io6 



NATURE 



[November 24, 1910 



north — as a drift with the currents. The negative fact 

 that none of the smaller larvae have appeared north of the 

 Azores, and none of the larger ones south, seems to favour 

 such an explanation. The further fact that none of the 

 transformation stages, previously found so abundantly on 

 the continental slope, were found in mid-ocean supports the 

 same view. Nevertheless, I consider it dangerous to form 

 any definite opinion from negative facts concerning such 

 vast ocean expanses, where so few investigations have as 

 yet been made. 



As a provisional working hypothesis I should be inclined 

 to regard the continental slope as the area where the 

 transformation of the larvae takes place, and the southern 

 central part of the North Atlantic ocean as the probable 

 spawning area of the eel. 



Fig. 5 gives information as to the depths at which the 

 Michael Sars caught the eel-larvae. The youngest speci- 

 mens were mainly found by towing a net with loo metres 



Depths 

 In mrttr 



300. 



o Jndividuds from Ihe Northern Section 

 + Jndividuals from the Southern Section 



■•• + ■♦■+♦•♦•♦ + + + + + + + 00 



++++0000000000 



4 00000000 



500. +00 



Fig. s- 



of wire out, or in a depth of about 50 metres. The 

 eldest stages were found in nets towed with 200 metres of 

 wire out, or at a depth of about 100 metres. The Michael 

 Sars employed for these depths mostly silk nets with 

 mouths of I m. in diameter, and no trawls. Otherwise 

 larger catches of eel-larvae might have been procured. I 

 should recommend that future investigators look for the 

 eggs and the smallest larvae from the surface down to 

 100 metres, say between the Azores and Bermudas, in 

 winter. I hope that this information will in this way be 

 found useful. Johan Hjort. 



Bergen, November 7. 



Are Mules Fertile ? 



In the Nuevo Mundo of Madrid for October 27 it is 

 stated that a mule, belonging to Don Carlos Gimenez, of 

 Argamasella de Calatrava, recently gave birth to a foal. 

 From India, South Africa, and America reports have 

 reached the writer about fertile mules, but in no single 

 instance has the evidence of fertility been altogether satis- 

 factory. In the present case the information thus far 

 submitted is very meagre. Nothing is said about the 

 breeding of the reputed parent of the foal. She may be 

 a she-ass with the conformation of a mule, or a mule in 

 milk which succeeded in stealing a mule foal from a 

 mare. A Przewalsky-horse hybrid bred at Penycuik 

 proved fertile, but all the ass and zebra hybrids experi- 

 mented with during the last twelve j-ears proved sterile. 

 The male zebra-horse hybrids were sterile because they 

 never succeeded in maturing perfect sperms. The hybrid 

 " Romulus," e.g., had all the instincts of a pony stallion, 

 and, so far as one could judge with the naked eye, he 

 was capable of getting foals. When, however, a micro- 

 scopic examination was made, it was ascertained that the 

 sperms were quite or almost tailless: — at the most the 

 length of the flagellum was, never more than three or four 

 times the diameter of the head, and it was immobile. 

 Why female mules are infertile has not yet been deter- 

 mined. 



Sterility in birds seems sometimes to be due to struc- 

 tural changes in the germ cells induced by in-and-in-breed- 

 ing. It is conceivable that similar changes may sometimes 

 result from intercrossing. It must be admitted that the 



photograph reproduced in the Nuevo Mundo supports the 

 view that the Calatrava foal is a mule, and that the 

 reputed mother is also a mule. 



But further and more definite information is wanted 

 before a decision can be arrived at. 



J. C. EWART. 



The Origin of Dun Horses. 



The cases quoted by Prof. Cossar Ewart from Mr, 

 J. B. Robertson in Nature of November 10 would be 

 good evidence against the theory that every dun horse 

 must have at least one parent dun or grey if the data 

 in the Thoroughbred Stud-book were absolutely trust- 

 worthy. This they are not, and all the cases quoted by 

 Prof. Ewart have in them a very considerable element of 

 doubt. Let me indicate these elements by placing the 

 cases quoted in one column, in reversed chronological 

 order, and the necessary remarks in another column 

 parallel. 



Remarks. 



This filly is registered 

 " b. or dun." 



This colt is registered 

 " b. or dun." 



In vol. xvii. Cellulites* 

 foal of 1892 was said to 

 have died, but in vol. xviii. 

 the alleged dead foal be- 

 comes Sarah Curran. 



This filly is registered 

 "bay." 



The breeder had doubts 

 as to this filly's colour. 



This filly died when two 

 days old. 



This filly had eight foals 

 the colours of which were 

 registered, and not one was 

 dun. 



This colt's sire was grey, 



Cases Quoted. 



Bay-dun filly, foaled 

 1907, dam. Unexpected. 



Dun colt, foaled 1897, 

 dam, Lobelia. 



Dun filly, Sarah Curran, 

 foaled 1892, dam, Cellu- 

 lites. 



Light dun filly, foaled 

 1886, dam, Danseuse. 



Dun or chestnut 

 Saneta, foaled 1884. 



Dun filly, foaled 

 dam, Octavia. 



Dun filly, foaled 

 dam, Miss Thigh. 



filly, 

 1829, 

 1763. 



i 



Dun colt, foaled 1730 

 dam, Young Kitty Burdett. 



The last case quoted is the mare Silverlocks, from which"" 

 nearly all the duns in the Stud-book are descended. 

 Silverlocks is credited with five foals, the first of which 

 was foaled in 1738, and four of these were dun. Three 

 of these four were by a bay or brown horse. So Silver- 

 locks herself was presumably a dun. The Stud-book 

 assumes that this mare Silverlocks was identical with a 

 chestnut mare Silverlocks foaled in 1825. Either the 1825 

 Silverlocks was a dun, not a chestnut, or the two mares 

 were different animals. James Wilson. 



Royal College of Science, Dublin, November 15. 



The Cocos-Keeling Atoll. 



In stating the depths to which the bores in the Funafuti 

 lagoon were carried, and in drawing his deductions from 

 them, the reviewer (Nature, November 10) has fallen into 

 a very curious error. He states that the first bore was 

 driven to a depth of 41 fathoms, and the second to nearly 

 36 fathoms, but he overlooks the fact that he is giving 

 the measurements from the surface of the lagoon water, 

 and not from the lagoon floor. 



The bores were started in loi feet of water at low-water 

 spring tide, and therefore, of the 41 and 36 fathoms men- 

 tioned by the reviewer, the top 17 fathoms in each case 

 consist of nothing but lagoon water. The actual bores 

 made into the lagoon bed penetrated no more than 24 and 

 19 fathoms respectively, or, as I pointed out in my last 

 communication, a maximum of 144 feet. 



F. Wood-Jones. 



My depths of 41 and 36 fathoms were not intended in 

 any way as a correction of Mr. Wood-Jones's letter. The 

 important point is that lagoon debris only occurred above 

 27 fathoms; there was 10 fathoms of it. Below this depth 

 we get coral rock. 



It is a long time since any discussion has been held in 

 this country on coral-reef formation, while much work 



NO. 2143, VOL. 85] 



