472 ME. H. LTSTEE JAMESOK ON 



of the Soutli wall from the Pigeon-house to Poolbeg, which took 

 place about 1790.) 



Kocque's map, revised by Bernard Scale (1773), is a very 

 careful and elaborate chart of the environs of Dublin. The 

 North Bull Island is not figured, and certainly would have been 

 if it existed, as the map goes into very great detail. Though 

 the South wall is not yet built, the Pigeon-house and Poolbeg are 

 connected by a row of stakes, which Haliday informs us (' Scan- 

 dinavian Kingdom of Dublin,' pp. 235-236) were put there in 

 1717. It is needless to add that none of the many maps before 

 this date that I have examined, going as far back as 1610, show 

 any land, though the submerged bank is generally figured, and 

 in some stated to be exposed at low tide. On the other hand, 

 Dalkey Island and " Clontarf Island " (now, I believe, part of 

 the mainland) are figured in most of them. 



McKenzie's chart, 1775, does not bear a figure of the island. 



Evidently between 1775 and 1800 the island first arose, and 

 unfortunately between these two dates I cannob find any maps. 



If the South wall was not built until 1790, it is probable that 

 the island (if it existed before that date) was too small to support 

 a colony of mice until the wall was finished ; especially when we 

 consider Broughton's statement in 1810, when the island was 

 only 5 mile long. These data allow us about 100 years for the 

 evolution of the pale race, with a maximum limit of 120 years ; 

 and certainly, when one considers the exceptional circumstances, 

 one cannot but think that a century or less would be more than 

 sufficient to produce the form with which we are dealing. 



A very interesting observation, which emphasizes the import- 

 ance of Isolation in intensifying the eff"ect of Natural Selection, has 

 recently been recorded by Kane (3). He discovered, on a small 

 island off the South-west coast of Ireland, a melanic race of the 

 Greometer Camptogramma hilineata, which he names var. isolata ;, 

 and he holds that this melanic form owes its existence to the 

 weeding-out of the light ones by the Gulls, Pipits, and Bats that 

 frequent the island, and that the darker ones, harmonizing with- 

 the colour of the rocks, escape more readily. On the adjoining; 

 mainland, as well as upon other parts of the coast where the 

 rocks are of a dark colour, more or less melanic forms occur, but 

 along with the type ; but on this particular island the type is not 

 to be found. 



I do not myself know any case of the kind where a protective 



