96 SCIENCE PROGRESS 



down to the sea in the preceding autumn, and that the breeding 

 places were in the sea not far from the mouths of the rivers. 

 In 1896 came the discovery by Grassi and Calandruccio that a 

 Uttle fish named Leptocephalus brevirostris, then known only 

 from the Straits of Messina, is the larva of the Common Eel ; 

 this they demonstrated by capturing a series of transitional 

 forms, showing the transformation of the transparent leaf- 

 shaped Leptocephalus into the pigmented cylindrical elver 

 {cf. Fig. i). The Italian naturalists concluded that the Eel 

 bred in deep water near the coast and that the larvae lived in 

 the depths, but in the Straits of Messina were brought to the 

 surface by the peculiar currents, and so were captured there 

 and nowhere else. This hypothesis was perhaps reasonable at 

 the time, but, as we shall see, it was far from the truth. 



In 1904 Dr. Schmidt was on board the Danish research ship 

 Thor, engaged in investigations in the Iceland and Faroe 

 waters, when a specimen of Leptocephalus brevirostris was 

 captured at the surface. This clue was followed up, and in 

 1905 and 1906 cruises were made in the spring and summer by 

 the Thor in the Atlantic west of Europe, and large numbers of 

 eel-larvae were captured, all in the upper layers of the water ; the 

 larvae caught in spring and early summer were full-grown, about 

 three inches long, but the majority of those taken in August 

 and September had begun the metamorphosis. In succeeding 

 years Schmidt took the Thor into the Mediterranean for winter 

 and summer cruises, the results of which were as follows : no 

 eel-larvae were taken in the Eastern Mediterranean, a large 

 number of full-grown or nearly full-grown larvae and meta- 

 morphosing stages were captured at or near the surface in the 

 Western Mediterranean, and no early larvae were found. This 

 led to the conclusion that the Eel did not breed in the Mediter- 

 ranean at all, but that all the eels of the Mediterranean countries 

 — as well as those of Western Europe — resorted to the Atlantic 

 to breed, and that the Straits of Messina, instead of being the 

 breeding ground of the Eel, was in the Mediterranean the 

 farthest away from the breeding ground of any places where 

 eel-larvae could be caught. This conclusion, based on the dis- 

 tribution of the larvae, seemed so extraordinary that Schmidt 

 tested it by an elaborate statistical investigation ; in large 

 samples of eels from Iceland, Denmark, England, the Azores, 

 and Italy the number of vertebrae was counted, and it was 

 found that in each sample the numbers when plotted gave a 

 normal curve with a mean of 1x4-7, the range being from 11 1 

 or 1 12 to 1 18 or 1 19. This identity in the number of vertebrae 

 in eels from such widely separated localities made it clear that 

 there were no local races of the Eel, and suggested that it had 

 only one breeding area. The correctness of this view was 



