184 The Irish Naturalist* October, 



were unable to do so. Taking the whole body of evidence 

 there is a practical unanimity of opinion among the 

 observers, resting on varied evidence derived from many 

 different groups of animals, that the narrow strait of sea 

 which separates Clare Island from the mainland represents 

 a very serious barrier to migration. The existing fauna as 

 a whole of Clare Island could not have crossed this barrier. 



The Survey has thus led to a definite advance in our 

 knowledge of the conditions of dispersal of animals and 

 plants. We can now affirm, as the result of these investiga- 

 tions, that the biological evidence points to the existence 

 of a former bridge of land between Clare Island and the 

 mainland We need not imagine this bridge of land to 

 have looked like a bridge such as we construct nowadays. 

 The term " land-bridge" has been used biologically in the 

 sense of a former continuous land-surface connecting two 

 areas now separated by the sea. The study of animals and 

 plants does not reveal to us whether this land connection 

 was narrow or broad or whether the whole coast -line 

 formerly lay far westward of its present site. In the latter 

 case Clare Island would have formed part of the mainland, 

 being surrounded on all sides by land. In any case we may 

 assume, that after these events had taken place the land 

 either subsided or the sea rose so as to produce the geo- 

 graphical features of the present time. 



Now it seems as if we had not devoted sufficient attention 

 to the importance of the absentees in our fauna. Especially 

 is this the case among the aquatic species. It is a striking 

 fact, for example, that the Frog is absent from Clare Island, 

 whereas it occurs abundantly on the mainland as well as on 

 Achill Island. Less than half the fresh -water mollusks of 

 Ireland are recorded from Clare Island by Mr. Stelfox, 

 although this does not appear to be altogether due to the 

 absence of suitable habitats. Mr. Balfour Browne dwells 

 on the poverty of the aquatic beetle fauna of Clare Island ; 

 but he beHeves that the lack of variety of habitat is accoun- 

 table for it. As regards fresh -water entomostraca, the 

 island fauna is mainly noticeable for negative rather than 

 positive characters according to Mr. Scourfield. Of the five 



