TEMPERATURE 193 



of swimming a little, are closely dependent on the 

 bottom of the deep sea either for food or for resting. 

 For their capture tow nets are attached to the trawl- 

 heads or dredge, streaming a little behind them so as 

 to catch the animals disturbed by raking up the 

 bottom. 



The Plankton, then, is not always the same every- 

 where, but the reasons why it is different in different 

 places are only partly understood ; it seems, however, 

 certain that the temperature has more to do with this 

 than anything else. Some species seem able to stand 

 much greater extremes of temperature than others ; 

 but for every species there is a (maximum) temperature 

 above which it cannot live, a (minimum) temperature 

 below which it cannot live, and an (optimum) tempera- 

 ture at which it thrives best. To take a well-known 

 case, characteristic animals of the Gulf Stream and 

 North Atlantic Drift rarely reach as far east as our 

 islands, although the actual water in which they 

 lived travels as far as the North Cape ; they die out as 

 the temperature falls. The slaughter is particularly 

 heavy where hot and cold currents meet. 



Supposing that we knew precisely the maximum and 

 minimum temperatures of a species — say 6o° and 45 ° F. 

 for a North Atlantic species — then by drawing lines on 

 the map through all the points where 6o° and 45 F. 

 occurred at the surface, we couid run a surface boundary 

 round the animal, and map its " horizontal distri- 

 bution." Further, if this species can tolerate so low 

 a temperature as 45 ° F. at the surface, there seems to 

 be no reason why it should not live at 45 F. at any 

 point below the surface ; consequently, in addition to 

 the horizontal distribution, we want to be able to 

 map also the distributions at different depths, and 

 thus the space occupied by the species will form an 



13 



