CHARACTERISTICS OF ASSOCIATIONS 15 



departure from this balance will result in changes in the 

 association. Great variation will be found in the relative 

 representation of the various phyla in different associations, 

 according to their type. In rocky pools Coelenterates 

 may be well represented owing to the presence of a suitable 

 substratum for attachment ; on sand they may be practically 

 (or even entirely) absent ; in both, Molluscs will probably 

 be numerous, but in the former case Gasteropods, and in 

 the latter Lamellibranchs, will predominate. In the pool 

 there will be Anemones and creeping Molluscs ; in the 

 sand, bur rowers. 



THE LITTORAL AREA IN RELATION TO THE EVOLUTION 

 OF LIFE 



The problem of the evolution of life has for long exercised 

 the minds of scientists, and in view of the importance 

 usually attached to the shore in this connection it may be 

 useful to give it a small share of our attention. 



Amid much doubt and disagreement one fact remains 

 certain, namely, that life began in the water : whether in salt 

 or fresh water has not been decided. The earliest forms 

 of life probably were nitrifying bacteria, since they alone 

 are capable of deriving their energy and nutrition from 

 inorganic chemical compounds and are independent of 

 the sun or of other forms of life. It is less the actual origin 

 of life which concerns us, however, than the manner in which 

 it was able to spread from its original home and has come 

 to occupy the various habitats we have been dealing with 

 above. Of this, various explanations have been put forward 

 based upon the varying conceptions of the conditions 

 prevalent at the time when the temperature of the earth 

 had dropped to the highest limit at which we believe life 

 to be possible. Some of these may now be quoted. 



Church (1919) believes ** the beginnings of Botany are 

 in the sea," the first phase of plant life being pelagic. Church 

 conceives of an early world entirely covered by water — 

 a world-ocean some two miles deep in which life was 



