In order to make this matter quite clear, I will give a familiar example as an instance of the manner in which the present distribution can be explained without taking recourse to direct migration from the Alps to Scandinavia or _vice versâ_. The example I will take is that of a family of birds, not only of extreme interest from the fact of its northern range, but also from the pleasure it gives to those addicted to sport. This is the grouse family, the _Tetraonidæ_. Let us commence with our British Grouse (_Lagopus scoticus_), which is peculiar to the British Islands. In Norway we find a Grouse (_L. albus_) which differs in habit, and in the fact of its turning white in winter; otherwise it is so closely allied to our Grouse that many ornithologists do not separate them specifically. No doubt the British Grouse is a descendant of this Scandinavian Willow-grouse. The latter is known also to inhabit Greenland and Arctic North America, and it is even found beyond Behring Straits in Northern Siberia. _En route_ between Scandinavia and Asia, travelling in a westward direction, we meet with two other very local species of Grouse, which may be looked upon as offshoots of _L. rupestris_--viz., _L. hyperboreus_ of Spitsbergen, and _leucurus_ of Western North America. In Asia we then again find two kinds of Grouse, very closely related, and by some indeed regarded as belonging to the same species. These are _L. rupestris_ and _L. mutus_. Mr. Ogilvie-Grant tells us of the former (p. 49), that it is merely a more northern rufous form of _L. mutus_, and that it goes through similar changes of plumage. In summer the males are readily distinguishable, but in winter it is impossible to tell one from the other. "_L. rupestris_ taken as a whole," says Mr. Ogilvie-Grant, "appears to us barely specifically distinct from _L. mutus_." _L. rupestris_ occurs not only in Northern Asia, but crosses the Behring Straits to Arctic America, being still found on the Aleutian Islands, which represent the last remains of the former land-bridge between Asia and North America, then eastward to Greenland and Iceland. However, while this form does not cross the confines of Asia in a westerly direction, its near relative _L. mutus_--better known as the Ptarmigan--does; and may perhaps have entered Europe as a Siberian and also as an Arctic migrant. It is still found in the Ural Mountains, in Finland, and the highlands of Scandinavia. It is gradually being driven out of the Alpine lowlands, while it has long ago disappeared from Germany, France, and Austria--in fact, from all the lowlands of Europe. It has also been met with in the Pyrenees and in some of the Spanish mountains. Similarly, the bird has become extinct in England and Ireland, while it is becoming more and more scarce in Scotland. The centre of distribution of the genus lies in Arctic America, and from there the genus has spread to Europe and Asia. _L. albus_ and _L. mutus_