BIOGEOGRAPHICAL BOUNDARIES 



213 



Rhizosol<2nia styliformis 



Fig. 4. The average distribution and abundance of the diatom Rhizosolenia 

 stvlifonnis in June and November. The charts are based on the combinations 

 of these months during the nine years 1948-1956, in the Continuous Plankton 

 Recorder Survey. Crosses indicate the centers of sampled squares in which Rhi- 

 zosolenia was absent or scarce. The three sizes of circles are drawn at abundance 

 intervals of approximately X3 (G. A. Robinson, unpublished). 



Although most organisms were distributed in a similar way 

 in each successive year, there were differences between years, some 

 of them abrupt and some showing trends of changing distribution. 

 One example is provided by the mollusc Clione limacina which, 

 between 1948 and 1952, was found over the deep waters of the 

 Atlantic. Thereafter, in each successive year, its distribution be- 

 came progressively restricted toward the continental shelf until 

 in 1956 it was almost entirely confined to the shallow waters 

 around the British Isles. Figure 5 shows the two extremes of this 

 trend and, like Fig. 4, offers a convincing argument for the repe- 

 tition of routine sampling over long periods of time. A striking 

 instance of changing distribution on the seashore is provided by 

 the barnacle Elminius modestus in European waters (Crisp, 1958). 



Another aspect of the shape of distributions concerns the sea- 

 sonal timing of the biological cycle of each species. Many organ- 

 isms showed a consistent history with the same seaso?i of abundance 

 in each year and about the same level of abundance in each season. 

 But some species underwent emphatic changes such as those il- 



