312 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1924 



the causes which lead the two Atlantic species of eels to find each its 

 own side of the ocean, despite the close proximity of their breeding 

 grounds. 



That fish should undertake migrations of considerable extent while 

 in the pelagic larval stage is nothing unusual. I need only call to 

 mind the young of the Gadoids and their migration round Iceland, 

 which I had an oj^portunity of studying in 1903-1905 (loc. cit., 1909). 

 The point which makes our eel an exception among fishes, and among 

 all other animals, is the enormous extent of its journeyings in the 

 larval stage. This is indeed a niigratory stage par excellence, the 

 unusual duration of which must be regarded as an adaptation — 

 effected by selection — to the distances of many thousand miles to be 

 traversed. So great are these distances that the class of one year can 

 not reach its goal, the fresh waters of Europe, until a second and a 

 third have started on their way. As a matter of fact, we have in 

 early summer three j^ear-classes of larvae on their journey; the 

 youngest in the western, the next in the central, and the oldest in the 

 eastern waters of the Atlantic, off the coastal banks of Europe (cf., 

 pi. 6, fig. 2, and fig. 7). There can be no doubt that a great wastage 

 of individuals takes place in the course of these years of migration, 

 but it is in all probability insignificant in view of the enormous pro- 

 duction of larvae, of which the Dana stations in the western Atlantic 

 give evidence. 



The Anguilla species, in contrast to other mursenoids, are usually 

 termed fresh-water eels, and are reckoned among the fresh-water 

 fishes of Europe and North America. From what we have now 

 learned this is far from literally strict. Both from their history and 

 their actual manner of life, these " fresh- water eels " are true oceanic 

 fishes, and the remarkable point in their life history is not so much 

 the fact of their migrating out into the sea to spawn as in their leav- 

 ing it in order to pass their period of growth in an environment so 

 unusual for mursenoid fishes as fresh water. 



I have in the foregoing pages described the course of our investi- 

 gations and set forth their results. In conclusion I will endeavor 

 very briefly to give an outline of the life history of our eel, as indi- 

 cated by the facts now ascertained. 



During the autumn months the silvery eels leave the lakes and 

 rivers and move out into the sea. Once beyond fresh-water limits 

 the eels are, in most parts of Europe, outside our range of observa- 

 tion. Exceptions are, however, found, as in the case of the Danish 

 sounds and belts and adjacent waters, which are passed by great 

 quantities of eels on their Avay to the Atlantic, and form the site of 



