112 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 122 



genus from South America. Menard (1964) reported that reef debris 

 of from late Tertiary to Recent age has been dredged from the Nasca 

 Ridge and that there were some Tertiary islands on the Nasca Ridge 

 which no longer exist today. While there is no data available on 

 the adjacent Sala-y-Gomez Ridge for the same period, the bathymet- 

 ric similarity of the two ridges would tend to confirm similar geologic 

 histories. 



In this scheme the populations of Entomacrodus at Clipperton and 

 Clarion Islands would be derivative of the mainland populations. 

 The resemblance of the Clipperton Island and Clarion Island in- 

 dividuals to specimens of E. sealei or E. caudojasciatus would then be 

 a secondary, parallel development. Some support for the derivation 

 of the Clipperton Island (farthest offshore tropical eastern Pacific 

 island) population of Entomacrodus from the eastern Pacific mainland 

 form is to be found in the primarily New World salarine genus Ophi- 

 oblennius Gill (eastern Pacific, western and eastern Atlantic in dis- 

 tribution — Springer, 1962). In the eastern Pacific, 0. s. steindachneri 

 Jordan and Evermann is distributed widely from Mexico to Peru, while 

 the population at Clipperton, 0. s. clippertonensis Springer, is endemic. 

 In view of the predominant offshore flow of the currents from the 

 mainland and the very large size attained by the larvae of Ophio- 

 hlennius (up to 66 mm SL), specimens of which have been taken off- 

 shore over deep water (Springer, 1962), I believe it quite possible 

 for Ophioblennius (and Entomacrodus) to have made the 600-mile 

 journey from the Mexican mainland to Clipperton. The North 

 Equatorial Current, which flows at the constant rate of 0.3 knot per 

 hour (Wyrtki, 1965) would be the vehicle of transport and the time 

 involved only 83 days. The main objection to this hypothetical 

 origin of the Clipperton population is the question of how the popu- 

 lation is able to maintain its distinctiveness in the face of a probable 

 continuing inflow of larvae from the mainland. A conjecture pro- 

 posed by Gosline (1957) to explain endemism of Hawaiian fishes may 

 answer this objection. Gosline believes that once a popidation of 

 a species colonizes a new area and becomes adapted to that area 

 (evolved), it either competitively excludes subsequent colonization 

 by the less adapted parental form or genetically swamps the subse- 

 quent arrivals. This conjecture requires a low intensity of continuing 

 immigration of the parental form. 



The alternative to the previous method of introducing Entomacrodus 

 to the eastern Pacific is to bring the colonizers across open ocean. 

 If the introduction was effected over the shortest route from the 

 central Pacific, it is still necessary to explain the mechanism of trans- 

 port. Such transport could be effected by the Equatorial Counter- 

 current, which occasionally skirts Clipperton Island (for discussion, 



