50 Transactions.—Miscellaneous. 
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 astreans 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 
& 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 
