— 232 — 



thinking had to be quite changed. Numbers of years are- undoubtedly necessary ere the 

 true connection has entered into the general consciousness; examples of this have often 

 been seen in like cases. That many scientists should also be doubtful is perhaps more 

 remarkable but to a certain extent also explicable. The reason was partly that some time 

 had to be taken in forgetting Günther's peculiar theory — that the Leptocephali are eel- 

 larvae which on account of the external conditions have developed abnormally and are not 

 able to develop further into eels. Partly also I believe, that Grassi and C.alandruccio were 

 themselves a little to blame, because they did not publish the whole of their undoubtedly 

 large material, by means of which they had reached such epoch-making results, as completely 

 as could be desired in all regards. On the contrary, they restricted themselves to giving 

 short preliminary notices, in spite of the requests often addressed to them to give informa- 

 tion to the many interested in the matter in Europe and the whole world. Lastly, it was 

 difficult for people to understand why the eel-larvae could practically never be found at other 

 places than just at the Messina Straits, since eels occur almost everywhere in Europe, 

 because even though Grassi and Calandruccio answer the question by saying that 

 "there are currents at Messina which disclose the secrets of the sea elsewhere so unap- 

 proachable" (1. c. 1897, p. 15), this still fails to give an explanation as to what the 

 conditions are which control the occurrence of the Leptocephali. 



So long as the larvae of the eel were practically only known from the Mediterranean 

 and no information was to hand regarding their normal mode of occurrence, it was quite 

 useless to discuss the reasons why they were not found on the coasts of North Europe. 

 As it has now been proved that they are common at places in the Atlantic we are able 

 with quite a different security to turn to the discussion of this question. 



We have seen in an earlier section what the depths and the hydrographical conditions 

 were at the places in the Atlantic where the larvae of the eel were found. We may now 

 for the sake of comparison consider the conditions in the waters lying, nearer the North 

 European Continent, so as to understand what causes the eel to be unable to breed there. 



I I, 2 and 3 are taken from the "Bulletin des résultats .icquis pendant les courses périodiques" publié par 

 le Bureau du Conseil Permanent International pour l'Exploration de la mer. 4 and 5 are from observations 

 taken on board the "Thor" by the hydrographer, Cand. mag. J. N. Nielsen, 



