The Making of Species 



barriers which oppose change in certain directions, 

 but that there are positive tendencies to develop- 

 ment along certain special lines. In a bird which 

 has been kept and studied like the pigeon, it is 

 difficult to believe that any remarkable spontane- 

 ous variations would pass unnoticed by breeders, 

 or that they would not have been attended to and 

 developed by some fancier or other. On the 

 hypothesis of indefinite variability, it is then hard 

 to say why pigeons with bills like toucans, or with 

 certain feathers lengthened like those of trogons, 

 or those of birds of paradise, have never been 

 produced." 



There are certain lines along which variation 

 seems never to occur. Take the case of the tail 

 of a bird. Variable though this organ be, there 

 are certain kinds of tail that are seen neither in 

 wild species nor domesticated races. A caudal 

 appendage, of which the feathers are alternately 

 coloured, occurs neither in wild species nor in arti- 

 ficial breeds. For some reason or other, variations 

 in this direction do not occur. Similarly, with the 

 exception of one or two of the "Noddy" terns, 

 whenever a bird has any of its tail feathers con- 

 siderably longer than the others, it is always the 

 outer pair or the middle pair that are so elongated. 

 It would thus appear that variations in which the 

 other feathers are especially lengthened do not 

 usually occur. The fact that they are elongated 

 in two or three wild species is the more signifi- 



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