io6 



DISCOVERY REPORTS 



travels south-eastwards across the line of the isotherms he will meet with important 

 changes, especially while crossing a belt running approximately from the South Shetland 

 Islands to South Georgia. This zone of rapid qualitative change will be mentioned again 

 in a later section. The point to be emphasized at the moment is that the chart indicates 

 a connection between the distribution of species and the surface temperature of the 

 water within the Antarctic Zone. This temperature ranges from nearly - 2° C. to about 

 + 5 C, and a quantitative comparison of the different species in respect of their 



Fig. 22. Approximate positions of the mean summer isotherms. 



occurrence in water of different temperatures will provide a clearer distinction between 

 the warm- and cold-water species than is given in the notes on general distribution. 



It must be supposed that plankton organisms which have an optimum temperature 

 cannot make active movements to avoid changes in temperature caused by the seasons, 

 the weather, the approach and departure of ice, etc. We should therefore expect more 

 reliable results from the mean temperature of the locality than from the actual tempera- 

 ture of the water in which a species is taken. It would be desirable if possible to compare 

 the distribution of a species with the temperature of the water at the mean depth at 

 which it lives. But at a great many of the stations at which the N ioo B was used the 



