No. 647] PROGRESSION OF LIFE IN THE SEA 489 



posterior end carries the nucleus and most of the chloro- 

 phyll and food reserves. The whole organism has a 

 specific gravity of 1.016, being slightly heavier than the 

 fresh water in which it lives. When dead, or when the 

 flagellum is not moving, it takes up, under the action of 

 gravity alone, a vertical position in the water, with the 

 pointed anterior end uppermost, and the heavier, rounded, 

 posterior end below, and sinks gradually to the bottom. 



In a very crowded culture a curious phenomenon is 

 seen, because the organisms tend to aggregate into clus- 

 ters beneath the surface film, and when they are crowded 

 together in these clusters the flagella cease to work. This 

 makes the whole cluster sink to the bottom under the 

 action of gravity. When the bottom is reached the in- 

 dividuals are spread out by the action of the downward 

 current, and, when they are sufiiciently widely apart, the 

 flagella again begin to move, carrying the organisms in 

 a more diffuse stream once more to the surface. The 

 whole culture vessel becomes filled with a series of ver- 

 tical lines of closely aggregated falling organisms, sur- 

 rounded by a broad cylinder of disseminated swimming 

 ones, rising to the surface by the action of their flagella. 

 If the conditions are kept uniform, such a circulation of 

 Euglenas, falling to the bottom by gravity wlion tho fla- 

 gella are stopped and returning to the snifact' iiiulcr 

 their owti power, will continue for days. 



The flagellum in this species, therefore, retains its 

 most primitive function of drawing the organism to the 

 light in the surface layer. With the establishment of the 

 flagellum an organ is produced which shows remarkable 

 persistence in both the animal and vegetable kingdoms, 

 and from the existence of the flagellated spermatozoon in 

 the higher vertebrates, in accordance with Haeckel's bio- 

 genetic law that the individual in its development repeats 

 or recapitulates the history of the race, we conclude that 

 they also in their earliest history passed through a plank- 

 ton flagellate phase. 



Exactly at what stage in the history of the autotrophic 

 flagellate the first formation of chlorophyll and its allied 



