CHAEADEIU8. 



141 



the sub- 

 genus. 



that repeated colonies were obliged to seek fresh homes. One of these became a resident 

 in South China, and is now known as C. dealbatus, which in its turn crossed over to CaUfornia 

 and is now C. nivosus. These were probably post-glacial emigrants which have not yet 

 become completely differentiated. C. peroni probably represents a second case of the 

 extinction of a species by interbreeding, as explained by Menzbier, and appears to be the 

 result of the fusion of a party of the ancestors of C. cantianus with one of those of 

 C. hiaticula before they were sufficiently differentiated to make interbreeding impossible. 



C. riiJicapiUm represents a colony of C. cantianus which emigrated to Australia; 

 C. marginaius and C. tenelhis others which crossed over to South Africa, whilst the 

 ancestors of C. coHaris crossed the Pacific to South America. 



C. asiaticus appears to have foresworn the sea-coast and to have become differentiated 

 in Central Africa, whence it migrated to the basin of the Caspian, and during the warm 

 post-glacial period it sent a colony across the Polar Basin to America, where it retained its 

 inland habits and became slightly differentiated into C. montanus. 



C. pecuarius appears to have been isolated in West Africa, whence it colonized 

 St. Helena at a comparatively recent date, C. sanctcB-helence being probably a post-glacial 

 form. 



There are about a score of species and subspecies of Sand-Plovers, all very closely Division of 

 related to each other, so closely indeed that only four out of the twenty have been removed 

 by the genus-splitters on the ground that their bills differed in shape from the others. 

 The form of the bill is frequently an important character, but in the genus Charadrius the 

 variations are so slight that it is impossible to beheve that they have any taxonomic value ; 

 they are obviously the result of pure accident, such, for example, as the nature of the food 

 and the facilities for obtaining it in the various localities where the species have been 

 dispersed. To unite C. geoffroyi with C. wilsoni is reasonable; but at the same time to 

 separate C. geoffroyi from C. mongolicm is a reductio ad absurdum which goes far to prove 

 that in this group of birds the slight differences in the shape of the bill are of very recent 

 origin, and are of no value whatever in determining relationship. 



For convenience of diagnosis it is advisable to divide the species comprised in the 

 subgenus jEgialopJiilus into two or more groups. 



The presence or absence of a white collar might be chosen as a character were it not 

 for the inconvenient fact that out of the eight species which possess that feature, in three 

 of them (C. wilsoni, C. nivosus, and C. tenellus) it is frequently so obscure as to be practically 

 useless. 



The colour of the feet is equally uncertain as a character ; regarding some species Incon- 

 ornithologists differ in opinion : the colour of the feet of C. nivosus and of C. montanus is charloters. 

 doubtful ; whilst in other species it would divide such forms as C. cantianus and C. dealbatus, 

 which are evidently conspecific. 



All the larger species have the central upper tail-coverts nearly the same colour as the 

 rest of the upper parts, but in all the small species they are very much darker, almost black. 



