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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