XXXV111 GEOGRAPHIC DISTRIBUTION. 



214, Eulota 200, Vallonia 282, Helicodonta 

 284, Helicigona 296, Helix 311, Plectopylis- 

 143, Gorilla 147. 



IV. DISTRIBUTION OF HELICES IN TIME AND SPACE. 



The bare facts of distribution of the several genera and species- 

 are sufficiently stated in the systematic portion of this work ; it 

 remains to draw the more obvious conclusions which they indicate. 

 As to means of distribution, there is much reason to believe that 

 upon continental areas, land snails, like mammals, Lave been mainly 

 dependant upon their own powers of locomotion, although rivers 

 with their flood-carried debris have doubtless been effective. Such 

 island faunas as are not traceable to former land connections, are 

 probably due to drift wood and " floating islands " swept from rivers ; 

 for although in rare cases the agency of birds or cyclones may have 

 been efficient, still the evidence of such means of transport of land 

 snails is extremely slight, and the facts now known do not warrant 

 or call for any extensive invocation of means so extraordinary and 

 exceptional, and so completely hypothetical. It will readily be 

 understood that the case with freshwater snails is quite a different 

 subject. 



The key to the wide distribution of many genera or super-generic 

 groups of terrestrials, is the known fact of their vast antiquity, 

 which has enabled them to take advantage of the various land com- 

 binations of several geological periods, and also of the rarely occur- 

 ring means of transport mentioned above. 



The fact must constantly be borne in mind that the evolution of 

 Pulmonates has been excessively slow ; and although the terrestrial 

 forms have changed more rapidly than the freshwater mollusks, 

 they cannot be compared with mammals or birds in this respect.. 

 Many genera of Helices dominant to-day, are known to have existed 

 in the early Miocene, and apparently as distinct then as now. In 

 the Eocene, forms less close to the recent occur, but in many cases 

 they cannot be generically different. In the mammalia we find the 

 roots not only of families, but of orders in Eocene strata, while 

 even the genera of Helices have scarcely changed since that time. 

 The super-generic groups must, therefore, strike deep into Mesozoic 

 time. As the means of transport of land snails are very limited 

 and slow, they lag far behind such freely mobile creatures as mam- 

 mals and birds ; and, therefore, we do not find, nor can we expect to- 



