468 ME. H. LTSTER JAMESON ON" 



passages leading from them. Two of them were made entirely 

 of blades of bent-grass {AmmopMla \_JPsamma] arenarid), the 

 third of tidal refuse, straw, paper, and feathers. I found one 

 mouse in a hole only a few inches deep, outside which was a 

 heap of freshly thrown-up sand ; there cnn be uo doubt it was in 

 the act of burrowing, which proves that they make their own 

 holes ; indeed, on the North Bull there is no other mammal 

 to make them. This fact is of interest, as, if Mus musculus in 

 the wild state uses the burrows of other animals as Thomas 

 suggests (1), we have here a habit that has been acquired by 

 these mice since they colonized the island, to enable them the 

 better to escape from their enemies. 



They are very numerous ; the sandhills are in places riddled 

 with their burrows, and the island is probably stocked with as 

 many mice as it can support, for I found in several cases the 

 mice I trapped had been devoured more or less completely by the 

 survivors. 



Presumably they live on the seeds of such plants as grow on 

 the sandhills, and on the varied diet of city offal cast up on the 

 sands from the mouth of the Liffey. At dusk I more than once 

 observed mice among the reiuse left at high-water mark. 



They are probably quite as prolific as their darker domestic 

 ancestors ; the uterus of one female contained nine foetuses. 

 The numerical increase must be very much accelerated by the 

 absence of terrestrial scent-hunting enemies. 



Running very rapidly over the sand, these mice are almost 

 impossible to follow with the eye, so perfectly do they harmonize 

 with their surroundings. 



When we come to enquire into the factors which have led to 

 the evolution of this race, we have not far to look. The short- 

 eared owls, which in autumn and winter always frequent the 

 sandhills, as well as the numerous hawks that come over from 

 the mainland, and can be seen any day in pursuit of prey on the 

 North Bull, most readily capture those mice which contrast most 

 strongly with the sand and the arid vegetation peculiar to such 

 places ; in short, they pick out the darker mice. The very 

 scanty cover, and the probability that the mice have to travel 

 some distance for their food over bare sand, make the dangers 

 to dark and conspicuous mice much greater. 



Under these circumstances, coupled with the fact that the 

 hawks and owls hunting by sigJd are the only enemies these- 



