NATURAL SELECTION AS APPLIED TO BIRDS 319 



we must remark that the phenomena which are apparently due 

 to this cause are both varied and complex, and subtly inter- 

 woven. 



While in some species the victims of this struggle succumb 

 completely, in others they escape, either by adopting new 

 habits or by undergoing a more or less radical change in 

 external appearance. Inability to respond to new conditions 

 means extinction ; adaptability, on the other hand, means evolu- 

 tion, though all evolution is not to be attributed to this cause. 



Though extinction does not necessarily imply an inability 

 on the part of the exterminated to combat the ravages of 

 predatory species, but may be due to climatic or other causes, 

 there are many cases on record where one species has been 

 ousted by another over areas of greater or less extent, and this 

 in some cases may have led to extinction. 



This replacement of one species by another is well illustrated, 

 for example, in the case of the Common Sparrow {Passer 

 doinesticus). Throughout England this pest is ousting both the 

 House- and the Sand-martin, whole colonies of which have 

 been dispossessed in this way; and similarly, the Purple Martin 

 {Progne sulns) is being crowded out in the United States. In 

 like manner the Puffin {Fratercula arcticd) has replaced the 

 Manx-Shearwater {Piiffinus angloruvi) on some of the Islands 

 of the Inner Hebrides. The latter, according to Yarrell's 

 British Birds, " on St Kilda and on Pabbay . . . was formerly 

 very common, and the young ones . . , were *so highly es- 

 teemed that a barrel of them formed part of the rent paid by 

 each crofter in Mingalay to the Macneills of Barra. About a 

 hundred years ago, however, the Puffins, which before were 

 not numerous, began to increase very much, and drove the 

 Shearwaters from the holes which they occupied in the cliffs : 

 and now they have completely supplanted them, so that only 

 a few pairs of Shearwaters are left on the Island of Pabbay." 

 Throughout Great Britain the Jackdaw is ousting the Cornish 

 Chough. In Greenland two forms of the P'ulmar Petrel 

 {Fulniaris glacialis) are met with, a light- and a dark-breasted 

 form. Major (now Col.) Fielden, when visiting this region 

 observed that the light-breasted birds domineered over the 

 dark-breasted, which are somewhat smaller. In Australia the 

 Common Starling {Stiirnus vulgaris^ was introduced some 



