GEOGRAPHICAL DISTRIBUTION 361 



(two separate species closely related morphologically and yet widely 

 separated geographically) are known and even vicarious generic 

 groups. The genus Hormothamnion in the Cyanophyceae, Micro- 

 dictyon and Neomeris in the Chlorophyceae (cf. fig. 206), all have a 

 Caribbean-Indo-Pacific discontinuity, whilst there are several 

 vicarious pairs in the genus Udotea. The explanation of these 

 discontinuities which has been advanced by Murray, namely 

 change of climate in former epochs, would only appear to explain 

 certain cases, e.g. certain species in the Laminariaceae (cf. below), 

 whilst it is equally obvious that the factors operating at present do 

 not provide an adequate explanation. The only feasible hypothesis 

 would be to postulate migration during an earlier epoch when there 

 was a sea passage through the Panama isthmus, and this involves a 

 migration not later than the Cretaceous. 



(2) There are some species which are common only to the 

 Western Atlantic and the western part of the Indian Ocean around 

 Madagascar e.g. Chamaedorus penicidum and three species of 

 Cladocephaliis. (Cf. fig. 207.) Although there is at present no very 

 adequate explanation for this distribution three possible hypotheses 

 may be suggested, but there does not appear to be any evidence 

 which supports one of them more than the others : 



(a) Migration via the Cape. 



{b) Migration via the Pacific and Panama, the related species 

 perhaps still existing in the Pacific but not yet recorded. 



{c) The related species or representatives in the interzone have 

 died. 



(3) There are some genera which are common to the Mediter- 

 ranean and the Indo-Pacific region, e.g. C odium Bursa group, the 

 vicarious pair Halimeda tuna in the Mediterranean and H. 

 cuneata in the Indo-Pacific, Acetahidaria mediterranea and other 

 species of Acetabularia in the Indo-Pacific (cf. fig. 208). In this 

 case also the only satisfactory explanation is the existence of a 

 former sea passage across the Suez isthmus. In the flora of the 

 northern part of the Arabian sea, out of a total of 137 species and 

 varieties, 22% are endemic, 52% are Indo-Pacific and 59-6% also 

 occur in the Mediterranean and Atlantic Ocean, the most striking 

 example being Cystoclonium purpureum which does not now exist 

 between its widely separated stations along the southern shores of 



