266 DISCOVERY REPORTS 



The marked difference in the shallow-water faunas of the two areas represented in 

 groups (i) and (2) is not surprising considering that they are separated from each other 

 not only by the sudden hydrographical change represented by the Antarctic Con- 

 vergence but also by Drake Strait. This channel of water, which reaches a depth of over 

 2000 fathoms, may prove to be a barrier of even greater importance than the con- 

 vergence to the bottom-living forms. It is interesting to note that this difference in the 

 shallow-water faunas of these two areas has been noted by other investigators. Barnard, 

 for example, states that the species of Amphipods in the two areas are entirely different. 



An examination of the geological history of the area shows that, during Eocene times, 

 the South Polar Continent, Antarctica, was united on the one hand with South America 

 through Graham Land, and on the other with Australia. It is probable that the genus 

 Serolis was already represented in the shallow- water fauna of the northern coast of this 

 southern land-mass, and that its centre of distribution lay in the waters off the shores of 

 Graham Land. This seat of origin would account for the northward spread of the genus 

 and the almost entire absence of species from the shores of the west coast of South 



America. 



During the Older Quaternary period, Australia broke away from the main southern 

 continent, and this resulted in the isolation of certain species of Serolis, the further 

 modification of which may be seen in the species of Australia to-day (group (4) above). 

 It is significant that the existing species are found off the coast of South-east Australia 

 in a ret^ion which was the last to lose its connection with the southern continent. 



In the course of time, the formation and gradual deepening of the channel between 

 South America and Graham Land would be sufficient to account for the separation of, 

 and variation in, the species found in the shallow waters represented by the areas in 

 groups (i) and (2) above. 



It is possible that the presence of the same species in the two localities represented by 

 groups (2) and (3), may be accounted for by the fact that conditions of life are much the 

 same in the two areas. As already mentioned, the southern shores of Kerguelen lie 

 within the convergence, and Marion Island, and Crozet Island, are respectively 120 

 and 160 miles north of it. According to information supplied to me by Mr G. E. R. 

 Deacon of the Discovery scientific staff, the two latter islands are so close to the con- 

 vergence that upwelling, which must take place somewhere near them, will bring to the 

 surface Antarctic water which has not been below a depth of 200-300 m. St. 42, in 

 47° 53' S, 61° 25' E, and St. 43 in 47° 53' S, 66° 26' E, of the Gauss Expedition, are 

 both north of the convergence. The former, 200 miles north, has Antarctic water at 

 a depth of about 400-600 m.; the latter, 100 miles north, at a depth of 200-300 m. 

 From these data it seems quite probable that the shallow water off the shores of these 

 islands is partly of Antarctic origin, in which case the presence of similar species in 

 the groups (2) and (3) above is made possible. 



The distribution of the various species mentioned by Nordenstam (1933), with the 

 exceptions of those occurring at stations 33 and 34 b, agrees with the grouping of species 

 as shown above. 



