98 BULIMULUS-NAESIOTUS. 



the Galapagos, but in this the Panama current seems to have pre- 

 dominated, not only because it has a shorter traverse, but because 

 around the Gulf of Panama, and on the banks of the rivers falling 

 into it, a luxuriant fauna and flora are found close to the sea while 

 along the Peruvian coast only in time of freshet could any large 

 quantity of debris be expected to reach the waters of the current, 

 owing to the aridity of the immediate shores. The two currents 

 join forces at some distance eastward from the islands, and pour 

 through the passages between them with considerable force. Pro- 

 fessor Alexander Agassiz has shown how much terrigenous material 

 the Panama current bears, and that there is no reason to doubt that 

 trees still bearing leaves and with some of their branches above 

 water might be carried from the Gulf and cast upon the islands, and 

 that, at least during the rainy season and in favorable years, there 

 would be opportunities for animals so carried, especially land shells 

 glued by the epiphragm to the bark of branches, to gain vegetation 

 on the shores where they could support life and propagate their 

 kind. Though unproven, yet there can be little doubt that in this 

 way the land mollusk fauna of the islands was introduced and pre- 

 served " (Dall). 



That these and other means of transport have been only in rare 

 instances successful in introducing new members to the fauna is 

 evident when we consider that the whole of the Bulimulidce belong 

 to one group and apparently owe their existence to one importation, 

 and the other strictly terrestrial genera yet known are Helidna (1 

 species), Leptinaria (2), Succinea (4), Pupa (2 or 3), Conulus (1), 

 Vitrea (1) and one species resembling what is generally known as 

 Microcystis. In all, seven or eight genera, most of them represented 

 by but few species. Further investigation will doubtless increase 

 the number of genera and species, but will hardly modify the con- 

 clusion that the mollusk fauna of these islands was originally intro- 

 duced by the chance means of transport alluded to above, and that 

 there were very few successful immigrations. 



The large element common to several of the islands lends color to 

 the belief of Dr. Baur that the archipelago has resulted from the 

 disintegration of a single large island; but while this seems ex- 

 tremely probable, the theory that this island was formerly connected 

 with the continent derives but slight support from what is now 

 known of the land mollusks. 



