110 
THE MEDITERRANEAN NATURALIST 
Geographical distribution of species is such an 
enigma in many cases that one feels reluctance in 
launching forth any theory whatever. Some 
species, at far as our present knowledge of them 
extends, appear to have an almost unlimited range; 
whilst, on the contrary, other allied forms seem 
to be equally restricted. As examples. I may 
instance Area lactea and A. olivacea. The former 
little species ranges through the Mediterranean 
Into the Atlantic as far north as this country, 
southward along the West of Africa past the 
1 flan tic Islands to Ascension Island, on to the 
Gape of 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 
eases of this kind, but the one mentioned is suffi- 
cient to demonstrate the unaccountable difference j 
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 Mediterranean. How- 
ever, taking all points into consideration, I think 
this supposition is likely to be as correct as the 
view usually entertained. 
Some support to i,his theory is derived from a 
A ; f - he emigration of species from the Red 
'Sea to tlw k xditerranean and vice versd since the 
opening ot the Suez Canal. From the reports 
upon- the subject by Fuchs (1), Keller (2), Kru- 
( J ) Die geologische Beschaffenhe.it der Landenge 
von Suez. Wien , 1877. 
( ') Neue Denkschrift. allgem . Sceweiz. Qesellsch. 
1883 , vol, xxviii . pt. 3. 
kenberg(l), 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 movemeut 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 may 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 
I 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 Ac.) and 
the Mediterranaan are absolutely identical, where- 
as, in some instances at all events, in the Red 
Sea equivalents some slight modifications are 
noticeable. 
The following table also lends some support to 
this proposition. It will be noticed that, starting 
I from Australia (2) and the Philippine Islands, all 
are found in the Red Sea, four at the Cape, one 
has been recorded from St. Helena, one from 
Ascension, six from the Atlantic Islands, and all 
vin the Mediterranean. 
(1) Yergl-physiolog. Studien , 1888, 2nd ser., 
5th part , 1st .half \ 
(2) Euthria cornea was recorded from New 
Caledonia by Brazier in 1889, and the \Challenger' 
dredged off Sydney about 10 species of Afollusca. 
which are inseparable from N. Atlantic forms ... 
