8 Panama Shells. INTRODUCTION 



Fissurella nigropunctata ; on stones and rocks, at and be- 

 low half tide level. 

 - Siphonaria gigas ; on rocks near half tide level. 



Thus it appears that a larger proportion of the littoral 

 than of the pelagic known species are common to the Galla- 

 pago Islands and the mainland : and of the pelagic species, 

 all which are known inhabit moderate depths. Locomotion, 

 therefore, beneath the sea cannot be assumed as a probable 

 means of dispersion. 



If, however, the distribution is due to dispersion by the 

 floating of the spawn westward, along with the prevailing 

 winds and currents, it is remarkable that there are no well 

 authenticated examples of clearly identical species which are 

 common to the Gallapagos and to the Polynesian Islands. 

 In respect of species, which so nearly resemble each other as 

 to be sometimes confounded, and which are by some authors 

 regarded as varieties produced by local conditions, the "West 

 Indies furnish as many of these analogues to the Panama 

 species as have been found in Polynesia. There is, therefore, 

 no reason for referring the Polynesian analogues to a conti- 

 nental origin. 



If such dispersion westward were a fact, then we ought 

 to find, as we go westward from the Pacific shores of America, 

 a continual accumulation of species. For, in addition to the 

 full complement of aboriginal species in any region, there 

 would be a per centage of immigrant species. If such dis- 

 persion were a common fact, the Gallapagos should be much 

 richer in species than the continent, and the western regions 

 of Polynesia and the Australasian Archipelago should contain 

 a great number of immigrant species in addition to their ab- 

 original Faunae. But there are r no facts which correspond 

 with such hypotheses. So far as is known, the Bay of Panama 

 is as rich in species as any region westward, with no more 



