746 



SCIENCE 



[N. S. Vol. XXX. No. 778 



ing a numerical accuracy which is not yet 

 attainable. The details of measurement of 

 catches and of computations of organisms 

 become useless, and the exact figures are 

 non-significant, if the hauls from which 

 they are derived are not really comparable 

 with one another and the samples obtained 

 are not adequately representative of na- 

 ture. If the stations are so far apart and 

 the dates are so distant that the samples 

 represent little more than themselves, if 

 the observations are liable to be affected by 

 any incidental factor which does not apply 

 to the entire area, then the results may be 

 so erroneous as to be useless, or worse than 

 useless, since they may lead to deceptive 

 conclusions. It is obvious that we must 

 make an intensive study of small areas be- 

 fore we draw conclusions in regard to 

 relatively large regions, such as the North 

 Sea or the Atlantic Ocean. Our plankton 

 methods are not yet accurate enough to per- 

 mit of conclusions being drawn as to the 

 number of any species in the sea. 



The factors causing the seasonal and 

 other variations in the plankton already 

 pointed out may be grouped under three 

 heads, as follows: (1) The sequence of the 

 stages in the normal life history of the 

 different organisms; (2) irregularities in- 

 troduced by the interactions of the differ- 

 ent organisms; (3) more or less periodic 

 abnormalities in either time or abundance 

 caused by the physical changes in the sea, 

 which may be grouped together as 

 "weather." 



[Illustrated by diagrams.] 



These are all obvious factors in the prob- 

 lem, and the constitution of the plankton 

 from time to time throughout the year must 

 be due to their interaction. The difficulty 

 is to disengage them from one another, so 

 as to determine the action of each sepa- 

 rately. 



Amongst the physical conditions coming 



under the third heading, the temperature 

 of the sea is usually given a very prom- 

 inent place. There is only time to allude 

 here to one aspect of this matter. 



It is often said that tropical and sub- 

 tropical seas are relatively poor in plank- 

 ton, while the colder polar regions are 

 rich. In fishing plankton continuously 

 across the Atlantic it is easy from the 

 collections alone to tell when the ship passes 

 from the warmer Gulf Stream area into the 

 colder Labrador current. This is the re- 

 verse of what we find on land, where luxu- 

 riant vegetation and abundance of animal 

 life are characteristic of the tropics in con- 

 trast to the bare and comparatively lifeless 

 condition of the arctic regions. Brandt 

 has made the ingenious suggestion that the 

 explanation of this phenomenon is that the 

 higher temperature in tropical seas favors 

 the action of denitrifying bacteria, which 

 therefore flourish to such an extent in 

 tropical waters as to seriously diminish the 

 supply of nitrogen food and so limit the 

 production of plankton. Loeb,^ on the 

 other hand, has recently revived the view 

 of Murray, that the low temperature in 

 arctic waters so reduces the rate of all 

 metabolic processes, and increases the 

 length of life, that we have in the more 

 abundant plankton of the colder waters 

 several generations living on side by side, 

 whereas in the tropics with more rapid 

 metabolism they would have died and dis- 

 appeared. The temperature of the sea- 

 water, however, appears to have little or 

 no effect in determining the great vernal 

 maximum of phyto-plankton. 



Considering the facts of photosynthesis, 

 there is much to be said in favor of the 

 view that the development and possibly 

 also the larger movements of the plankton 



" " Darwin and Modern Science " ( Cambridge, 

 1909), p. 247. 



