358 A HISTORY OF BIRDS 



absence of such isolation agencies as physical barriers, by the 

 development of intersterility between more or fewer sections of 

 a species occupying a common area, and this he called " physio- 

 logical selection ". 



"If no other form of isolation be present," he remarks, 

 " specific divergence can only take place when some degree of 

 cross-infertility has j^reviously arisen between two or more 

 sections of a species. 



" When such cross-infertility has arisen it may cause specific 

 divergence, either (a) by allowing independent variability in 

 each of the physiologically isolated groups ; (7^) by becoming 

 associated with any other cause of differentiation already 

 operating ; or (c) by both of these means combined. 



" As some degree of cross-infertility generally obtains be- 

 tween allied species, we are justified in concluding that this has 

 been the most frequent — or at any rate the most effective — 

 kind of isolation where the origin of species is concerned, and 

 therefore the kind with which, in the case of species formation, 

 natural selection, or any other cause of specific divergence, has 

 been most usually associated." 



Though among birds cross-infertility may be regarded as 

 the general rule among certain groups, this occurs freely wher- 

 ever two or more species overlap in the confines of their range. 



Cross-infertility among Passerine birds is the rule, but the 

 Gold-finch and the Hooded Crow are striking exceptions there- 

 to. The British Gold-finch {Cardnelis elegmis) ranges to the 

 east of the line of the Urals where it meets and inter-breeds 

 with a closely similar but larger species {Carduelis major), while 

 this last in Southern Siberia meets and inter-breeds with C. 

 caniceps, a species which has no black on the crown and nape, 

 and more white on the wing. The Hooded Crow [Corvus cor- 

 nix) in Scotland, on the one hand, and Siberia on the other, 

 meets and inter-breeds with the Carrion Crow {Corvus corone), the 

 resulting hybrids showing every gradation of plumage between 

 the two species, and on this account many Ornithologists 

 insist that these represent but two forms of one species. 

 There seems little justification for such a conclusion, especially 

 since more than one species of Hooded Crow is recognised, 

 which, like the British species just referred to, always breeds 

 true when isolated. The British Hooded Crow ranges east- 



