480 Proceedings of the Royal Irish Academy. 



itself in the question whether Ireland was only partially united to 

 Great Britain, as was supposed by Professor Leith Adams, or along 

 the whole of the east coast. To answer the last question by means of 

 the past or present distribution of the British mammals, seems to me 

 almost impossible; but some light is thrown on the subject by an 

 examination of the range of the British fishes. Still more useful in 

 solving this difficult problem are the Land and Freshwater Mollusca. 

 Their distribution is greatly influenced by the geographical conditions 

 of a country, and even a narrow channel of sea water forms an almost 

 impassable barrier to their migration. A minute study of the exact 

 range of the various species inhabiting the British Islands has given 

 me the greatest assistance in attempting to trace the extent of the 

 former land connection. 



It is evident that for the purposes I have in view, only those 

 species can be made use of whose claim to be considered indigenous 

 cannot be doubted. In commencing the enquiry, therefore, with the 

 twenty -two species of mammals known to inhabit Ireland, a certain 

 number, which may possibly have been artificially introduced, or have 

 arrived by some accidental means, should be eliminated : for instance, 

 all the bats, the rabbit, the rats, &c., so that only ten species remain. 

 Of these ten species it has to be ascertained whether they originated 

 in Ireland or elsewhere. The exact geographical distribution of most 

 -of the European species of mammals is well known, so that there is no 

 difficulty in constructing a table by means of which we can see at a 

 glance that all the ten Irish species also occur in Great Britain and 

 ■on the Continent. Some species even range beyond the boundaries of 

 Europe. That alone, however, does not prove that they did not origi- 

 nate primarily in Ireland, and only subsequently migrated to those 

 parts. How can we ascertain in what part of the world any particular 

 species originated? As probably every species of mammal has the 

 tendency to spread in all directions from its birthplace, the centre of 

 its geographical distribution must correspond approximately with its 

 original home. By determining the extreme limit not only of the 

 present, but also the past range of any species, we can thus approxi- 

 mately ascertain its original home. 



Let us take, for example, the Irish hare {Lepus variabilis). The 

 fact alone that in most countries its fur changes to white in winter is 

 suggestive of a northern origin, and this view is confirmed by its dis- 

 tribution. As we travel northward in Europe, Asia, or North America 

 we find this hare becoming more and more plentiful, and when we 

 reach the Arctic Circle it is almost the only rodent we meet with. On 



