CH. X] THE CONDITIONS OF LIFE IN THE SEA 245 



not float in it. These are indirect effects of changes in the salinity 

 of the sea water; but there is also evidence that slight changes 

 in the salinity per se may affect the metabolism of marine 

 organisms. How precisely this happens we do not know, but we 

 do know that very slight changes in the salinity and composition 

 of sea water, in which float the eggs or larvae of certain marine 

 animals, affect, in a very notable manner, the further development 

 of these things^. A priori we should expect that reactions of this 

 nature would take place, for in many marine invertebrates the con- 

 centration of the juices of the body is very much the same as that 

 of the salts in sea water. In these creatures the wall of the body 

 is a " semi-permeable membrane," and diffusion out of, and into the 

 blood, modified of course by " vital action," must take place. 

 Probablj^ the key to many of the effects which we have to consider 

 may be sought in these diffusion phenomena. 



However this may be there is good evidence that slight 

 changes of salinity have considerable influence in determining the 

 movements of marine animals. So far this evidence relates almost 

 entirely to the migrations of the edible fishes, and has been 

 collected by a comparison of the fishery statistics with the results 

 of the hydrographic study of the northern seas. 



(1) The migration of the herring. This is one of the older 

 problems of the natural history books. The great summer herring 

 fishery of the coasts of Britain begins about May off the coasts 

 of the outer Hebrides and Orkneys, extends, in June, to the sea off 

 the coasts of Aberdeenshire ; and then changes in such a manner 

 that the bulk of the fish taken in each successive month are caught 

 further to the south along the. east coast of Britain, until in 

 November the fishery is concentrated in the southern North Sea, 

 and in December and January off the coasts of Devon and Cornwall. 

 Nothing can be clearer than this orderly change in the situation 

 of the place of the maximum herring fishery. It used to be sup- 

 posed that there were great shoals of herring which came from 



^ The reader will remember the classical experiments of Loeb, who induced the 

 eggs of the sea-urchin to begin to develop merely by adding certain salts, in slight 

 traces, to the water in which these eggs were contained. See also some very 

 interesting observations by Moore, Eoaf and Whitley on the influence of slight 

 traces of acids, alkalies, &c. on the development of the eggs of the plaice and 

 sea-urchin. Proc. Roy. Soc. London, January 6, 1906. 



