BOYCOTT : DISTRIBUTION OF BRITISH NON-MARINE MOLLUSCA. 165 



tliat costs of carriage do not compel attempts at its production in 

 unsuitable localities. The maps show that wheat is south-eastern, 

 oats nortli- western, and it is difficult not to believe that climate 

 is the chief factor determining this distribution. 



Turnips have to be dealt with in a different way because they are 

 not particularly valuable, and are relatively bulky, so that they 

 cannot be economically transported from a district where they 

 flourish to one which is not very well suited for them. The map 

 shows accordingly the produce of turnips and swedes per acre, the 

 three areas being 13 tons or less (plain), 14 or 15 tons (dotted), and 

 16 tons and over (black). Turnips are evidently north-western, 

 their relative failure in the remote parts of the north of Scotland 

 and the west of Ireland indicating that they want good farming as 

 well as plenty of rain. 



Is it possible that Helix j^omatia and other south-eastern species 

 (see maps on PI. VI) live where they do because the climate is 

 congenial and not because they have recently arrived from Europe ? 

 The position of the north-western species may also be comparable 

 to that of oats : J. W. Taylor would indeed find a parallel and say 

 that they do not live in the south-east because the more highly 

 specialized species, which are characteristic of that area, crowd them 

 out in the same way that oats predominate where wheat cannot be 

 profitably cultivated, and alders grow in swamps where they escape 

 competition with other trees that cannot tolerate such places. The 

 idea of direct competition on land between snail species which this 

 view involves is, however, highly problematical, and it is significant 

 that Pwpa anglica and Acanthinula lamellata are found in holocene 

 deposits in the south-east, where they are now extinct. Whether it is 

 the complex of circumstances which we call civilization, or increasing 

 dryness which has destroyed them there we cannot tell : the two 

 are not mutually exclusive, for surface dryness is one of the attain- 

 ments of English civilization both in our streets and in our cultivated 

 land. I should say, therefore, that these north-western species 

 require wetness without too much summer heat : note particularly 

 that the summer temperature of south Ireland is that of Yorkshire. 



(5) It is natural to suppose that different species will be differently 

 affected by temperature in relation to their varying seasonal 

 activities. In one case the summer clim.ate may be most important, 

 in another the winter weather. Species which breed in the late 

 autumn and winter may be either mainly south-eastern {Helicella 

 virgata, H. ca])erata), north-western {Hygromia fusca), general and 

 local {Limax tenellus), or general and common (Vitrina pellucicla), 

 and at present I cannot trace any definite correlation. The -range 

 of another autumn breeder, Cochlicella harhara, from. Sussex to the 

 north of Scotland round the southern and western coasts, and 

 more generally in Ireland, is suggestively coincident with the January 

 isotherm of 39°. Another one, Hi/gromia revelata, is restricted to 



