Molluscan Fauna of tlie Gulf of Suez. 381 



water. In the latter the great distance between the Sandwich 

 Islands and the opposite coasts of America, and the cold polar 

 current which pours down from the north in a course parallel 

 to those coasts, form obstacles too great for the fry of tropical 

 Mollusca to overpass ; and, so far as I am aware, it cannot be 

 shown that a single species has succeeded in crossing them in 

 this direction. It is quite true that a few Suez species (not 

 more, perhaps, than half a dozen in all) are found on the West- 

 Mexican coast ,• but there is no evidence to show that they 

 got there via the Sandwich Islands. On the contrary, their 

 existence (speaking more particularly of Triton pilearis and 

 Lucina quadrisulcata) in the West Indies tends to show that 

 their appearance on the W.-Mexican coast dates from a time 

 when the Isthmus of Panama was not yet closed. And it 

 appears to me that, though the distance may be far greater, yet 

 the presence of East-Indian species in the West Indies and on 

 the V/est-Mexican coast is far more easily accounted for by a 

 trans-Atlantic than by a trans-Pacific migration, especially 

 when it is borne in mind that the Isthmuses of Panama and 

 Suez have both been open, each of them perhaps more than 

 once, within late geological times. For in the case of the 

 Atlantic we have a series of warm currents trending generally 

 from the west coast of Africa towards Brazil * ; while in the 

 case of the Pacific the cold polar current sweeping down from 

 the north parallel to the American coast, and the enormous 

 distance without any perceptible set of current to the east, 

 present just as insurmountable a barrier as the Antarctic current 

 in S. Africa. 



Of the enormous area of distribution whose normal limits 

 have been indicated above, there is a district which may 

 probably be regarded as the nucleus. If it is possible to 

 approximate in any degree to the original birthplace of a 

 single species, whether vertebrate or invertebrate, the same 

 must be true, though of course in a wider and less specific 

 sense, for groups of species also. If it is granted that a 

 particular species develops at a particular place (using the 

 term in a wide sense), and not, whether contemporaneously or 

 at different periods, at different places, it is evident that indi- 

 cations may be discovered which will tend to show where that 

 place was, and the process may be extended to groups of 

 species as well. It appears to me that the Philippine district 

 may be regarded as the centre of distribution of those species 

 which radiate westward as far as Suez, northward as far as 



* Studer states {KVa. Ak, Berlin, 1882, p. 5) that out of 541 marine 

 shells from the west coast of Africa 55 are common to the opposite 

 coast of America. 



