600 SCIENCE PROGRESS 



from the work of the Italian naturalists ; and, indeed, these 

 investigators did not actually obtain the larvae of the eel from 

 the depths of the Straits of Messina, but from the surface, where 

 the little eels were brought up by reason of the exceedingly 

 strong currents which prevail in that region. It remained for 

 investigators on the Danish international exploring steamer to 

 place the hypothesis of the Italians beyond the region of mere 

 probability by discovering the larvae of the European fresh- 

 water eel in the open Atlantic Ocean south-west of the Faroe 

 Islands, and at depths of over 1,000 metres. There is no 

 doubt that the eels of North Europe must traverse the North 

 Sea, or the English Channel, before they can reach the depth 

 of water, and the other essential surroundings, for their repro- 

 duction. In a paper which is likely to be one of the classics of 

 marine natural history, Schmidt has worked out the migrations 

 of the eel for the purpose of the act of reproduction, and the 

 converse migration of the young and immature fishes, or 

 Leptocephali, from their birthplace in the deep Atlantic to the 

 rivers of Northern Europe. 



We have seen that an important part of the work of the 

 International Fishery Organisation has been the study of 

 the hydrographical changes in the waters of the seas of the 

 north European area — that is, the determination of the yearly 

 cycle of variation of temperature and salt-contents in the waters 

 of selected representative areas. I leave aside the discussion of 

 the value of such investigations for the student of the climate 

 of North Europe, and will refer here very briefly to the con- 

 nection which we now know to exist between these changes 

 in the physical characters of the sea and the abundance and 

 migrations of fishes. Some years ago, in the case of the 

 migration of the herrings of the Swedish coast fishery, it was 

 shown beyond doubt that this fish regularly inhabited a stratum 

 of water of definite constitution — not Atlantic or strictly coast 

 water, but a mixture of the two known as " bank water." The 

 herring came and went with this stratum of bank water. 

 Within the last few years there has been reason for the belief 

 that the herrings which form the great summer fishery off the 

 east coasts of Britain are also influenced in their annual 

 migration and shoaling movement by the annual influx of 

 Atlantic water into the North Sea from the European 

 Stream. Not only so, but we have evidence that other well- 



