Willow Grouse (_L. albus_) (compare p. 91), constitutes in some respects
a curious case of parallelism with the Arctic Hare, since the latter, in
its more southern station, generally retains the summer fur throughout
the year. The allied Ptarmigan (_L. mutus_) inhabits Scandinavia, the
Ural Mountains, and some of the Asiatic mountain ranges. It is also
found in the European Alps and in the Pyrenees. The North European range
of the Ptarmigan suggests that we are dealing with an ancient species
which came south from the Arctic Regions at about the same time as the
Arctic Hare; but it is more probable, as I have shown in a subsequent
chapter (p. 334), that this species has entered Europe more recently
with the Siberian migrants from Central Asia, where indeed the genus had
its original home. The Black Cock (_Tetrao tetrix_) and the Capercaillie
(_Tetrao urogallus_) have also come to us from the east, and have even
penetrated into Ireland. They are therefore some of the few instances of
members of the Siberian invasion having become temporarily established
there.

Reptiles and amphibia are altogether unknown in the Polar Regions, but a
large number of fish, chiefly marine, have taken their origin there. The
Salmon family are of Arctic origin, as also are the Sticklebacks and the
Perches, many of the Cod family, the Herrings, and several of the Flat
fish.

It would lead me too far to refer to the invertebrate fauna of the Polar
Regions, but a few remarks on the Arctic plants may not be out of place.

The principal Arctic genera are Salix, Ranunculus, Draba, Pedicularis,
Potentilla, Saxifraga, Carex, Juncus, Luzula, Eriophorum, and others.

Among the most characteristic Arctic plants may be mentioned _Dryas
octopetala_, to which I have already referred as occurring in the west
of Ireland; _Saxifraga oppositifolia_, another British species, occurs
in the higher mountains of Scotland, Ireland, and Wales; _Braya alpina_,
_Papaver nudicaule_, _Lychnis apetala_, _Diapensia lapponica_, and
_Lobelia Dortmanna_, which is found in the lakes of Scotland and
Ireland. The dwarf birch (_Betula nana_) also, which still occurs in
Scotland and the North of England, and which had formerly a wider range
in the British Islands, should be included among these; but there are
other plants probably of Arctic origin, though not now occurring in the
Arctic Regions, and to these may be classed the so-called American
species of plants which are fo