CHAPTER VI. 



MEANS OF DISPERSAL. 



A CLUE to the almost universal distribution of many 

 families and genera of land-shells is to be found, 

 Mr. Wallace observes, in their immense antiquity : in 

 the Pliocene and Miocene formations, he says, most of 

 the remains of these creatures are either identical with 

 or closely allied to living species, while even in the 

 Eocene almost all are of living genera ; no true land- 

 shells have been found in the Secondary formations, 

 but they must certainly have abounded, for in the far 

 more ancient Palaeozoic coal measures of Nova Scotia 

 two species of the living genera Pupa and Zonites have 

 been discovered in considerable abundance. Types 

 having thus "survived all the revolutions the earth has 

 undergone since Palaeozoic times," are hardly likely to 

 be confined by now existing arms of the sea, mountain 

 chains, and other similar barriers which have effectually 

 limited the ranges of many groups of higher animals.^ 

 But it is obvious, of course, that antiquity in itself, how- 

 ever great, could have effected nothing without migra- 



1 "Geographical Distribution," ii. p. 528; "Island Life," pp. 

 76-7, ed. 2, p. 79- 



I 2 



