1911] Michael: Chaetognatha of the San Diego Region. 103 



second haul, 1872, made with the same net at the same location 

 and lasting from 7 :40 to 8 :15 p.m. contained 4300 of the same 

 species. In this instance • the number of specimens per unit 

 volume of water was approximately 1 to 430 but, during the 

 first haul, the temperature of the water was 19° C. and the 

 salinity 33.589, while during the second haul the temperature 

 was 18?8 C. and the salinity 33.630. Obviously a change of 0?2 

 in temperature and 0.041 in salinity took place within 37 min- 

 utes, and these changes possibly involved an alteration in cur- 

 rent and other physical conditions of the w^ater. 



Other instances might be cited, but enough have been given 

 to show that the surface distribution of S. hipunctata is not 

 constant for any length of time even in very small areas. The 

 objection will be made that hydrographic and meteorological 

 conditions change rapidly near the coast but remain constant 

 on the high seas. I doubt the validity of such an objection. In 

 the first place, owing to variations in wind, rain, light, baro- 

 metric pressure, heat, etc., it is very improbable that hydro- 

 graphic and meteorological conditions even approach constancy 

 on the high seas. In the second place, some of the above 

 examples show that *S'. hipunctata varied in abundance even 

 when these conditions, so far as known, remained constant dur- 

 ing the period within which the contrasted hauls were made. 



Hydrographic and meteorological conditons of the region in 

 which collections are made certainly affect the quantity of 

 plankton present, but does this warrant the assumption that 

 other conditions do not also affect it? According to Johnstone 

 (1908), who has championed the cause of the "Kiel school," 

 we are justified in making this assumption. He says (p. 157) : 

 "The validity of all conclusions as to the general abundance of 

 microscopic life in the sea depends on the truth of the postulate, 

 that wherever in the sea the physical conditions [italics mine] 

 are uniform, there also the composition and abundance of the 

 plankton is uniform." 



The truth of this postulate depends upon what is implied by 

 the term "physical conditions." Obviously Johnstone does not 

 mean merely physical conditions for, without chemical and 



