No. 647] PROGRESSION OF LIFE IN THE SEA 



491 



We have tried to picture the gradual building up from 

 elements occurring in sea-water of a chlorophyll-bearing 

 flagellate, capable of manufacturing its own. nourishment 

 and able to multiply indefinitely by the simple process of 

 dividing in two. If we assume only one division during 

 each night as a result of the day's work in accumulating 

 food material, such an organism would be able in a com- 

 paratively short space of time to occupy all the natural 

 waters of the world. But here we are met by a difficulty 

 which is not easily overcome. Chlorophyll, the photo- 

 catalyst, the most essential factor in the building up of 

 the new organic matter, is itself a highly complex organic 

 substance, and in any satisfactory theory its original for- 

 mation and its constant increase in quantity must be 

 accounted for. Lankester'° has maintained that chloro- 

 phyll must have originated at a somewhat late stage in 

 the development of organic life, and has suggested that 

 earlier organisms may have nourished themselves like ani- 

 mals on organic matter already existing in a non-living 

 state. An alternative hypothesis, which in view of the 

 recent work seems more attractive, is to suppose that the 

 earlier organisms were either activated by some simpler 

 photocatalyst, or that they received the necessary energy 

 at suitable frequency directly from some outside source. 



It must not be forgotten, also, that at the time these 

 developments were taking place the conditions of the en- 

 vironment would in many ways have been different from 

 those now existing in the sea. One suggestion of special 

 interest that has been made^' is that the concentration of 

 carbon dioxide in the atmosphere, and hence also in nat- 

 ural waters, was very much greater than it is to-day. 

 Free oxygen, indeed, may have been entirely absent, and 

 all the free oxygen now present in the air may owe its 

 existence to the subsequent splitting up of carbon diox- 

 ide by the action of plant life. With such possibilities 

 of differences in the conditions in this and in so many 



1912, p. 107: 



