NO. 11 GARTH : DISTRIBUTION STUDIES OF BRACHYURA 611 



While the great majority of the 38 Galapagos species having Atlantic 

 analogues occur among the large group of species common to the Panamic 

 province, not all Galapagos Brachyura possessing such analogues are to be 

 found in this category. One or two species having analogues are to be 

 found among those common to the Galapagos and to north or south 

 temperate waters alone. This is understandable in view of the fact that 

 the Rathbun designations of analogous Atlantic and Pacific species repre- 

 sented comparisons of the two faunas as a whole, irrespective of the lesser 

 provinces into which each might ultimately be divided, and that morpho- 

 logical resemblance, rather than continuity of habitat, was the determining 

 factor in establishing relationship. 



It is assumed, to begin with, that the analogous species, Pacific and 

 Atlantic, were one species occupying a continuous range at the time of the 

 Isthmian closure and that the slight morphological differences which 

 separate them have arisen since that time. Consequently, species now 

 occupying the Bay of Panama which do not possess Atlantic analogues are 

 believed to represent recent arrivals. It follows that their more strongly 

 differentiated characteristics, requiring greater time in development, have 

 been attained in another portion of their present or immediate past range. 

 In the case of Panamanian species now common to both Gulf of California 

 and Galapagos, this place of origin might have been either the Gulf of 

 California or the Galapagos ; in the case of Panamanian species common 

 only to the Galapagos, it would seem to have been the Galapagos alone. 



Galapagos species without analogues now inhabiting the Bay of 

 Panama, 30 in number, may be broken down into the following groups : 

 (a) species belonging to genera excluded from the Atlantic, and therefore 

 without analogous possibility, 11 ; (b) species having one or more Pacific 

 congeners, one of which is analogous to the single Atlantic member of the 

 genus, 4; (c) species belonging to certain genera, mostly oxystomatous, to 

 which recent new species have been added, leaving species on both sides 

 of the continent for which there is no accepted pairing, 6. Thus but nine 

 Galapagos species remain whose presence without analogues in the Bay of 

 Panama must be otherwise explained. 



It is reasonable to expect that a large number of current-borne species 

 would have been drawn into the vortex of the Bay of Panama from out- 

 lying regions in response to the shift in oceanic circulation which must 

 have taken place with the union of the two continents by a land bridge. 

 That some of these might have arrived from the Galapagos Islands with 

 the assistance of a current of the required magnitude and direction is even 

 more credible than that species should have reached the Galapagos from 



