Geographical Distribution of Mollusca. 315 



the plant associations are single examples among several. 

 The floral composition of a particular association of plants is 

 to a large extent the register of the operation of a complex 

 of factors working upon the plant individuals as material, 

 which in the main is previous to and independent of the vegeta- 

 tion. Forms of hfe, other than plants, which, hke them, are 

 more or less ' spot-bound,' would seem to be scarcely less 

 dependent upon the definite physical conditions of these asso- 

 ciations. Thus, in taking the various plant associations and 

 formations as the basis of a geographical study of mollusca, 

 there is something more than a convenience. A further bond 

 between the molluscan fauna and the florula holds in the 

 holo- or semi-parasitism, for food and home, of snails upon the 

 vegetation. The many plant associations of first importance 

 which have been recognised in the district of South Lonsdale 

 are here recognised as life-associations and made to serve for 

 a preliminary molluscan survey. It appears to us that the 

 loan of the results obtained in one field of natural science to 

 problems in another is amply justified in the present treatment, 

 as well as in the hope for future work on new lines. 



The area taken for consideration lies broadly, as a naturally 

 defined tract of diversified country of hill and valley encircled 

 between the sea in the west and the ranges of highlands in the 

 other quarters — to the north the Cumbrian watershed, to the east 

 the Pennine axis, and to the south the Wyresdale escarpment 

 and slope. It comprises not only the lower basin of the Lune, 

 but also of the several rivers which are properly its tributaries 

 entering the shallow waters of Morecambe Bay. Using the 

 usual bio-geographical terms, it covers much of the vice- 

 counties 60 (West Lancashire), and 69 (South Lake District). 



The altitude ranges between sea-level and 3000 feet O.D., 

 so that within the limits of the district there is a wide variation 

 within the climatic factors of mean monthly and diurnal tem- 

 perature of soil and air, of rainfall, of air humidity etc. There 

 may be instanced the contrast in cHmate between Grange — 

 suggesting the geniality of a Devon sea-village — and the summit 

 of Ingleboro or High Street, little removed from the Arctic 

 tundra. 



The soils have abundant variety. The Silurian and Ordovi- 

 cian rocks of the northern buttress, ecologically, stand inter- 

 mediate between the Carboniferous Limestone of the eastern 

 scars and the Millstone Grits of the southern fells. On the coast 



1909 Sep. I. 



