Atkins and Lebouij — Hydrogen Ion Concentration of the Soil. 239 



Site A appeared to be the best soil, growing good grass with leguminous 

 plants, and conductivity measurements indicate that it was less washed out 

 than were the other soils. The three-hour values show that none of the soils 

 have more than small amounts of salts, the figures found being less than one 

 hundredth part, of that for sea-water. So the soils from B are not markedly 

 salt, as it was thought they might be. Chlorides are, however, present in B in 

 larger traces than in the others, and give an opalesence with silver nitrate 

 though inland soils often give nearly as much. 



It is evident that though the pH values are all between pll 75 and 80 

 there is no definite sequence in them. All the sites may be considered as 

 more or less uniformly of slight alkalinity. 



With regard to exposure to wind, B is most exposed, followed by A, C, and D. 

 The distribution of the snails is strikingly different, for example, in B and C, 

 where the physico-chemical values are almost identical. Again, though D has 

 twelve species out of the total fourteen, //. virgata and C. barhara are absent, in 

 spite of the fact that they constitute 73 per cent, of the population of B and 

 H. virgata 79 per cent, of A. There appears to be no specially marked difference 

 in temperature or moisture in the sites, all being dry and very warm, A probably 

 retaining moisture a little better than the others. Of all the characteristics 

 of the sites perhaps the force of the wind is the most noticeable difference, 

 though it is not easy to see why on this score Oreston should have no H. virgata. 



According to the " Age and Area " theory of Willis the extent of the 

 distribution of a species depends upon the interval of time since its introduction 

 into the region under consideration. This conception is obviously modified by 

 the existence of ecological factors inimical to any particular species. On the 

 other hand ecological factors alone must be incapable of explaining the distri- 

 bution of newly arrived species. On turning to the map given in Roebuck's 

 census it is seen that CochUcella harhara is widely distributed in Ireland, 

 with inland as well as coastal records. In England, Wales, and Scotland, 

 however, it is only found as a maritime species on the west coasts, and along 

 parts of the south of England. These facts point to its having a south-western 

 origin — from Europe or by Atlantic drift possibly. The area colonised by 

 Theba cantiana is to be accounted for similarly in all probability, for it occupies 

 the east of England, extending some way down the south coast and up as far 

 as Northumberland, where it has been found by one of us. This species is 

 absent from western England, Wales, Scotland, and Ireland, and may be 

 considered to have an eastern origin. 



SUMMARY. 



1. The hydrogen ion concentration of the soil is a factor limiting the 

 distribution of snails. 



2. Snails are more numerous at pH 7-8 than they are elsewhere. 



3. The number of species of snails found in the districts studied increases 

 from pH 5 four species, to pH 7 twenty species, falling at pH 8 to fourteen 

 out of the total twenty-seven species found. 



4. Snails with hyaline shells ma}^ be found in any portion of the range, but 

 those with calcareous shells are limited to the more alkaline end. Granite 

 and quartzite regions have few species, basaltic districts have a more numerous 

 fauna, and in limestone areas both species and numbers of individuals give 

 high values. 



