EVOLUTION OF THRUSHES 42J 



on some of the palsearctic genera. The reader may be 

 interested in hearing upon what data such theories are 

 based. Let us select the Thrushes as an example. They 

 are almost cosmopolitan. They are found on all the 

 great continents, on many of the Pacific Islands, and 

 almost all over the world except in New Zealand, Western 

 Australia, part of New Guinea, and Madagascar, and we 

 must remember that these countries are by no means fully 

 explored yet. But in spite of their near approach to being 

 cosmopolitan, they belong to a palsearctic genus or genera. 

 A large proportion of their nearest allies are palsearctic, 

 and the formation of their wings — flat, long, pointed, and 

 with the first primary very small — is such as is principally 

 found in palsearctic birds who acquired wings capable of 

 powerful flight to enable them to migrate during the 

 glacial epoch. Before this time we may assume that 

 the Thrushes were residents in Europe and North 

 Asia. 



The Thrushes are divisible into three tolerably well- 

 defined genera. The genus Geocichla, or Ground- 

 Thrushes, contains about forty species. The genus 

 Turdus, or true Thrushes, contains about fifty species, 

 and the genus Merula, or Ouzels, contains rather more, 

 about fifty-three. Zoologists have come to the conclusion 

 that the history of the individual is more or less an 

 epitome of the history of the species. Now the young 

 in first plumage of all thrushes have spotted backs, but 

 the only thrushes which retain this peculiarity through 

 life are to be found in the genus Geocichla ; and we 

 therefore assume that the ground-thrushes are the least 

 changed descendants of their pre-glacial ancestors. In 

 fact we come to the conclusion that before the glacial 

 period there were no true thrushes and no ouzels, and 

 that the ground-thrushes inhabited Europe and North 



