Page:Darwinism by Alfred Wallace 1889.djvu/52

 from the West Indies, which appears to have found in Ceylon a soil and climate exactly suited to it. It now covers thousands of acres with its dense masses of foliage, taking complete possession of land where cultivation has been neglected or abandoned, preventing the growth of any other plants, and even destroying small trees, the tops of which its subscandent stems are able to reach. The fruit of this plant is so acceptable to frugivorous birds of all kinds that, through their instrumentality, it is spreading rapidly, to the complete exclusion of the indigenous vegetation where it becomes established.

The not uncommon circumstance of slow-breeding animals being very numerous, shows that it is usually the amount of destruction which an animal or plant is exposed to, not its rapid multiplication, that determines its numbers in any country. The passenger-pigeon (Ectopistes migratorius) is, or rather was, excessively abundant in a certain area in North America, and its enormous migrating flocks darkening the sky for hours have often been described; yet this bird lays only two eggs. The fulmar petrel exists in myriads at St. Kilda and other haunts of the species, yet it lays only one egg. On the other hand the great shrike, the tree-creeper, the nut-hatch, the nut-cracker, the hoopoe, and many other birds, lay from four to six or seven eggs, and yet are never abundant. So in plants, the abundance of a species bears little or no relation to its seed-producing power. Some of the grasses and sedges, the wild hyacinth, and many buttercups occur in immense profusion over extensive areas, although each plant produces comparatively few seeds; while several species of bell-flowers, gentians, pinks, and mulleins, and even some of the compositæ, which produce an abundance of minute seeds, many of which are easily scattered by the wind, are yet rare species that never spread beyond a very limited area.

The above-mentioned passenger-pigeon affords such an excellent example of an enormous bird-population kept up by a comparatively slow rate of increase, and in spite of its complete helplessness and the great destruction which it suffers from its numerous enemies, that the following account of one of its breeding-places and migrations by the celebrated