17 14 Proceedings of the Royal Irish Academy. 



seems simpler to regard the common genera as widespread types formerly 

 inhabiting the whole area between Ireland (eastwards) and New York, and 

 intermingling in the New and Old "Worlds by means of a bridge across the 

 Pacific Ocean at or near Bering Sea. Such a bridge is easier to imagine than 

 one across the Atlantic ; and in fact we find on examination that all the above 

 genera, excepting only Elephas, are of eircunipolar distribution in the above 

 area, and there is a strong probability that Elephas also will prove to be so. 

 A point telling against the Atlantic land-bridge is that England has actually 

 more mammals of American type than Ireland, and western Europe even 

 more than England ; also that some of our commonest types, e.g., all the true 

 Muridae, have never reached America. Apodemus (Wood-Mouse) is found in 

 many British deposits from pliocene to late pleistocene times (Forest Bed to 

 Ightham), aud would surely have been one of the first forms to utilize a land- 

 bridge enabling it to reach North America. There seems no need, then, to 

 suppose that our present Mammal-fauna owes any of its features to a recent 

 land-bridge extending to America. The influence of earlier land-bridges, 

 though they may still be evident in the distribution of invertebrates and 

 plants, is no longer traceable in mammals. 



