90 JOTTINGS ABOUT BIRDS. 



mcnuhtr'nia, a species which is never so pure a black 

 as our own. Our Robin is replaced in Caucasia 

 by Erithnciis lujrcaims, which is distinguished by 

 its chestnut upper tail coverts ; whilst the British 

 Nightingale is replaced in South Sweden, Russia, 

 and Western Asia by Erithaciis philomela, and in 

 Turkestan and Caucasia by E. golzii, both of 

 which differ in important and constant characters. 

 In Algeria our Wheatear is represented by the 

 closely allied Saxicnla seebokmi, a species specially 

 remarkable for its black throat. 



And so w^e might proceed right through the list 

 of British species, giving instance after instance where 

 our native birds are replaced or represented by per- 

 fectly distinct yet closely allied forms. The most 

 important distinction between these representative 

 forms or sub-species, and those entitled to full 

 specific rank, appears to be that the latter have been 

 isolated more completely, or for a longer time, from 

 the typical species than the latter, and have conse- 

 quently become differentiated. We find that in 

 the case of mere forms or races, interbreeding with 

 the typical forms is taking place, and thus prevent- 

 ing the completion of the segregation ; and so soon 

 as this interbreeding is checked by the more 

 effectual isolation of the form and its parent species, 



