8 Proceedings of the Royal Irish Acadenvi. 



is iucumbent upou the theorist who peoples the Mid-Pacific islands by such 

 means to show why such domiuant groups as the Helicidae, Buliminidae, 

 Ehytididae, Streptaxidae— in fact, the whole Holopoda and Agnathomorpha 

 with the higher members of Aulacopod families, as well as the higher 

 operculates — should have utterly failed to take advantage of these means of 

 transport."' 



In counexion with the well-known fact that many invertebrate species 

 have acquired an immensely wide distribution, it is well to recall what 

 Darwin already told us, that within each great class the lower organisms 

 change at a slower rate than the liigher. Consequently they will have had 

 more opportunity for dispersal, while still retaining the same specific 

 character.' 



But even Darwin had no conception of the remoteness of the date of origin 

 of most of our common genera, and even of species of land and freshwater 

 shells. Pahi'ont" 'logical discoveries have revealeil e\'en in ilesozoic deposits 

 certain species which are still living at the present day. Genera like 

 C'lausilia have now been traced to the Cretaceous period. Their dispereal to 

 remote regions may have taken place in tlic dawn of tlie Tertiary era, when, 

 as we know frDUi independent rcscarche-s, the allocation of land and water 

 was vastly ditlerent from what it is now. 



And yet the influence of accidental or occasional means of dispersal upon 

 the fauna and flora of a country is considered by many zoologists and botauists 

 to i»e of profound importance. Until recently tlie e^^dence tliat could be 

 adduced in favour of their theory was almost unsupported by any actual 

 demonstration in the field, as we might say. At last the longed-for evidence 

 has been discovered in the shape of an island whose fauna and flora are 

 alleged to have been completely destroyed by a volcanic eiuptiou, and subse- 

 quently entirely reintroduced by accidental means. Dr. Ernst's account of 

 it made quite a sensation. The terrible outburst of fire and ashes from what 

 was lookeil upon as an extinct volcano on the i.sland of Krakatau (Krakatoa) 

 occurretl not verj- many years ago — in 1883. Half the island sank beneath 

 the ocean ; the remainder was covereil with a layer of glowing stones and hot 

 ashes, reaching an average depth of 100 feet. In some parts of tlie island, 

 however, as the author tells us, not more than two mouths after tlie eruption, 

 subaerial ilenudation had alreatly carved out of the loose strata deep valleys 

 and gorges. " In the \acinity of the peak," as he puts it, " where the newly- 

 formed deposit must have been thinnest, patches of the original rock-surface 



■ PiUbry, H. A., '• OenesU of Mid-Pacific Faunas," p. 572. 

 ' Darwin, C, " Origin of Spede*," p. 369. 



