VI PKEFACE. 



effects, in very distinct genealogical lines. I have found that in many cases the colour or 

 the pattern of the colour of such parts of the plumage as are unaffected by age, sex, or 

 season, and which is therefore presumably of ancient origin, is apparently of much greater 

 value in ascertaining the relationship of many birds than the so-called structural characters, 

 which are compelled by the laws of evolution to change with the changing habits or 

 environment of the species. 



In order to split up a species it must be dispersed. The chief causes of the dispersal 

 of the ancestors of the Charadriidse have probably been two Glacial Epochs. The Prse- 

 Pliocene Glacial Epoch compelled the ancestral species to emigrate from its old home in the 

 Polar Basin. It emigrated in various directions, and a score of parties were thus isolated 

 in a score of localities, where they met with difficulties of various kinds. Emigration 

 produced Isolation, and Isolation in more or less different environments caused Evolution 

 to proceed on different lines, the final result being Differentiation. 



Half the species thus differentiated remained in their new homes ; but the other half 

 followed the retreating cold to the old home in the Polar Basin, where most of them lived 

 long enough to become again circumpolar. The Post-Pliocene Glacial Epoch again dis- 

 persed them with similar results, until finally many of them returned again to the Polar 

 Basin, which, for the third time, became the great breeding-ground of the Charadriidse. 



The habits of Migration, originally formed for the purpose of seeking light, and 

 strengthened by the experience of emigration, became an annual necessity when a semi- 

 arctic condition of the Polar Basin again ensued, and culminated in the catastrophe which 

 exterminated the Rhinoceros, the Hippopotamus, the Elephant, the Ma mm oth, and Palaeo- 

 lithic Man in the Palsearctic Region. This catastrophe I believe to have been the floods 

 caused by the sudden melting of vast accumulations of snow, which must have begun when 

 the climate became cold enough to allow the excessive rainfall of the later Pleistocene Age 

 to accumulate in winter in the form of snow on the mountains, and which must have 

 periodically occurred so long as the excessive rainfall continued. In Chapter VI. I have 

 described the great annual catastrophe which, even with the present reduced rainfall, takes 

 place in Siberia, and to a lesser extent in the valleys of the Nile and the Danube. This 

 semi-arctic condition of the Polar Basin still exists, and the partial isolation thus produced 

 has caused and is causing the production of subspecies. 



To each of these great factors in the Differentiation of the species and subspecies of 

 the Charadriidse I have endeavoured to devote as much space as the limits of the work 

 would allow. A large part of the book is occupied with details respecting each species 

 or subspecies, the various names by which it has been called, the characters by which 

 it may be recognized, its summer and winter range, and the probable course of the 

 emigration of its ancestors. 



In discussing the Glacial Epoch I have endeavoured to place before the reader a 

 clear idea of the main features of CrolPs theory, which appears on the whole to be more 

 plausible than any other that has hitherto been advanced. Croll lays great stress upon 



