544 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 2 



than 1,500 fathoms) extends from the Galapagos Islands past Cocos 

 Island almost to Panama, while deeper water lies between the Galapagos 

 Islands and South America. Thus, a possible former shallow-water con- 

 nection with the present Gulf of Mexico is suggested. 



While the present fauna and flora of the Galapagos Islands are doubt- 

 less of multiple origin, affinities to the West Indian region have been 

 frequently noted. Agassiz (1892, pp. 74-75), discussing the deep sea 

 fauna of the Panamic district, says: ". . . we found, in the first place, a 

 great many old West Indian friends. In nearly all the groups of marine 

 forms among the Fishes, Crustacea, Worms, Mollusks, Echinoderms, and 

 Polyps, we brought up familiar West Indian types or east coast forms." 



Swarth (1934) found the avifauna of the Galapagos more like that 

 of the West Indies than like that of any other region. He states (p. 215) : 

 "There is an important element clearly recognizable as of West Indian 

 affinities, that could have had no other derivation. There are other species 

 that might have come from either a West Indian or a Central American 

 source, and there are a few species that definitely do not belong to the 

 West Indian avifauna. There is not one species that can be recognized as 

 having necessarily come from the adjacent South American coast." Later 

 (p. 233) Swarth states ". . . the relationship of the fauna of the West 

 Indies and the Galapagos is to be regarded in the same light as relation- 

 ships from one to another of the West Indian Islands." 



Fisher (1938) finds somewhat similar evidence regarding the hydro- 

 corals. He states (p. 495) : "I have also examined material from the 

 Galapagos Islands, which is well represented in the National Museum. 

 This fauna has nothing to do with that of the north Pacific. If it points 

 anywhere it is to the West Indian region." 



Thus, conditions revealed by the distribution of trematodes support 

 conclusions arrived at by various other lines of evidence. It might be noted 

 that there is greater similarity among the trematodes of the two oceans 

 than among the hosts involved. Such a condition might be expected from 

 the more uniform environment of the endoparasites and from their rather 

 specialized form. 



The interoceanic similarity of these trematodes could be explained in 

 four different ways : ( 1 ) a continuous distribution around the entire 

 South American continent: (2) recent transfer through the Panama 

 Canal; (3) transfer by fish-eating birds; (4) the influence of a foniier 

 continuity between the two oceans. The first two of these explanations are 

 very unlikely if not impossible. Although no collections have been made 



