while by their growth they partially exclude the inner ones from the same influence,—­the rapid growth of the latter being also impeded by any impurity or foreign material washed away from the neighboring shore and mingling with the water that fills the channel between the main-land and the reef.  Thus the Coral Reefs, whether built around an island, or concentric to a rounding shore, or along a straight line of coast, are always shelving toward the land, while they are comparatively abrupt and steep toward the sea.  This should be remembered, for, as we shall see hereafter, it has an important bearing on the question of time as illustrated by Coral Reefs.

I have spoken of the budding of Corals, by which each one becomes the centre of a cluster; but this is not the only way in which they multiply their kind.  They give birth to eggs also, which are carried on the inner edge of their partition-walls, till they drop into the sea, where they float about, little, soft, transparent, pear-shaped bodies, as unlike as possible to the rigid stony structure they are to assume hereafter.  In this condition they are covered with vibratile cilia or fringes, that are always in rapid, uninterrupted motion, and keep them swimming about in the water.  It is by means of these little germs of the Corals, swimming freely about during their earliest stages of growth, that the reef is continued, at the various heights where special kinds die out, by those that prosper at shallower depths; otherwise it would be impossible to understand how this variety of building material, as it were, is introduced wherever it is needed.  This point, formerly a puzzle to naturalists, has become quite clear since it has been found that myriads of these little germs are poured into the water surrounding a reef.  There they swim about till they find a genial spot on which to establish themselves, when they become attached to the ground by one end, while a depression takes place at the opposite end, which gradually deepens to form the mouth and inner cavity, while the edges expand to form the tentacles, and the productive life of the little Coral begins:  it buds from every side, and becomes the foundation of a new community.

I should add, that, beside the Polyps and the Acalephs, Mollusks also have their representatives among the Corals.  There is a group of small Mollusks called Bryozoa, allied to the Clams by their structure, but excessively minute when compared to the other members of their class, which, like the other Corals, harden in consequence of an absorption of solid materials, and contribute to the formation of the reef.  Besides these, there are certain plants, limestone Algae,—­Corallines, as they are called,—­which have their share also in the work.

I had intended to give some account of the Coral Reefs of Florida, and to show what bearing they have upon the question of time and the permanence of Species; but this cursory sketch of Coral Reefs in general has grown to such dimensions that I must reserve a more particular account of the Florida Reefs and Keys for a future article.