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 soon. But this is not the case ; no bird has 
one szmgle 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; ¢.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; eg., although most 
terns have the outer-tail feathers elongated, in 
some of the Noddy Terns (Axous, 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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