The Making of Species 



birds in which the tail feathers are greatly 

 elongated. Were variations indeterminate, we 

 might reasonably expect to find that the 

 elongation occurred in one particular feather or 

 pair of feathers in one species, in another pair in 

 a second species, in a third pair in a third species, 

 and so on. But this is not the case ; no bird has 

 one single long feather in its tail, and when two 

 are elongated, as is so commonly the case, these 

 are almost invariably the middle or the outside 

 pair; e.g., in the European bee-eater and pheasant 

 it is the former, in the swallow and blackcock, 

 the latter. 



Exceptions are so rare that they may almost 

 be said to prove the rule ; e.g., although most 

 terns have the outer-tail feathers elongated, in 

 some of the Noddy Terns (A nous, Gygis) the 

 third pair, in others the fourth pair, of tail 

 feathers are the longest. This must mean one of 

 two things, either that the variation, as regards 

 length in tail feathers, other than middle or outer, 

 does not ordinarily occur, or that it occurs, but is, 

 in some way, inimical to the welfare of the 

 species. The latter hypothesis does not seem 

 probable, as the Noddies are particularly 

 abundant birds where they occur, that is to 

 say, in the tropical seas ; therefore, we can only 

 conclude that that particular variation has not 

 occurred in birds as a whole. 



We have adduced abundant evidence to show 

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