MoUuscan Fauna of the Gulf of Suez. 391 



been proved in every case, of the enormous range of distri- 

 bution shown by some of the Mollusca, not as varieties, but 

 as constant typical forms. It is not to be imagined, for 

 instance, that the whole extent of coast-line, littoral or sub- 

 littoral, from Suez to Hawaii, offers a suitable home to Gypvcca 

 erosa. But if the larval form is free-swimming, and if, when 

 it returns from the open sea to the shore, it perishes at unsuit- 

 able lines of coast, but lives and flourishes on those which 

 offer conditions of existence similar to those in which its 

 ancestors have lived, we can understand why the typical form 

 is preserved unvaried over a distance of many thousand miles. 

 The distribution in a typical form of several of the large 

 Tritons will, I have no doubt, have to be explained in a 

 similar way. 



Again, Issel's theory of " equivalent species " and " equi- 

 valent varieties " seems open to serious objection. He has 

 made a list, as we have seen, of twenty-six " equivalent 

 species " and of only four " equivalent varieties." But what 

 is to determine the difference between the one and the other ? 

 What is to prevent our classifying Tapes Deshayesii as an 

 equivalent variety and Area erythrcea a.^ an equivalent species? 

 Issel tells us that equivalent varieties are Mediterranean 

 species " only slightly modified," and equivalent species are 

 Mediterranean species " modified to a larger extent ;" but 

 who is to draw the line, and how ? For since certain genera 

 are notoriously liable to variation while others are remarkably 

 constant, the amount of variation from the type which in some 

 genera suffices to constitute a species, will in another genus 

 be regarded as unimportant. The question is something more 

 than a mere matter of words, if we go on to assume, as Issel 

 does, that the " equivalent species " entered the Hed Sea 

 during the Pliocene period, while the " equivalent varieties " 

 did not effect their passage from the Mediterranean till a later, 

 or Postpliocene age. 



Issel s grounds for refusing to classify his four " equivalent 

 varieties " {Nassa costulata, Solecurtus strigilatus, Gastro- 

 chcena duhia^ and Area lactea) as " equivalent species " are 

 two in number: — (1) the distinctive characters are not suffi- 

 ciently constant ; (2) the groups in which they occur are 

 subject to considerable variation in form. These two reasons 

 are really only two different ways of stating the same fact, 

 namely, that the genera concerned are remarkably liable to 

 variation. I should dispute this statement at the outset with 

 regard to half the genera in question. Whatever may be 

 said of the capacities of variation in Gastrochcena and Arca^ 

 I should be inclined to select the Solenidge and Nassidee, parti- 



27* 



