PEEFACE. vii 



the effect of changes in the elevation of land and in the direction of oceanic currents ; 

 and it seems very probable that the apparently simple action of the changes of the 

 eccentricity of the earth's orbit has been greatly complicated by these and other causes. 



It must, however, be left to competent Geologists to decide in what way the Glacial 

 Epochs were caused. All that the ornithologist requires is a knowledge of the existence 

 of two long-continued breaks in the continuity of bird-life in the Polar Basin, during which 

 the birds were dispersed and isolated for a sufficient length of time to give them an 

 opportunity of being modified to suit the various conditions of their temporary homes ; so 

 that the one species which lived on the shores of the Arctic Ocean before the Prae-Pliocene 

 Glacial Epoch became differentiated into nearly two hundred species after the Post- 

 Pliocene Glacial Epoch had passed away, and for at least the third time opened the gates 

 of Paradise to the Charadriidae. 



There seems to be a tendency amongst modern Biologists to modify the theory of 

 Evolution as propounded by Darwin in two directions. One party, headed by Weismann, 

 desires to eliminate the effects of Use and Disuse ; and another, of whom B.omanes is the 

 exponent, wishes to minimize the importance of Isolation. In each case it is thought that 

 too much importance was attached by Darwin to the point under consideration. I think 

 exactly the opposite. I think that the relative importance of the hereditary effects of Use 

 or Disuse, and the necessary part which Isolation plays in the Differentiation of species, 

 appear to be much underestimated by Darwin, and I have endeavoured to bring some 

 evidence to show that this is the case. 



Finally, I beg to commend my book with all its faults, which I am sure are many, and 

 with all its blunders, which I hope are few, to the careful consideration of ornithologists. 

 It possesses at least the merit of originality, and (if an author may be permitted to pass 

 sentence on his own work) it does not quite deserve the critical remarks once made to a 

 writer : " Your book is both good and new ; but that part which is good is not new, and 

 that part which is new is not good." 



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