W. K. BROOKS ON THE GENUS SALPA. 157 



diffused through the ocean from the surface, where it is absorbed from 

 the air, is gradually exhausted by oxidizable substance, both inorganic 

 and organic, and it diminishes with the distance from the source of 

 supply at the surface. The oceanic circulation tends to equalize its dis- 

 tribution, and no part of the ocean now seems to be totally without 

 oxygen. Oxygen has been shown to be reduced to a minimum at the 

 bottom of some of the great depressions of the sea-floor, and it is clear 

 that a slight change in the conditions which influence it might render 

 the sea-bottom unfit for life. 



In early palaeozoic times the sea-floor was perhaps more level than it 

 is now, and there may have been no deep hollows like those in which the 

 oxygen is now found to be deficient, but the average depth must have 

 been considerably greater, when all the water which is now locked up in 

 the sedimentary rocks of the bottom and of the shores was still free in 

 the ocean. The circulation may also have been less active when geo- 

 graphical conditions were more simple, and the air was undoubtedly less 

 rich in oxygen in early palaeozoic times than it is at present. 



It is therefore easy to understand that long after the crust of the 

 earth had acquired essentially its present character, there may have been 

 a period when the supply of oxygen was so scanty that the activities of 

 pelagic organisms and the products of their decomposition used it up in 

 the surface water, so that life on the bottom was impossible at a time 

 when the superficial water supported a luxuriant fauna and flora. 



During this period the proper conditions for the production of large 

 and complicated organisms did not exist, and while the total volume of 

 life was probably very great, it consisted of the organisms of minute size 

 and simple structure which I have termed the primitive pelagic fauna 

 and flora. 



The Primitive Pelagic Fauna. 



In using this term I do not, of course, intend to imply that these 

 organisms are the beginning of life, or to express any opinion as to the 

 way in which life first came into existence. I use it merely as a con- 

 venient designation for the total sum of the organisms which have been 

 evolved by purely pelagic influences from a starting-point which is 

 absolutely unknown at present. 



The attempt to reconstruct in imagination the primitive pelagic 

 fauna and flora is most fascinating, but all the available evidence is 

 indirect, and as we can have little hope of finding any record of it in the 

 rocks, we must trust to deduction rather than observation. 



