HOW SPECIES PERISH. 229 



especially those whose geographical area of distribution 

 has been a restricted one, and those confined to islands, 

 mountains, and valleys. Another cause of extinction is 

 the immigration of carnivorous animals and rapacious 

 birds into districts tenanted by species unfitted to cope 

 with such a danger ; in a similar way plants might soon 

 suffer extinction from the arrival of herbivorous animals 

 in their habitat. One more cause of the extinction of 

 species must be noticed, more especially so as it is one 

 that many naturalists are inclined to ignore. This is the 

 gradual absorption of a species by regular inter-breeding 

 with one or more allied forms. I have already entered 

 at some length into this profoundly interesting phe- 

 nomenon.* The instances known to naturalists are at 

 present few, but doubtless many yet remain to be 

 discovered. What I believe is a hitherto unrecorded 

 instance of this mode of extinction is presented in the 

 Common Pheasant (Phasianus colchicus) and its Ring- 

 necked ally (P. torquatus). At one time the typical 

 Pheasant of our English coverts was P. colchicus from 

 the Caucasus, with no white ring round the neck ; but 

 since the introduction of P. torquatus from China the 

 two races have inter-bred, with the curious result that 

 the ringless variety is fast disappearing, and the white- 

 ringed form is becoming the dominant one in all parts 

 of the kingdom. The evidently more vigorous Chinese 

 * See " Evolution without Natural Selection," p. 67. 



