BRITISH VERTEBRATES. 
41 
belonging to groups whose powers of locomotion are limited to 
the land or fresh water, are not numerous compared with those 
inhabiting large continental tracts. Their numbers can only 
increase under exceptional circumstances, and have a tendency 
to diminish, as the growth of human population and increase 
of the area of cultivated land gradually circumscribe their 
native haunts. In this way, the Brown Bear, the Wolf, the 
Beaver, and the Wild Boar have disappeared from Britain 
within the historic period, while others, as the Badger, Marten, 
and Wild- Cat, with difticulty maintain a somewhat precarious 
existence. These have all been originally derived from the main- 
land of Europe, probably before the formation of the channel 
which now makes our country an island. The wider and older 
channel which separates Ireland from Great Britain has beeiL 
a greater barrier to the emigration of animal life than that 
between the latter and the Continent, many species (as the 
Polecat, Wild-Cat, Mole, Squirrel, Dormouse, Harvest-Mouse, 
Water and Land Vole, Common Hare, Eoedeer, as well as 
Snakes and Toads) never having crossed, unless by human 
agency. 
On the other hand, those species that have the powder of 
travelling through the air or traversing the ocean are far less 
fixed in their habitat, and thus the list of so-called " British 
birds " receives accessions from time to time from stragglers 
which find their way from the European continent or even across 
the Atlantic, and doubts as to the authenticity of some of 
the recorded occurrences make the list somewhat vague and 
uncertain. 
Slight but permanent variations from the continental type 
may be recognised in a fev/ of our indigenous species, some of 
the most marked among vertebrated animals being the Irish 
Stoat, the Squirrel, the Bed Grouse, the St. Kilda Wren, and 
several species of fresh-water fishes, mostly belonging to the 
genera Salmo and Cor eg onus. Some of the latter have an 
extremely local distribution, being only found in some small 
groups of mountain lakes. 
Of the Seals, only two species are really indigenous to Britain, 
the Common Seal {Phoca vitulina) and the Great Grey Seal 
(Hcdiclicerus gryirus). 
