WALKER ON ORIGIN AND DISTRIBUTION OF MOLLUSCA. 51 



and moisture along the south Appahichians has enabled a number of 

 species to extend their range into the Southern Atlantic states. 



In a similar manner some species of Campeloma and Goniohasis among 

 the Univalves and Unio and Margaritina and Anondonta among the 

 bivalves, have spread out to the northern reaches of the Missouri and 

 Mississippi and even into the St. Lawrence drainage and from thence 

 into the waters of eastern New^ York and New England. 



Toward the south a few of the species range into Mexico and Central 

 America where the_y mingle with the northern outposts of the tropical 

 fauna of South America. And in' a similar manner, a few stragglers 

 from the West Indies and South America have obtained a foothold along 

 the gulf states and the Mexican boundary. To Binney, treating the 

 North America fauna as .separate eiitirt'ty, the occurrence of these 

 species along our southern borders justified the establishment of a. 

 southern region or province. But a broader generalization based upon 

 the zoological relations of the two continents, requires its union with 

 the mixed fauna of Central America and Mexico, which is now con- 

 sidered a transition region between North and South America. 



Taken as a whole, however, there is very little in common, so far as 

 the existing mollusca are concerned, between the two great divisions 

 of the New World. Indeed it would not be far from true to say, that 

 not only are there no common species, which would scarcely be ex- 

 pected, but that common genera as well, are almost wholly lacking. In 

 almost every class of molluscan life, the corresponding place in the 

 economy of nature, is filled by radically different groups. Thus the 

 northern indigenous Helices are replaced by the tropical Bulimuli, the 

 Viviparas by the Ampullarias, the PU'iiroceridw by the Melaniadw and the 

 Unionidw (largely) by the Mutelidw and so on, almost indefinitely. In 

 short the differences are quite as great as between the fauna of North 

 America and Asia. There is one remarkable exception, however, which 

 must not be passed by unnoticed. The peculiar helicoid fauna of the 

 I*acific coast, which is so conspicuously absent from eastern North 

 America, is found not only through Mexico and Central America, but 

 all over South America as far south as Argentina. The importance of 

 this fact as bearing nj>on the evolutionary history of our fauna will be 

 referred to later. 



In striking contrast with this radical separation between the existing 

 fjiunas of North and South America is the close relationship between 

 lliose of the great continental areas of the Northern Hemisphere. With 

 the exception of the American melanians and the peculiar polygyrine 

 Helices of the eastern states, not only the families, but the character- 

 istic genera, are in the main the same. The minor groups peculiar to 

 each are but differentiations of types common to both. Moreover, in 

 addition to their general generic resemblance, according to one recent 

 authority, thei'e are no less than thirty-five species common to them all. 



These are the great elemental facts of present distribution, and to ac- 

 count not only for them, but for the many peculiarities of the provincial 

 faunas, which have been indicated, upon a basis of acceptable scientific 

 theory, is the probh^m which is now engaging the attention of all students 

 interested in tlie study of the origin and distribution of animal life. 



Before attem])ling (o pres(>nt the leading facts and theories which bear 

 upon the origin and introduction of the existing fauna of Noith America, 



