INTERBREEDING 501 



extreme term of a somewhat complicated series. English 

 skins are the brownest, and have the black on the head 

 extending only to the nape, and are scarcely distinguish- 

 able from examples of Parus palustris from the South 

 of France, Italy, and Asia Minor. This form turns up 

 again in China. Examples of P. borealis from Norway 

 differ in having the back grey instead of brown. Ex- 

 amples from Archangel are greyer still, and have the 

 black on the head extending beyond the nape. Both 

 these characteristics are more pronounced in skins from 

 the Petchora, the Ob, and the Lower Yenesei, and still 

 more so in those from the Upper Yenesei — the true 

 P. camtchatkensis ; whilst in Japan a fourth form, to 

 which I have given the name of P. japonicus, is found, 

 which combines a greyish-brown back with the great 

 development of the black on the head. 



The Siberian form of the nuthatch, to which Lichten- 

 stein gave the name of Sitta uralensis, is another case in 

 point. Examples from the Yenesei, and also from the 

 north island of Japan, have the under parts almost pure 

 white. 



Other examples of slight variations between our birds 

 and those of Siberia might be given, in some cases 

 where intermediate forms are known to exist, and in 

 others where they have not yet been discovered, or may 

 possibly not exist. The subject of the interbreeding 

 of nearly -allied birds in certain localities where their 

 geographical ranges meet or overlap, and the almost 

 identical subject of the existence of intermediate forms in 

 the intervening district between the respective geographical 

 ranges of nearly-allied birds, is one which has not yet 

 received the attention which it deserves from orni- 

 thologists. The older brethren of the fraternity have 

 always pooh-pooh'd any attempt to explain some of these 



