Molluscan Fauna of the Gulf of Suez. 395 



Besides the above seventeen species I should add the fol- 

 lowing two (which MacAndrew did not find at Suez) on the 

 strength of Issel regarding them as varieties : — • 



Nassa costulata, Ren. Solecurtus strigilatus, L, 



Thus, while holding to the undoubted fact that variation 

 must be due to modification of physical conditions, I should 

 maintain with Semper* that the converse is not necessarily 

 the case, and that modification of physical conditions does not, 

 in some cases, produce a measurable amount of variation. 

 Why this should be the case must remain unexplained. It is 

 possible that different conditions of temperature, different 

 chemical constituents of water, &c., act less on certain species 

 than on others, and that while a particular genus or a parti- 

 cular species would be profoundly modified by such differences 

 as exist between the waters of the Red Sea and the Mediter- 

 raneaUj other genera and other species would remain practi- 

 cally unaffected. The facts seem to point in this direction,, 

 for how otherwise can we account for the extraordinary paral- 

 lelism of species exhibited in Issel's list of equivalent species, 

 and the simultaneous similarity of the species enumerated in 

 the list given above. It has been shown that earlier or later 

 dates of migration cannot be relied upon to explain these 

 facts ; the only true explanation must be that altered physical 

 conditions act very unequally upon different genera, and even 

 upon different species of the same genus. 



It may at the same time be remarked that, for pm-poses of 

 comparison between the water of the two seas, it will not be a 

 fair test to take the mean amount of saltness, temperature, &c. 

 of the Mediterranean generally and compare it with that of 

 the Eed Sea. The mean surface-temperature of the Mediter- 

 ranean is, of course, considerably below that of the Red Sea, 

 probably as much as 10° F., and, on a priori grounds, it 

 would seem unlikely that Mollusca could endure a change of 

 10°F.in the temperature of the water in which they live without 

 undergoing considerable modification. But the comparison 

 must be made between the water from which the species actually 

 migrated and that where they now are, viz. the water at Port 

 Said and Suez, and then, as far as temperature at least is con- 

 cerned, there is no very marked difference. The mean annual 

 surface-temperature f of the sea at Port Said is 70*^-71° F,, 



* " Hence every change, as for example in the composition of the water 

 of a lake or river, will not affect the fauna inhabiting it equally and as a 

 whole, but will act on individuals ; some will bear the change without 

 being in any way affected by it, othei's Avill die, Avhile others again Avill 

 survive ; but their habits uf life will be changed, and at the same time 

 their structure will be modiffed." — Semper, ^Animal Life,' p. 177. 



t Taken from the publications of the Meteorological Office. 



