286 Uitif( rsH ij of California Fubliiaficnis in Zoology [Vol. 18 



uiinmg their whereabouts? Does this self-induced movement — the 

 locomotion of the egg — count for nothing in its distribution? Is the 

 egg carried along passively by the current ? 



It would seem necessary, in the light of this investigation, to 

 discard completely this dognui of passivity, and to replace it by a 

 conception more in accordance with fact. For Salpa dcmocratica is 

 admitted by all to be a most typical plankton organism, and, if the 

 facts revealed in the foregoing jjages are trustworthy, it is evident 

 that this plankton species, to a very large extent, does control its 

 own distribution. It is not drifted about passively ; it is not a particle 

 in the physical sense and it does not behave as such. How explain 

 the differentials in distribution of the two generations on any such 

 basis ? The hauls were the same ; the currents were the same ; the 

 tides were the same; every conceivable condition of and in the sea 

 was the same, during the collecting of one generation as during the 

 collecting of the other. Yet they were distributed differently. Ob- 

 viously, the activity of the organisms and that alone can have caused 

 the differentials in their distribution. Further, the data strongly 

 suggest that the main type of activity involved is locomotion. If 

 so, it necessarily follows, not only that this plankton species influ- 

 ences its own distribution, but that it does so just as certainly, just 

 as definitely, and by much the same means as does any fish or other 

 animal included under the general term, nekton. 



Salpa probably does not accomplish this by forcing its Avay against 

 a current as does a fish, but the solitary forms manage to get them- 

 selves onto the surface in largest numbers when the temperature of 

 the water is high and to avoid the surface when the temperature is 

 low, while the reverse is true of the aggregate forms. Even granting 

 them to be transported by surface currents, as they doubtless are, 

 these data demonstrate that solitary forms are found for the most 

 part in the warm curi-ents and aggregate forms in the cold currents. 

 Is it not, therefore, as illogical to credit the entire control of their 

 horizontal distribution to the currents as it would be to claim that John 

 Smith had nothing whatever to do about getting himself to New York 

 because he Avas carried there on a Pennsylvania Pullman? 



Foni a strictly biological point of view, it would seem necessary to 

 disregard entirely, as Fowler (1911, 1912) has done, the distinction 

 between plankton and nekton. There seems to be no natural line of 

 demarcation between the two. Surely, there is far less difference in 

 activity between sardines and jelly fishes or the larger copepods than 



