Page:Geology and Mineralogy considered with reference to Natural Theology, 1837, volume 1.djvu/338

 334 Polype they remain permanently attached to the bottom. Thus one generation establishes the basis whereon the next fixes its habitation, which is destined to form the foundation of a further and continual succession of similar constructions, until the mass, being at length raised to the surface of the sea, a limit is thereby put to its further accumulation.

The tendency of Polypes to multiply in the waters of warm climates is so great, that the bottom of our tropical seas swarms with countless myriads of these little creatures, ever actively engaged in constructing their small but, enduring habitations. Almost every submarine rock, and submarine volcanic cone, and ridge, within these latitudes, has become the nucleus and foundation of a colony of Polypes, chiefly belonging to the genera Madrepora, Astrea, Caryophyllia, Meandrina, and Millepora. The calcareous secretions of these Polypes are accumulated into enormous banks or reefs of coral, sometimes extending to a length of many hundred miles; these continually rising to the surface in spots where they were unknown before, endanger the navigation of many parts of the tropical seas.

If we look to the office these Polypes perform in the present economy of nature, we find them acting as scavengers of the lowest class, perpetually employed in cleansing the waters of the sea from the impurities which escape even the smaller Crustacea; in the same manner as the Insect Tribes, in their various stages, are destined to find their food by devouring impurities caused by dead animal and vegetable matter upon the land. The same system