NO. 11 garth: distribution studies of brachyura 615 



Revilla Gigedos, an archipelago of a sort; similar in origin to the 

 Galapagos and supporting an authentic, if limited, endemic population. 

 Each island group stands athwart a great oceanic current, the two so 

 nearly comparable that they have been called mirrored images of one 

 another (see Chart IV). Some hundreds of miles west of the Galapagos, 

 waters of both streams, on coming into contact with the edges of the 

 Equatorial Countercurrent, are subjected to a transverse circulation 

 (Sverdrup, op. cit., p. 711) and may actually interchange to some degree. 

 That planktonic forms of Galapagos origin could be cast back on the 

 Revilla Gigedos in a period of instability involving the invasion of 

 Northern Hemisphere waters (between 10° and 20° N. Lat.) by the 

 Equatorial Countercurrent, with accompanying displacement of the 

 California Current, is at least a theoretical possibility, although such a 

 shifting of the meteorological Equator would involve a deflection oppo- 

 site to that known periodically to occur. Such a phenomenon would 

 account most plausibly for the occurrence of Galapagos endemic species at 

 Socorro and Clarion, and would allow for the analogous Atlantic, or 

 even the direct Chilean, relationship claimed for L. cooksoni. In any 

 event, the Galapagos Islands cannot well be overlooked as a factor in 

 considering the faunas of other Eastern Pacific islands, either as a site of 

 possible origin or as a relay station between them and more remote 

 localities. 



DISTRIBUTION OF GENERA 



As pointed out by Finnegan (1931) : "The entire circumtropical, or 

 Indo-Pacific-American and American-African faunas, are knit together 

 not so much by their species as by their genera. . . The explanation of 

 this would seem to be that genera, being much older than species, were 

 established at a time when barriers now in existence, such as the Isthmus 

 of Panama, did not impede their migration." Clearly Galapagos Brach- 

 yura are no exception to this principle, for it is immediately apparent that 

 the relationship of the Galapagos fauna to the Indo-Pacific is better indi- 

 cated by genera than by species, there being 10 of the former common to 

 the two regions and to none other, as against but 5 of the latter. An 

 additional 20 genera were found to be common to western Pacific, 

 Galapagan, and Caribbean waters. Obviously these were established on 

 the American coast while the Panamanian portal was in existence, the 10 

 genera excluded from the Caribbean having put in their appearance after 

 its closure. 



