into two, and these again divide in a process called fission, and as each mouth is formed the polypifer becomes larger. This is the mode by which astrœans increase in bulk. The porites increases by budding, and the bud becomes a mouth and tentacles, and each species has its peculiar mode of growth. The number of animals in one mass, produced from a single polyp, is many millions, and when the many-coloured tentacles are all spread over a surface of living coral, the effect is said to be very beautiful. There is nothing more wonderful in a polyp secreting coral than in an oyster secreting his shell, or the higher animals their bones. All alike are largely, or for the most part, composed of carbonate of lime, whether animal bones, oyster shells, or polyp coral. But that a small gelatinous animal should raise a barrier, in the midst of dashing waves, against the force of an ocean current, is one of the most wonderful examples of the power of vital over the most mighty mechanical force. The polypifera always commence the reef in the neighbourhood of some land;—if the shores are steep, it must build near the shore, but if the land slopes gently, water will be shallow for a good distance from shore, and the reef-builders must therefore commence further from land in water ten or twelve fathoms deep. In due time the distance between the reef and the shore will become almost filled up with the different varieties of coral. If all coral reefs were of this description they would not have excited so much interest, but by far the greater number of reefs differ very essentially from this description. Instead of the reef being fifteen fathoms thick, it is often 200 fathoms on the ocean side. Instead of being close to a steep shore, it is often many miles away from it; instead of the water within the reef being shallow, it is often more than forty fathoms deep. In order to account for these reefs the most contradictory theories were advanced by naturalists until Mr. Darwin studied the structure of these reefs when he accompanied the English exploring expedition under Captain Fitzroy. After comparing together hundreds of islands with coral reefs, both in the Indian and Pacific Ocean—after taking soundings within the reef and without—after an amount of labour in surveying, mapping, and collecting information that an ordinary person would shrink from, he arrived at a very simple explanation of the scientific problem. He observed that where islands had not reefs close to the shore or fringing reefs, that the reefs varied in distance from the land, and that, as a rule, the size of the enclosed island diminished in proportion to the distance of the reef; and that the island also became lower in height. Some reefs again surround one solitary rock, as in Nanuka, in the Fijis; others, again, enclose a shoal; and, lastly, similar reefs have been found by sounding to be at present submerged many fathoms deep. It was also