Page:Evolution of Life (Henry Cadwalader Chapman, 1873).djvu/193

143

Every one knows that the life of an individual plant or animal depends on a proper supply of food, is affected by changes of climate, and is constantly endangered by disease and enemies; few are, however, aware of the extent to which individual life is dependent on the existence of some other kind of life, and of the extremely complex nature of the struggle for existence. Thus, according to Prof. Haeckel, " There are small oceanic islands whose inhabitants live essentially on a species of Palm. The fructification of these Palms is effected principally through Insects, who carry the pollen from the male to the female Palm. The existence of these useful Insects is endangered through Insect-feeding birds, who in turn are pursued by Rapacious birds. But the Rapacious birds often succumb under the attacks of a small parasitic Mite, which develops by millions in their feathery coats. These small, dangerous Parasites can be killed through parasitic Fungi. Fungi, Rapacious birds, and Insects in this case would favor, Bird-mites and Insect-feeding birds, on the contrary, would endanger, the growth of the Palms, and consequently of men." Thus the existence of entire populations may be indirectly dependent on the presence of a highly insignificant plant or animal form. If one considers the millions of eggs laid by fishes, and that a pair of elephants, the slowest of breeders, would reproduce in five hundred years fifteen millions, the importance of the struggle for existence in checking over-population will be appreciated. According to Mr. Darwin, the red clover never produces seed if the humble-bees be prevented from visiting it. For the bee, in sucking the honey out of the flower, brings the pollen in contact with the pistil, and by this means the clover is fertilized. Now, it is well known that the bees are destroyed by the field-mice, and that the number of