NO. 11 GARTH : DISTRIBUTION STUDIES OF BRACHYURA 617 



of the formation of the islands by subsidence of the mainland mass (Baur, 

 1891, 1895, 1897), G. planatus having found its way to Socorro, San 

 Benedicto, Clipperton, and Malpelo. However, had the islands been so 

 formed it is scarcely conceivable that one or both families would not have 

 persisted. Their absence, therefore, is in keeping with the theory of inde- 

 pendent, volcanic origin proposed by Darwin (1839, 1845), in which 

 chance has been largely responsible for subsequent faunal accretions. 



To one who inclines toward the latter view, the assemblage of brach- 

 yuran species, genera, and families which now inhabits the Galapagos 

 littoral appears as definite a faunal entity as do those of Hawaii, the 

 Azores, or any group of oceanic islands. The confluence of mighty ocean 

 currents which converge upon the archipelago from each of three 

 American faunal provinces and from trans-Pacific islands determines that 

 the small but stable endemic population shall occupy a subordinate position 

 to the large and diverse increment of current-borne species, while the 

 geographical proximity of the islands to the continental mass decrees that 

 a relatively large proportion of these shall be drawn from the more highly 

 developed fauna of the tropical American shore. As these vagrants be- 

 come established and their differentiation proceeds, barring always changes 

 in oceanic circulation which would force them back in numbers, the 

 Galapagos brachyuran fauna will become increasingly distinctive, and 

 will no longer be considered merely an extra-territorial extension of 

 either the Panamic or Peruvian faunas, as has been previously advocated. 



SUMMARY OF DISTRIBUTION 



(1) The system of oceanic circulation observed in the Galapagos 

 Islands, plus that known to exist in the greater Pacific area, are together 

 capable of accounting for the now recognized distribution of brachyuran 

 species within the archipelago on the basis of oceanic transportation of 

 larval stages alone. 



(2) The 11 species of Galapagos Brachyura common to Caribbean 

 waters and the 38 Galapagos species represented by analogous Atlantic 

 species are the largest group to which a common origin can be assigned. 

 They indicate the former transportation of larval stages by a westerly- 

 directed current flowing through the Panama Portal over the present 

 Isthmus of Panama. 



(3) The 63 Galapagos species common to the American mainland 

 from the Gulf of California to the Bay of Panama or to the Bay of 

 Panama alone are evidence of the predominant influence of the Panamic 

 Province upon the Galapagos brachyuran fauna. Except in the cases of 



