86 DISCOVERY REPORTS 



ments taken at the time of the survey after the samples had settled for over 24 hours. 

 There was a belt of phytoplankton along by the South Shetlands, and an area of low 

 phytoplankton production on the southern side of the Bransfield Straits. Our hydro- 

 logists have shown that there is an influx of water into the Bransfield Straits from the 

 Weddell Sea. Here we see the warmer Bellingshausen Sea water meeting the colder 

 Weddell Sea water, and, as at South Georgia, at the region of their mixture there is the 

 denser production of phytoplankton. St. 161 in mid-ocean showed a dense production 

 of phytoplankton, and this station, although in Bellingshausen Sea water, lies close 

 against the border separating it from Weddell Sea water, and along this border there 

 may be a wide area of mixture. In the southern North Sea dense patches of phyto- 

 plankton are often produced in the region where the water flowing from the north meets 

 and mixes with that entering from the south through the English Channel. The hypo- 

 thesis I would tentatively suggest, to be considered with others, is that the mixing of two 

 different water systems, in providing a change of environment, is a cause of high pro- 

 ductivity (provided of course that sufficient nutritive salts are present, as in the Antarctic 

 is certainly the case). It is well known that Protozoa will go on multiplying in cultures 

 for a period of time, but that then a period of depression sets in when they divide less 

 and less frequently and the culture dies down to a low level in spite of the fact that con- 

 ditions of the culture medium are kept constant. Calkins (1902-4) showed that in 

 cultures of Paramoechim the periods of depression might for a time be overcome by a 

 change in the nature of the culture medium, or by temporary treatment with simple salt 

 solutions. Control animals not so treated died out at the end of about six months, whilst 

 those treated with a change in environment entered upon a new cycle. He expresses the 

 view that such depressions can probably be overcome " in nature by opportune changes 

 in the immediate environment". The view is held that the protoplasm becomes too 

 stabilized in relation to environmental conditions that persist unchanged for a long time, 

 and that from time to time a change of environment is required to enable metabolism to 

 proceed at its fuller rate leading to active division. 1 Possibly over stretches of ocean 

 where conditions become comparatively uniform diatoms may decline into a phase of 

 inactivity, to be awakened to an outburst of new life on coming into a changed environ- 

 ment by mixing with water of slightly different constitution, either by meeting another 

 surface current or by an upwelling of water, or perhaps by the effects of melting ice. 

 If the hypothesis should be proved correct it would of course only carry one a step 

 further. It would remain to investigate exactly what sort of changes are necessary in the 



1 It has been suggested that sex, before it became of evolutionary significance in bringing about a re- 

 assortment of the genetical factors, arose as a fusion of two cells to bring about a change in the protoplasm in 

 relation to its environment. It is true that since this idea was put forward certain strains of Paramoechim 

 have been kept alive by Woodruff and others for a very long period without being allowed to conjugate, but 

 nevertheless in other strains periodic conjugation or the process of endomixis appears to be necessary. 

 Referring to the auxospores of diatoms Lebour (1930) states that "These are most often, if not always, 

 asexual in the centricate forms. . .but in the pennate diatoms the auxospores may be formed sexually by 

 conjugation". The majority of oceanic diatoms belong to the centricate division, and whilst highly specu- 

 lative it seems possible that the mixing of different water masses by the flowing together of ocean currents 

 might bring about a change in the protoplasm-environment relationship which might be necessary for con- 

 tinued propagation in the absence of syngamy. 



