April 30, 1885] 



NA JURE 



599 



sufficient, still the method is only available on a com- 

 paratively clear night ; and though the same sights may 

 possibly be also used for the determination of longitude, 

 it will more commonly happen that the complete position 

 may be satisfactorily determined by Sumner's method 

 applied to two stars having a considerable difference in 

 azimuth. 



The pages in which Mr. Rosser treats of Sumner's 

 method are of themselves sufficient to establish what has 

 been already said as to the practical nature of the book. 

 In an admirable monograph published two years ago, 

 under the title of " Stellar Navigation," Mr. Rosser has 

 shown himself alive to the very great value of this method 

 of determining a ship's position, and to the necessity of 

 shortening the calculation by the use of Sir William 

 Thomson's special tables, or by Burdwood's and Davis's 

 azimuth tables. But no remark in the " Self-Instructor" 

 ition to this, and the problem is left, in its native 

 clumsiness, in the form suitable to the questions of the 

 examination room. The same might indeed be said of 

 Mems, which are given without any 

 hint of the little artifices which, in practice on ship-board, 

 render the computation quicker and easier. In saying 

 this, however, we attach no blame to Mr. Rosser, unless 

 ailing his book : ' practical," or " adapted for use 

 at sea." The book is meant to meet the demands of the 

 examinations ; and for this, at least, it appears sufficiently 

 well adapted. J. K. L. 



LETTERS TO THE EDITOR 



[The Editor does not kold himself responsible for opinions expressed 

 by his correspondents. Neither can he undertake to return, 

 jr to correspond with the writers of, rejected manuscripts. 

 No notice is taken of anonymous communications. 



[ The Editor urgently requests correspondents to keep their letters 

 as short as possible. The pressure on his space is so great 

 that ii is impossible otherwise to insure the appearance even 

 of communications containing interesting and novel facts. t 



On the Cause of the Dissimilarity between the Faunas 

 of the Mediterranean and Red Seas 



The republication by Mr. A. H. Cooke of the list of Testa 

 M I ca obtained by the late Mr. Robert MacAndrew 



during a dredging excursion (in [869) in the 1 lulf of Suez, 1 

 for comparison with that of the Mediterranean over 



Lin part, and of which the late Mr. J. Gwyn Jeffreys has, 

 amongst other writers, given an account.- The 



■ ill, upon such a compt 

 - ind. 3 I propose briefly to sketch out the process 

 by which this dissimilarity may lie e been brought 



about. 



' ack to the Eocene period, we know that the 

 whole of the region bordering the Levant, and im Ii 

 large lie bed of the 



we may presume ihat a community of genera and 

 species existed over the whole tract represented by those of the 

 Nummulite limestone of the Middle Eocene ; 



During the Upper Eocene period there was a shallowing of 

 the sea-bed in many places, and corres] 1 ning in 



other-', and thus the first division of the submerged area into 

 11 'I shall iw ba boitl with a 



certain influence on the animal and plant life ; but the general 

 result may not have 



hiring the sure :i id that the differen- 



tiation of the fauna and flora of the two seas really I 

 Recent observations on the geology of Northern Africa, Arabia, 

 I destine by Zittel, I rs, leave little cloubl 



of Natural History, vol. xv. p. 322 

 dI. vi. p. 65 (fourth 



has been recognised by i Visil to Ceylon" 



and his "Arabi^rhe Koratt'-v 



that the Miocene period was one during which the main lines 

 of the future lands and seas were marked out ; and the absence 

 of depo its belonging to this epoch (except a few scattered tracts 

 formed of shallow-water and littoral beds) over the region re- 

 ferred to, leads to the conclusion that land-conditions prevailed 

 very much where we now find them, and that the submerged 

 areas of the Mediterranean and Red Seas were dissevered by the 

 Isthmus of Suez. It was during this period of elevation that 

 the differentiation proceeded ; the original forms of the Eocene 

 period developing in each basin independently of one another, 

 and becoming more divergent as time went on. The process 

 seem- to have been continued well into the Pliocene epoch, but 

 at a time which may be indicated perhaps as "Newer Pliocene" 

 there occurred a re-submergence of the land to the extent of 

 220 to 250 feet below the present level of the sea, marked by 

 the occurrence of raised sea-beds containing shells, &c, of species 

 still living in the adjoining waters, and of old coast-cliffs per- 

 forated by Pholas borings, like that discovered by Oscar Fraas in 

 the cliffs of Jebel Mokattam, near Cairo, at an elevation of 220 

 feet above the surface of the Mediterranean, and recently de- 

 scribed by Dr. Schweinfurth (Zeitsck. d. deutschen geolog. 

 Gesellschaft, 1883). Daring this depression Africa became an 

 island, and the waters of the two seas were united. 



With this union of the Mediterranean and Red Seas there 

 must have been brought about a certain commingling of the 

 forms inhabiting their waters respectively, and hence it \~ some- 

 what surprising that there should at the present day be found 

 such an almost entire dissimilarity as that already stated. The 

 explanation, it seems to me, is to be found in the fact that the 

 strait was, in its shallower portion, very shallow ; and that con- 

 sequently, except for the purely littoral and shallow forms of 

 marine life, a commingling really did not take place to any great 

 extent. To the north of Lake Timsah there occurs a ridge of 

 ground called El Guisr, which rises 70 feet above the i 



sea-level, and another called Tiinam, which rises 25 feet. These 

 ridges would have caused a shallow ing of the strait to the extent 

 of their elevation, so that over the former ridge the depth of the 

 strait would only have amounted to 180 feet or less during the 

 greatest submergence. It is impossible to say whether these ridges 

 are higher, or the contrary, than they were at that period ; but it 

 is a remarkable fact that the'sub-fossil shells in the gra\ 

 south of Tunum are those of the Red Sea, and to the north 

 those of the Mediterranean ; other ridges, like that of Tel-el- 

 Kebir, produced similar shallows. Asa general result it is clear 

 that the submersion of the isthmus during the later Pliocene 

 period did not produce a general commingling of the forms 

 of the two seas ; and when ultimately the seas were again 

 by the re-elevation of their beds, and the present 

 isthmus esta lished, those forms which may have passed across 

 from sea to sea would succumb to the altered conditions of their 

 environment. It can scarcely be doubted that the temperature 

 of the water of the Red Sea differs considerably from that of the 

 Mediterranean by several degrees, and the forms which belong 

 ner would perish in the latter, and vice vend. It would 

 ing to ascertain which of the two faunas more closely 

 resembles that of the original Eocene stock. 



Hire, then, we have the remarkable zoological phenomenon 

 fectly distinct sets of marine forms originating in one 

 st ck only as far hack as the Middle Eocene period, inde- 

 such an extent that, at the present day, 

 there are scarcely p ci irding to Prof. 



Issel) common to both. Now, if the beds of the-: two >ea< (the 

 Levant and ire to be elevated into land and their 



by a geologist of the future, he would 

 probably assert on the pakeontological evidence that they be- 

 longed to two distinct periods of geological time ! This is sub- 

 1 at least for geologists of the present 

 day. I may add that I have been induced to try and solve to 

 my own satisfaction the problem here presented while engaged 

 on a work containing the scientific obser inclusions 



made during the r \ Petrea in connec- 



tion with the " Palestine Exploration I 



Edward 1 1 



Hybridization among Salmonidee 



ixi. p. 563) that the ' 

 Fi-h Culture Association" propose cross-breeding land-locked 

 salmon and trout as proposed by Prof. Brown Goode in "Forest 

 1884. L'efore doing so I would venture 

 to direct their attention to a few points. 



