292 COKAL AND ATOLLS 



in the form of large massive rocks which will float buoyantly 

 in the sea. This coral is a species of Astrcea, the zooid of 

 which is a very large one, and nothing but its dead form is 

 ever seen in the islands. The secret of its presence is the 

 fact that it floats to the atoll, and is in reality an intruder 

 upon the island fauna. The reason of its floating when dead 

 is [the presence of the numerous air-spaces, formed in its sub- 

 stance by the growing zooids depositing thin septa at the lower 

 limits of their bodies. Since the zooids are for ever growing 

 upward, they seal off cell after cell, and when they are dead, 

 their mausoleum is a great honeycombed mass of rock, which 

 leaves its old moorings — and after a long life spent in one 

 spot, sets out to sea. It is not likely that this floating coral 

 could be a very useful agent in the colonising of the islands 

 to which it floats, but it might easily increase the coral fauna, 

 for it is a regular thing in coral bionomics that a young colony 

 starts its growth upon an older one. 



These floating objects have almost certainly been the means 

 of introducing a portion of the fauna of the atoll — and the 

 group of water-borne colonists, which are not dependent upon 

 man for their transit, is the one upon whose coming some light 

 may be thrown by an attempt to trace their source. 



Two methods of investigation are open to the dweller on a 

 coral island. He may try to identify the objects which come 

 ashore, and so determine their place of origin ; and he may 

 cast bottles into the water, and try to find out where they go 

 ashore and are picked up. In this way some idea of the 

 ocean drifts which pass the islands may be arrived at ; we 

 may find out where they come from, and whither they go. 



It might be thought that a determination of the species of 

 those plants, the seeds and trunks of which we find washed up, 

 would give the clue to their native land ; but, unfortunately, 

 the information to be derived from this source is not very 

 definite. The distribution of the littoral plants of the tropics 

 is so very wide that even when a seed is definitely diagnosed 

 its exact place of origin cannot, in some cases, be limited even 

 to a continent. It is certain that many of them come from 



