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genus Lagopus, to which the Grouse belongs, and which 

 includes the Ptarmigan, the Willow Grouse, and other species. 

 The Grouse is a species considered to be peculiar to Britain, 

 though closely related to the Scandinavian Willow Grouse. 

 Belonging to a genus of strictly northern and even Arctic 

 range, it follows from the theory of centres of distribution 

 that it came from the north. Its history seems to have 

 been the dispersal southwards by the Ice Age of a bird 

 extremely like, if not identical with the Willow Grouse, 

 its gradual adaptation to the post-glacial climate of Great 

 Britain, and consequent loss of white plumage in winter 

 owing to the less abundant snowfall. In other words the 

 Red Grouse is simply a southern form of the Willow Grouse, 

 evolved during the complex movements of animals during 

 the glacial and post-glacial periods. 



Several other northern animals came with the Grouse, 

 including the Little Yellow Underwing Moth (Anarta myrtilli), 

 and various beetles. But the competition of southern forms 

 has driven most of the true arctics from Cleveland, and it is 

 now chiefly on the higher r^ountains of our island that they 

 are to be found. 



If the Ice Age drove out most of the pre-glacial species, it 

 would seem likely ( ;hat these northern animals constitute 

 the oldest elements of our fauna, but indications are not 

 w r anting that son? s forms may have survived from Pliocene 

 times. 



The beautiful Emperor Moth so abundant on our moon?, 

 belongs to the genus Satumia, which has its head-quarteis 

 in Asia. Hence, on the theory, Asia is to be regarded as the 

 home of the Emperor Moths which have gradually spread 

 westwards across the Old World. The present distribution 

 of the Emperor Moth supports this contention, as it occurs 

 all over Europe, except in the Islands of Sardinia and 

 Corsica, and extends far into Palaearctic Asia. 



Various other animals inhabiting the district can similarly 

 be traced back to centres of origin. An American element 

 is to be found in the Pearl Mussel (Unto margariti/er) ; 

 an Asiatic element in the Nightjar and the Death's Head 



