21 6 DISCOVERY REPORTS 



distribution is continuous in the sense that there are no longitudes from which the species are excluded. 

 This is not quite the same as to say that each species is represented by a single completely continuous 

 circumpolar population throughout the year, for it is not impossible that at a time of year when the 

 numbers of a species are reduced in the surface layer the population becomes broken up into isolated 

 local stocks. It seems safe, however, to conclude that there is no persistent discontinuity in any 

 longitudes. The distribution of the samples in time and space hardly leaves room for the possibility 

 of several discrete populations drifting round through all longitudes but remaining separate from each 

 other, and in any case it would be difficult to conceive of any mechanism in the water movements by 

 which they could be kept permanently isolated from one another. 



To demonstrate on taxonomic grounds alone that there can be no reproductively isolated population 

 of a species in different parts of the Antarctic it would be necessary to prove that there are no sub- 

 specific differences in different sectors ; and in this connexion it must be remembered that in a wide- 

 spread species distance alone may constitute a sufficient barrier to bring about genetic isolation and 

 the development of subspecific differences between remote parts of the range, even in a continuous 

 system of populations. No proof is offered here that the representatives of each species are everywhere 

 subspecifically identical, but it can be said that no such differences have been observed, and in view of 

 the uniformity of the properties of the Antarctic surface layer in a circumpolar direction, and the 

 absence of any known environmental barriers which might interrupt the continuity, there is good 

 reason to believe that each species has a circumpolar population forming a single unit in which little 

 if any geographical variation can be expected. This does not of course rule out the possibility of two 

 or more concentric circumpolar populations occupying different zones, partially isolated from one 

 another and perhaps exhibiting subspecific differences. 



Very much the same can be said of the species of phytoplankton. Here rather more gaps appear in 

 the histograms (in Fig. 2) than in those for the zooplankton, as Dr Hart adopted a rather more 

 rigorous method of selecting the samples. The distribution of all the species can be described as 

 circumpolar, it is certainly or almost certainly continuous in most of them (at least in the period of 

 relative abundance), and it is highly probable that apparent gaps in the remainder would disappear 

 with further sampling. 



The species dealt with in this paper do not include the smaller zooplankton forms, but they are an 

 arbitrary selection of the total plankton in the sense that they are the common species taken in certain 

 types of nets, and they include representatives of many taxonomic groups with different habits and 

 modes of life. Evidence has been given that all of them are circumpolar, and there is at least a strong 

 expectation that circumpolar continuity is a general rule for oceanic plankton species of the Antarctic 

 surface layer. 



SUMMARY 



It has been suggested that, as the Southern Ocean is an uninterrupted circumpolar belt with more or 

 less uniform conditions prevailing in east and west directions, the range of planktonic species of the 

 Antarctic surface water may be expected to extend as far as these uniform conditions persist, i.e. to 

 be circumpolar. 



Dr Hart has shown this to be true of the important phytoplankton species and discusses the reasons 

 for the few gaps that appear when the selected data are plotted. 



The same is shown for the larger zooplankton species and an indication of the degree of accuracy 

 of the specific identification is given. 



Histograms are drawn which demonstrate the occurrence of each species in a circumpolar direction 

 and show the percentage of samples in which the species occur. 



