BIOGEOGRAPHICAL BOUNDARIES 209 



liminary investigations suggest that it is more likely that they 

 were species with a number of geographically separate popula- 

 tions. 



Darlington (1957) pointed out that "wherever regional faunas 

 meet or are separated by partial barriers, there is transition, over- 

 lapping of faunal elements, with progressive subtractions in both 

 directions." He classified organisms as exclusive (belonging to one 

 fauna), transitional (belonging mostly to one but extending into 

 the neighboring fauna), and shared (belonging to both faunas). The 

 terms used in this list, oceanic, neritic, intermediate, and unclassi- 

 fied, can be related approximately, to fauna A, fauna B, transi- 

 tional, and shared in Darlington's Fig. 53. It is extremely difficult, 

 however, to define separate faunal or floral groups in the plankton ; 

 transition occurs in all directions within and between the oceanic, 

 neritic, and intermediate groups of organisms. The boundary be- 

 tween oceanic and neritic is usually read as the position of the 

 100-fathom depth contour. This criterion has been relaxed some- 

 what, in preparing this list, in order to preserve a distinction be- 

 tween those organisms found only over the North Sea plateau 

 and those found mainly over the Atlantic shelf to the west of the 

 British Isles. Some of the distributions described as intermediate 

 would probably be classified as neritic by other workers. Braarud 

 et al. (1953) and Smayda (1958), In their biogeographlcal studies 

 of marine phytoplankton, also found neritic and oceanic to be 

 unsatisfactory terms. Smayda came to the conclusion that they 

 should be discarded as being ill-defined and unrelated to the 

 factors controlling the distribution or the life cycle of the phyto- 

 plankton species. He proposed, instead, a binary classification 

 based on the life cycle (meroplanktonic or holoplanktonic) and on 

 the distribution. He classified distributions as anoictic (Greek, in 

 the open sea), paractic (Greek, of the coast) and adiaphoric (Greek, 

 indifferent). The operation of this scheme is dependent on the 

 recognition of those waters "modified by coastal Influence" or 

 "free from coastal influence." It Is doubtful whether the present 

 state of knowledge is sufficient to render this distinction less 

 ambiguous than the pre\'ious division between oceanic and neritic. 

 The three divisions used In the pre&ent series, however, are prob- 



