296 Proceedings of the Royal Iriah Academy. 



study of zoogeograpliy" in the determinatioii of past changes in the sur- 

 face of the globe. '' It is certainly a wonderful and unexpected fact," he 

 says (€, p, 14), " that an accurate knowledge of the distribution of 

 birds and insects should enable us to map out lands and continents 

 which disappeared beneath the ocean long before the earliest traditions 

 of the human race." 



That certain species are occasionally liable to be accidentally 

 can'ied away from their homes towards distant lands, has been noted 

 and referred to by Lyell, by Darwin, by Wallace, and many others. 

 Tet, owing to a variety of circumstances, and especially the great 

 difficulty such species have in maiataining a foothold in their newly 

 acquired quarters, such species do not persist as a rule in large 

 numbers in any country. Altogether I am convinced that their 

 influence in the permanent colonization of a country has been 

 exaggerated. Actual observations of accidental introductions, more- 

 over, have only been made in exceedingly few instances ; while there 

 are numerous records to show that such occasional intruders rarely 

 become established. In order to find out whether animals could 

 traverse oceans and thus populate islands, Darwin and others have 

 attempted to determine experimentally how long certain snails could 

 stand immersion in sea-water. For one of their experiments Cyclo- 

 stoma elegans was taken, a snail provided with a lid or operculum 

 which can be closed over the mouth of the shell. This provision of 

 nature enabled the creatui'e to withstand a fortnight's immersion in 

 sea-water ; and one would imagine such a species to be easily trans- 

 ported by sea to distant islands in that time. Cyclostoma elegans is 

 common on the western border of f ranee and England ; but though 

 dead shells of the species have been cast upon the shores of Ireland 

 repeatedly, and probably living ones as well, it has never become 

 estabKshed on this island. If a species so particularly favoured by 

 nature to resist the deleterious influences of sea-water is unable to 

 establish itself in a neighbouring island, what chances are there for 

 less suitably endowed forms to cross the ocean ? 



But in studying zoogeographical problems such as the one I have 

 endeavom-ed to solve, it is not at all necessary to base a theory on 

 species which can be accidentally transported by hunicanes or marine 

 currents. We can confine oui'selves to those whose habitats preclude 

 the possibility of occasional dispersal. As such I consider Plutonia 

 atlantiea — a slug-like creature living altogether underground in the 

 Azores ; the fresh- water crayfish, confined to South America and West 

 Africa; the blind woodlouse Platyarthrus, inhabiting ants' nests in 



