144 LECTUEES ON BIOLOGY 



each of which is again widely differentiated from the rock-dove, 

 some of the descendants revert to the wild stem-form, exhibiting 

 distinct characteristics of the species Columbia lima. 



The rock-dove is found throughout the greater part of 

 Europe, North Africa, and wide stretches of Asia. Its colour 

 resembles that of the ordinary bluish-grey field pigeon, but 

 is distinguished from it by the bright metallic sheen on the 

 feathers of the neck and breast, and a more delicate and elegant 

 shape. It lives socially in large communities, like all other 

 animals suitable for domestication, builds its simple nest in 

 inaccessible crevices of rocks out of dry twigs, straw, and fibres, 

 and, like its domesticated descendants, lays regularly two eggs, 

 and has a strong inclination against settling on trees. It shows 

 the same peculiar nodding of the head and clapping of the 

 wings, and delights in large swarms to circle round and round 

 high up in the air. By what magic has man been able to 

 produce these widely divergent birds ? We need only think 

 of a carrier, fantail, pouter and turbit to admit that the dif- 

 ferences between these various breeds are extraordinary, more 

 striking than the differences of many natural species for instance, 

 hare and rabbit, the common nightingale (Daulias luscinia) and 

 "sprosser" (Daulias philomela), &c. The distinctive features 

 are indeed so great that if he met them in the free state no 

 systematist would hesitate to describe each of the mentioned breeds 

 as well-defined, distinct species If, nevertheless, we continue 

 to describe the different forms of our domestic animals only as 

 varieties and not as species it is merely due to force of habit, 

 for there is no valid reason why they should not be regarded 

 as distinct species. If by nothing else, they prove their right 

 to the rank of species by the fact that as long as the protective 

 conditions remain under which they came into existence they 

 will continue to breed straight like any other natural species. 

 The magic which has effected this apparent miracle is called 

 artificial selection. (Compare Plate V.) 



The variability and plasticity of the pigeon is most remark- 

 able. A skilled and persistent breeder may in the course of a 

 few generations produce almost any desired variation, and there 

 is hardly a part of the body which cannot be altered at will. 

 In the carrier-pigeon the bill is very long and straight, and 



