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Show 1891.] MR. E. A. SMITH ON SHELLS FROM ADEN. 397 Good Hope and Natal, and finally it is known from the Red Sea and Philippine Islands. The other species, A. olivacea, the distribution of which, as far as we know, is as limited as that of A. lactea is extensive, has at present only been recorded from the Philippines. I could multiply cases of this kind, but the one mentioned is sufficient to demonstrate the unaccountable difference in the distribution of allied forms. There seems to be an unfathomable something in their nature which permits the one to live under very varied conditions, in temperatures greatly differing, and in waters of which the chemical composition is dissimilar, and on the other hand which does not allow the other to exist excepting under special and limited conditions. It is so in the vegetable kingdom. Do we not find some plants which will grow almost anywhere, in all kinds of soil, whereas to others existence appears to be possible only amid very special surroundings ? Being cognisant of such facts as these, it is with much diffidence that I have suggested the migration, so to speak, of the species in question, or some of them at least, from the Red Sea into the Mediterraneau. However, taking all points into consideration, I think this supposition is likely to be as correct as the view usually entertained. Some support to this theory is derived from a study of the emigration of species from the Red Sea to the Mediterranean and vice versd since the opening of the Suez Canal. From the reports upon this subject by Fuchs l, Keller2, Krukenberg3, and others, it is evident that there is a greater pilgrimage taking place northward than towards the south, and this, to some extent, is possibly attributable to the movement of the current from the Red Sea to the Bitter Lakes being faster than that from the Mediterranean southward, for there is a flow in both directions, owing to the great evaporation in the Bitter Lakes. At present two Red Sea forms, Mytilus variabilis and Mactra olorina, have been taken living at or near Port Said; on the contrary, no Mediterranean species has as yet got through to Suez, but Cardium edule (if correctly identified) is said to have almost reached there. Although certain species m a y extend northward and to the south, it yet remains to be seen if they become modified to any extent, supposing the altered temperature and chemical composition of the water into which they may have migrated permit their race to be perpetuated. I can well imagine that eventually it will be found that all the rest of the species have as wide and very nearly the same distribution as Area lactea, and therefore the possibility is suggested that their presence in the Mediterranean may have originated from the Atlantic end and not from the eastern or Red Sea extremity. Suggestive of this is the fact that specimens of the same species from the Atlantic Islands (Madeira, Canaries &c.) and the Mediterranean are absolutely identical, whereas, in some instances at all events, in the Red Sea equivalents some slight modifications are noticeable. 1 Die geologische Beschaffenheit der Landenge von Suez. Wien, 1877. 2 Neue Denkschrift. allgem. Schweiz. Gesellsch. 1883, vol. xxviii. pt. 3. 3 Vergh-physiolog. Studien, 1888, 2nd ser., 5th part, 1st half. PROC. ZOOL. Soc.-1891, NO. XXVII. 27 |