THE STRUCTURE OF THE BARRIER 159 



tlie statement that these assumptions would explam is a 

 statement that facts do not justify, and the assumption would 

 appear to be far from true. It is true that some corals pre- 

 dominate on the seaward edge for the very reason that, as the 

 area most commonly exposed to the surf is reached, many 

 species that cannot withstand the rough usage are left behind, 

 and the few dominate the picture. In that area on which the 

 surf beats — the seaward edge of the barrier — living coral 

 growth reaches its minimum of individuals as well as of species. 

 The most luxuriantly growing of all the corals are not even 

 represented far out on the barrier, for the simple reason that 

 they could not withstand the conditions which obtain there. 

 Towards the lagoon the corals increase in number and 

 variety, and in the lagoon itself are some of the most 

 luxuriant of all the growths, branching and laminated forms 

 whose habit of growth does not permit of a successful struggle 

 for existence in the perpetual tumble of rough water. 



It must be borne in mind that these remarks apply only 

 to the rock flat — the simple barrier in its whole extent — in 

 those gaps of the atoll ring in which no islands are formed. 

 Where an island is present, a disturbing factor comes in, for the 

 island is a piling up of debris which rises from the barrier. The 

 nearer the island therefore, the more the barrier becomes ex- 

 posed at low tide ; and although it is by no means true to say 

 that corals cannot withstand an exposure of some duration to 

 the sun and air, still they do not flourish best on any part of 

 the barrier normally exposed at low tide. It is true, therefore, to 

 a certain extent, to say that the farther one goes seaward on 

 the barrier from an island, the more luxurious becomes the 

 growth of coral ; but this is an after-effect of the formation of 

 the island, and must not be used as an argument in a theory 

 to account for its formation. 



I have stated that there is a roughly defined distribution 

 of the types of corals on the barrier, and this distribution is 

 one that is easily accounted for. The conditions of life on the 

 seaward edge of the barrier are peculiar and severe ; the com- 

 petition for life takes place in a perpetual crash and rush of 



