154 DISCOVERY REPORTS 



much of the error that has occurred in systematic work because of a too critical study 

 of museum specimens ; the population can be considered as a biological community, 

 which exists at the mercy of an environment whose chemical and physical factors are 

 liable to continual change. 



Interesting phenomena present themselves for consideration, such as the reaction of 

 organisms to their environment — the most important factor that controls geographical 

 distribution of species. Special modifications of form and methods of adaption, such as 

 the production of peripheral air chambers to increase buoyancy in tropical waters, or 

 the production of spines and bristles for increased stability in forms which have to 

 undergo the buffeting of a truly oceanic existence, throw interesting light on the 

 "rigidity" or "plasticity" of species. 



This plasticity, which is observed in all the truly planktonic genera, may be a feature 

 of some specialized part of the organism, or may take its origin in something deeper and 

 more fundamental. It is not yet known whether these structural modifications are 

 accompanied by or are the result of changes which take place in the cytological elements. 

 This field of research is entirely unexplored. 



Facts concerning the growth and extent of the algal population of the surface waters 

 in the southern seas have been dealt with by Dr Hart (1934), but the problem might 

 also be considered from the points of view of Verhulst, and Pearl and Reed. These views 

 have been ably expounded and extended by Longley (1932), who seeks to explain bio- 

 logical populations in terms homologous with the laws of gases, and suggests that they 

 are equally capable of mathematical expression. 



In considering the factors which govern the phenomenal increase in southern phyto- 

 plankton during certain seasons of the year attention must be paid to the chemical and 

 physical constitution of the water. In striking contrast to tropical and subtropical areas, 

 where extreme paucity in individuals is probably accounted for by the fact that the 

 surface waters are almost entirely devoid of nutrient mineral salts, the waters in polar 

 and subpolar areas never become completely exhausted of their store of inorganic food, 

 and become supercharged with salts every spring, with the breaking up of the ice fields, 

 and the influx of new water. 



Hart (1934) has shown that the phosphate content of water in the more northerly 

 parts of the Antarctic Zone never falls below 50 mg. per cubic metre, and he has sug- 

 gested that the nutrient salts cannot be regarded at any time as a factor limiting phyto- 

 plankton growth or controlling periodicity. There is, however, the possibility that 

 shortage of silica may sometimes limit diatom growth in the northern parts of the 

 Antarctic Zone. 



The interesting and highly provocative work of H. T. Barnes (1928) might also be 

 considered in this connection. Barnes utilized the work of Bayliss (1927) who showed 

 that water must be regarded as a system of polymers, that the degree of polymerization 

 is roughly inversely proportional to the temperature, and that the differences in state are 

 due to the varying proportions of the polymers present. In steam, monohydrol pre- 

 ponderates; in ice, trihydrol. Later, T. C. Barnes (1932) was able to show that in a 



