148 W. K. BROOKS. 



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 geographical 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 decompo- 

 sition 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 primi- 

 tive 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 convenient 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. 



The modern pelagic protophytes have probably retained nearly 

 their ancient form, but the modern radiolarians and pelagic fora- 



