1916] Iiitrofhirtioii xvii 



contradicts this generalization really signifies that the natural vari- 

 ability in plankton distribution is so large that the hydro-biological 

 relations will be masked unless large numbers of searching and fairly 

 continuous hydrographie observations are made which correspond in 

 time and place to net hauls. Says Herdman (1908, p. 56) : 



It is clear that samples taUen quarterly, monthly, or even fortnightly, are 

 quite inadequate to convex' a correct idea of the constitution and changes of 

 the plankton of a sea-area in any detail; and, consequently, conclusions ought 

 not to be drawn from such insufficient observations. Samples taken weekly 

 throughout the year, and almost daily during the . . . most critical months, 

 give by no means too much information, but will probably suffice to enable 

 one to make that detailed comparison between adjacent localities and dates 

 which are necessary for the purpose of determining the representative value 

 of such periodic samples. 



The complexity of hydrobiological problems is well illustrated by 

 Herdman (1907, p. 37). On April 1. 1907, one mile north of Port 

 Erin, Isle of Man, two "exactly similar" surface nets were towed side 

 by side for fifteen minutes. One net (which we shall designate as A) 

 was hauled in, emptied, and put out again at the expiration of the 

 first eight minutes, while the other net (B) was not touched. The 

 first half of the haul with A did not contain a single crab zoea, but 

 the last half contained essentially the same number as the uninter- 

 rupted haul with B did. In both there was "an extraordinary number 

 of crab zoeas, rendering the ends of the nets quite dark in color." 

 This is "incontrovertible evidence of the sharply defined nature of a 

 shoal of organisms . . . [and forms] ... an instructive example of 

 how nets hauled under similar circumstances a short distance apart 

 may give very different results." 



In spite of the many similar instances that might be cited, there 

 seems to be a widespread impression that one, or at most a very few 

 hauls with a net of known filtering capacity justify conclusions con- 

 cerning the number of each species present over large areas of the sea. 

 It is not fully appreciated that the number and kind of organisms 

 caught by any one net haid at any one depth, time, and place is 

 consequent not only upon the volume of water filtered, but upon the 

 temperature, salinity, gas-content, and chemical composition of the 

 water ; upon the effects of w'ind, clouds, fog, rain, snow, ice, light, and 

 season ; and upon many strictly biological influences, such as stage 

 of growth, period of reproduction, food relations, and periodic activi- 

 ties. Though collecting apparatus be as perfect as human ingenuity 

 can devise and the volume of water filtered be known to the smallest 



