100 CORAL AND ATOLLS 



barrier must of necessity cause great changes in the life- 

 surroundings of the corals in its passage ; and so a colony, 

 found in a calm pool, may for a part of its life be exposed to 

 violent wave action. This must always be borne in mind, for 

 the results obtained by careful collecting will not be pure. 

 Corals may be found of different forms, growing in close 

 proximity ; but far from being evidence that they are different 

 species, and not mere varieties, they demonstrate the fact that 

 the physical conditions of their surroundings are inconstant, and 

 that, for a cycle, each form is, in its turn, the most suitable. 



The very inconstancy of the environment is one factor in 

 showing the plasticity of the corals, for the partial death and 

 repair caused by changed conditions afford striking evidence of 

 the wonderful powers of varied building possessed by the zooids. 

 Experiment with constant physical surroundings must be 

 the ultimate test of variability, and in this connection a very 

 interesting case may be quoted. In the lagoon, a large 

 portion of a tree-trunk was floated, and made fast to an anchor 

 and chain ; the wood was used to float a ship's moorings, and 

 remained just two years in the water. When it was removed 

 in 1906, several colonies of Focillopora had started growths 

 upon it, and they had taken up different positions around its 

 circumference. The colonies growing above were flattened 

 bosses ; those on the sloping sides showed more tendency to 

 branch ; and those below its convexity were delicate branched 

 forms. 



Now the environments of these colonies were very different, 

 and they were absolutely constant ; at all states of the tide, 

 waves broke upon the upper surface of the log, whilst the sides 

 were in gently moving unbroken water, and the bottom was in 

 comparative calm. The growths might be referred to many 

 so-called species, and they represent many types found in the 

 atoll, and yet no one may justly doubt that they are identical, 

 and that their vegetative growth is entirely the outcome of 

 their differing environment. I believe that this natural ex- 

 periment indicates the lines along which the real understanding 

 of the "species" of the corals is to be arrived at. 



