and taste. In the mean time, fruits possessing unfavorable characteristics have for this very reason been exterminated, or else have attained a less degree of success than the others.

Insects are the lowest animals known to assist in seed dissemination. Mr. Darwin tells us of locust excrement containing seeds which grew when planted. Considering that locusts often