Having now shortly reviewed the whole grouse family, we have seen that, although some species live within the Polar Circle, the majority are more or less confined to the more temperate or rather the less arctic parts of the Northern Hemisphere. They are quite absent from Southern Asia and even the southern parts of North America, and almost so from the Mediterranean basin. The whole range of the family is therefore suggestive of a northern origin, and this view agrees perfectly with all the details of distribution. The centre of distribution lies in Northern Asia, or in Arctic North America. From there the great genus _Lagopus_ spread east and west, reaching Europe by these vastly divergent routes at a time when the physical geography was very different from what it is to-day. Several of the species common to the Alps and Scandinavia have migrated from Siberia direct to Eastern Europe. But we can now imagine how from a similar centre in Asia--perhaps at a rather more remote time--a species spread eastward across North America and Greenland to Scandinavia, and westward along the mountain ranges of the Tian Shan and the mountains of Asia Minor to Greece, and finally to the Alps. We should then have the same species in the Alps and in Scandinavia, not far removed from one another; but how different were their paths of migration! This, however, is not an imaginary instance. Such a migration must have actually taken place in a good number of instances among the terrestrial invertebrates and also among plants.