INTRODUCTION. 5 



several English pleistocene and holocene deposits. 

 The Stone-cutter can scarcely be looked upon as a very 

 recent immigrant in the light of this evidence, though 

 we have no proof of its having ever had a much 

 wider range in the British Islands than it has to-day. 



Among the lichens, which so abundantly cover the 

 rocks and trees in South-western Ireland, and which 

 impart such a characteristic feature to the scenery, 

 we find a beautifully spotted slug (Geomalacus macu- 

 losus}. 1 It is a stranger to the rest of the British 

 Islands, and indeed occurs nowhere else in Northern 

 Europe. We have to travel as far as Northern 

 Portugal before we again meet with it, and it is 

 there also that its nearest relations live. 



Many more similar examples might be quoted, but 

 enough, I think, has been said to show that the 

 British fauna is made up of several elements whose 

 original homes may lie widely apart and in different 

 directions. We have fossil evidence that some of 

 the northern species, and also a few of the southern 

 ones, have become extinct within comparatively 

 recent times ; others are apparently on the verge of 

 extinction, whilst many not only maintain their 

 position in the constant struggle for existence, but 

 are even extending their range. 

 /X^The problem of tracing the origin of the British 

 fauna, or at least that of some of the more 

 characteristic members of every. section or element, 



1 A map giving its exact distribution in Ireland will be found on 

 p. 300, and a figure of the slug on p. 298. 



