1918] Michael: Behavior of Salpa dcmocratica 281 



5. Validity of the Plankton Concept 



This is too large a subject to discuss fully, but the assumption of 

 passivity, and consequently of uniformity, in plankton distribution 

 so permeates the literature as to demand brief consideration in the 

 light of the facts revealed by this investigation. The fundamental 

 tenet of the prevailing plankton concept is, as later demonstrated, that 

 the organism is carried about passively by the currents of the sea ; 

 that the organism plays a negligible part in its own distribution. 

 Virtually, the inanimate is substituted for the animate, and the prob- 

 lem of plankton distribution thus becomes nothing more than a 

 problem in mechanics; a problem resembling that of the distribution 

 of dust in the air, or of salts in the sea ; a complicated problem, per- 

 haps, but none the less a mechanical one. 



This may be better appreciated, perhaps, from an analogy. Rain- 

 drops tend to be uniformly distributed. In any particular region 

 where the physical conditions of the air are the same, approximately 

 the same number of raindrops fall on one square foot of the earth's 

 surface as upon any other square foot, whence a single rain gauge 

 is sufficient to measure quite accurately the total precipitation through- 

 out that entire region. This is common knowledge. It is also com- 

 mon knowledge that the locomotive powers, say of small gnats, are too 

 feeble to permit headway against the wind ; they are carried hither 

 and thither by the currents of the air. Let it be assumed that they 

 are carried passively, that their own activities are negligible, and 

 they must of necessity be distributed in a very similar manner to 

 raindrops, or better to the dust of the air. In other words, wherever 

 in the air the physical conditions were uniform, there also the abun- 

 dance of gnats would approximate uniformity. 



Of course this sounds ridiculous, but it is the unescapable con- 

 sequence of an assumption of passivity. It is only necessary, there- 

 fore, to substitute plankton organisms for gnats and water for air 

 to realize that, if the fundamental tenet of the plankton concept be 

 true, there is no escape from the claim made by Johnstone (1908, 

 p. 157) that "the validity of all conclusions as to the general abun- 

 dance of microscopic life in the sea depends on the truth of the 

 postulate, that wherever in the sea the physical conditions are uni- 

 form, there also the composition and abundance of the plankton is 

 uniform." This postulate of uniformity, although rightly held by 

 many to be absurd, either must be true or else the fundamental tenet 



