j8o THE SCOTTISH NATURALIST 



to that of Aberdeen South, 2 to that of Kincardineshire, and 

 13 to that of Ross East. 



Investigation has for the most part had to be on the 

 coast-line, not merely because easier of access to the collector, 

 but because they are the natural lines of advance of the 

 Mollusca themselves ; for, as Mr Booth shows by implication 

 in his printed observations on habitats, the hard palaeozoic 

 rocks of the interior by the method of their weathering 

 afford but few spots in which Mollusca can maintain even a 

 precarious existence. 



Other material available for our study, previous to Mr 

 Booth's work, include the papers published by William 

 Macgillivray (1843 an ^ 1855), Robert Dawson (1870), and 

 Rev. Geo. Gordon (about 1853), these being for the counties 

 of Aberdeen, Banff, and Elgin. It is a pity that Macgillivray's 

 and Dawson's collections, left to Aberdeen University, seem 

 to have practically ceased to exist, but we have seen some 

 of Gordon's from the Elgin Museum. A list for Caithness 

 was published in 1864 by Mr C. W. Peach. Other material 

 has included various casual notes and observations by 

 Messrs W. Evans, A. Somerville, Revs. J. E. Somerville, 

 J. McMurtrie, and R. Godfrey. Also by Mr W. Baillie, of 

 Brora, who was of late years perhaps the only resident 

 collector in the extreme north. 



So far, our examination of the materials available which 

 are, however, insufficient by reason of the fact that we know 

 very little as to the two most northern counties, Sutherland 

 and Caithness we are able to note the line of advance 

 westward and northward of a few of what we may term 

 dominant species, those which are gradually taking possession 

 of the more desirable parts of these islands, and are extending 

 their range to the detriment and the eventual expulsion of 

 the older and more recessive species, the results of this 

 being shown by the gradually diminishing number of the 

 localities where the recessive species occur, and the lessening 

 number of individuals therein. 



The greater advance along the coasts of Great Britain 

 on the east side of the country, which is so striking a feature 

 in the distribution of most life, is, as in these species, 



