496 THE AMERICAN NATURALIST [Vol. LVI 



progress of animal life, we must pause for a moment to 

 ask in what direction plant life in the sea developed, 

 from which the increasing animal life derived its nourish- 

 ment. Here the striking fact is the lack of progress in 

 the free, floating, plankton phase. The plant life of the 

 plankton has never proceeded beyond the unicellular 

 stage, for the plankton diatoms, which with the peri- 

 dinians form the great, fundamental vegetable food sup- 

 ply of the sea, are only autotrophic flagellates which have 

 lost their flagella, having acquired other means of flota- 

 tion to keep them in the sunlit region of the upper water 

 layers. Deriving their food, as these plants do, directly 

 from molecules in the sea-water, the factor which is for 

 them of supreme importance is the exposure of maximum 

 surface directly to the water. Hence the minute unicel- 

 lular form has been the only one to survive as phytoplank- 

 ton. The marine region in which plant life has succeeded 

 in making some progress is the narrow belt along the 

 shores, where a fixed life is possible, but this belt, limited 

 by the amount of light which penetrates, extends only to 

 a depth of about 15 fathoms. The available area is 

 further restricted to rocky and hard bottoms, and is 

 therefore nowhere great. This is the wave-lashed region 

 of the brown and red seaweeds. In the brown seaweeds 

 a history can still be traced,^*' from the fixture of an auto- 

 trophic flagellate to the building up, by laying cell on 

 cell, of the essential structures which afterwards, on trans- 

 migration to the land, reached their climax in the forest 

 tree. 



But if the flagellate thus rose and gave origin to the 

 flora of the land, it also degenerated, for it adopted a 

 parasitic habit, living in and directly absorbing already 

 formed organic matter. In this way the bacteria arose, 

 whose activities in so many directions influence the life 

 of to-day. This view exceeds in probability, I think, the 

 suggestion often put forward,'' that it is to the simpler 

 bacteria we must look for the first beginnings of life. 



16 Church, Botanical Memoirs, No. 3. Oxford, 1919. 



i-Osborn, "The Origin and Evolution of Life," 1918. ^ Waksnian and 



