_North European Islands._--The British Islands are known to have been recently connected with the Continent, and their animal productions are so uniformly identical with continental species as to require no special note. The only general fact of importance is, that the number of species in all groups is much less than in continental districts of equal extent, and that this number is still farther diminished in Ireland. This may be accounted for by the smaller area and less varied surface of the latter island; and it may also be partly due to the great extent of low land, so that a very small depression would reduce it to the condition of a cluster of small islands capable of supporting a very limited amount of animal life. Yet further, if after such a submergence had destroyed much of the higher forms of life in Great Britain and Ireland, both were elevated so as to again form part of the Continent, a migration would commence by which they would be stocked afresh; but this migration would be a work of time, and it is to be expected that many species would never reach Ireland or would find its excessively moist climate unsuited to them. Some few British species differ slightly from their continental allies, and are considered by many naturalists to be distinct. This is the case with the red grouse (_Lagopus scoticus_) among birds; and a few of the smaller Passeres have also been found to vary somewhat from the allied forms on the Continent, showing that the comparatively short interval since the glacial period, and the slightly different physical conditions dependent on {198}insularity, have sufficed to commence the work of specific modification. There are also a few small land-shells and several insects not yet found elsewhere than in Britain; and even one of the smaller Mammalia--a shrew (_Sorex rusticus_). These facts are all readily explained by the former union of these islands with the Continent, and the alternate depressions and elevations which are proved by geological evidence to have occurred, by which they have been more than once separated and united again in recent times. For the evidence of this elevation and depression, the reader may consult Sir Charles Lyell's _Antiquity of Man_.