vi PREFACE 



their use has been extended. There is of course a 

 very great amount of such work still to be done but 

 we know enough even now to make it well worth 

 while to attempt to make a broad survey of life in 

 the ocean ; to consider the way in which the different 

 kinds of organisms affect each other ; and how they 

 are influenced by the great seasonal changes which 

 sweep across the sea. Such a discussion of the 

 general economy of the sea is attempted in this 

 book. 



It may seem to some readers who possess a first- 

 hand acquaintance with some of the subjects dealt 

 with, that many of the statements made are too 

 dogmatic. It is impossible to give all the evidence 

 that might be brought forward in support of them, 

 and I have therefore added some notes at the end 

 of the book, which indicate the main sources of 

 information. Finally one may admit that there is 

 difference of opinion with regard to some of the 

 views stated, but a good deal of evidence may be 

 adduced in their favour ; and at all events they 

 are permissible views in the present state of our 

 knowledge. 



JAS. JOHNSTONE. 



LIVERPOOL, 



September, 1911. 



