Jan., 1895. THE DISTRIBUTION OF FOOD-FISHES. 31 



The first question arising in such investigations is, naturally, 

 What special physical or chemical element is it which primarily affects 

 various forms of animal life? The actual influence exerted upon 

 different animals directly, favouring their healthy existence and repro- 

 duction or otherwise, or indirectly, affecting their food supplies, is a 

 biological problem : the chemist or physicist must for the present assume 

 that, given a certain chemical or physical variation, some corres- 

 ponding change takes place in the biological conditions, and that the 

 success or failure of certain fisheries is in some sense a measure of that 

 change. The first and most obvious element is, of course, that of 

 temperature. But, while it is undoubtedly true that most animals 

 are found within regions having more or less definite temperature- 

 limits, and that certain forms flourish under a wider range of 

 temperature-conditions than others, 1 it is also true that direct hard 

 and fast relations between temperature and the constant or periodic 

 occurrence of certain animals in certain localities cannot be found. 

 The same statement holds for salinity, and for density, the joint 

 function of salinity and temperature. In the absence of any simple 

 relation, it remains to explain the coincidence of the occurrence of 

 certain changes of temperature or salinity with migrations of animals, 

 which has been observed over and over again ; and we obtain a hint 

 from the fact that it is just these two physical elements upon which 

 the oceanographer chiefly relies in tracing the circulation of waters, 

 and in identifying the sources from which the water occupying any 

 given region is derived. The presence of a body of water at a 

 particular place is in general indicated by peculiarities of temperature 

 or salinity, which, although easily enough recognised by proper 

 physical methods, are certainly too slight to seriously affect animal 

 life ; and if the arrival or departure of such water coincides with 

 important changes in the fauna, we are almost forced to conclude that 

 it has some other property to which we must look for the direct 

 relation required. 



It is impossible to detail here the steps by which the Swedish 

 oceanographers have, by observations in and around the Baltic, built 

 up their case ; but a study of Professor Pettersson's papers in the 

 Scottish Geographical Magazine shows that he and his colleagues have 

 collected a mass of evidence to prove that the migrations of herring 

 and mackerel, and other variations in the distribution of not only 

 fishes but Plankton, are dependent on the amount of oxygen present 

 in the sea-water. Jacobsen, the chemist of the " Pommerania " 

 expedition in the North Sea in 1872, showed that the amount of air 

 absorbed by sea-water depended solely on the temperature and baro- 

 metric pressure to which it was exposed when at the surface. Water 

 passing from the surface to lower layers retains the amount of 

 nitrogen unchanged, while the oxygen may be diminished by the 

 action of organic matter or animal life. Sea- water shut up in enclosed 



1 See the paper by Dr. Otto Maas in Natural Science, vol. v., p. 276. 



