o'4 



ON THE GEOGRAPHICAL DISTRIBUTION OF 

 MOLLUSCA IN SOUTH LONSDALE. 



Rev. C. E. Y. KENDALL, B.A., J. DAVY DEAN, and 

 W. MUNN RANKIN, M.Sc, B.Sc. 



The distribution of mollusca over a given area, as of other more 

 or less sedentary forms of life, is noticeably discontinuous ; 

 the species constituting the fauna being limited within their 

 stations by life conditions favourable to the individuals. The 

 occurrence of a species within a district is not simply a function 

 of the organisation of the individuals, but to a large extent, 

 is one also of the purely physical conditions of the habitat 

 wherein they obtain. Thus regarding a particular area broadly 

 from the view point of an ecologist to whom the life conditions 

 of a species are of interest not second to the taxonomic rank 

 of the form, there is a closed patchwork of wide habitats 

 showing among themselves much diversity, but within an almost 

 uniformity of conditions, upon which the presence of living 

 forms depends. 



In addition to this concept of the grouping together or asso- 

 ciation of physical factors of biological value, there is the 

 further, of fne association of species, which in their individuals 

 find the physical inanimate nature more or less advantageous 

 to their manifold activities. The distinguishing of such natural 

 groups, shewing a biological or ecological uniformity, as im- 

 mediately concerning vegetation, has been the business during 

 many years of several workers, chief among whom in England 

 is Dr. W. G. Smith. Following the methods laid down in his 

 early papers, and those of his brother, the late R. Smith, not 

 only have the Pennine areas — ridge and flank, from the Peak 

 to the Cheviots been marked out into their vegetation associa- 

 tions — but also that district of South Lonsdale with which our 

 theme is immediately connected. The distinguishing of plant 

 associations, whether of the salt marsh, of the woodland, or 

 of the moorland, is distinct from the effort to make as com- 

 plete a floral list as may be — a worthy and profitable aim in 

 itself — but in its analysis of the physical factors of climate and 

 soil ruling in the station, and of those arising from the struggle 

 of individual with individual and species with species, as well 

 as of both with the station, there is the further effort to make 

 out the mechanism of the broad biological associations of which 



Naturalist, 



