COMPARISON WITH OTHER AREAS 357 



put forward simply as hypotheses, which may perhaps serve the purpose of suggesting 



experiments and further observation in the field, which in turn might lead us either to new 



conceptions or to establish more firmly the realization of the domination of temperature. 



The outstanding features of the Antarctic plankton may be summarized as follows : 



1. The wealth of phytoplankton. 



2. The small number of zooplankton species inhabiting the Antarctic surface layer, 

 i.e. the top 150 m., the species which do so being very prolific. 



3. The lack of pelagic larval stages of benthic animals (noting the exception of 

 Auricnlaria antarctica). 



4. The absence of a daylight surface fauna. 



5. The importance of vertical migration. 



The wealth of the phytoplankton in the surface layers is reflected in the rich deposits 

 of diatom ooze on the floor of the Antarctic Ocean. The limit of this great zone of 

 diatom ooze follows generally the line of the Antarctic Convergence. 



We have seen that all the metazoan animals, 1 which occur in the upper layers in 

 sufficient numbers to be investigated, show a marked vertical migration, except three 

 species of copepod and the chaetognath Enkrohnia hamata, four animals which are only 

 rarely taken in large numbers in the upper 50 m. The possibility is suggested that the 

 animals find the upper phytoplankton layers uncongenial, and, whilst making feeding 

 excursions into these layers at night, live for the greater part of the day below. If the 

 surface layers are less congenial to the animals, temperature is not a factor contributing 

 to this; we see that whilst the Antarctic surface layer — the top 150 or 200 m. — is as a 

 whole much colder than the lower intermediate layer, the actual surface layers — the top 

 50 m. — are the warmer part of the layer, being heated by the warmer atmosphere in 

 summer (see Figs. 5 and 7). The migrating animals spend the greater part of their time 

 in the colder portion of the Antarctic surface layers, and the four animals just mentioned, 

 which show little or no migration, live altogether in this colder portion. 



Temperature has usually been held to be the main factor in limiting the number of 

 zooplankton species in the polar seas. The line of the Antarctic Convergence separating 

 the Antarctic Zone from the sub-Antarctic Zone is one marked by a sharp change in 

 surface temperature between the two zones. This we may reasonably suppose is a very 

 important, if not the most important, factor in limiting the ranges of different animals 

 occurring on each side of this line, but we do not know that temperature is the most im- 

 portant factor in limiting the number of animal species which may successfully inhabit 

 the upper layers of the Antarctic Zone. Temperature does not appear to limit the 

 number of plant species ; we note that the phytoplankton of the Antarctic is not only 

 rich in its numbers of individuals but also in its variety of forms. The benthic fauna of 

 the Antarctic, correlated no doubt with the luxuriant phytoplankton above it, is re- 

 markably abundant, perhaps more prolific than anywhere else in the world; like the 

 Antarctic animal plankton, it is rich in numbers of individuals, but in contrast it is also 

 remarkable for the number and variety of the different species represented. This great 



1 The protozoa have not been investigated for vertical migration within our area. 

 dxi 46 



