EXTERNAL ANATOMY. FORKED TAIL. 
103 
( 96 .) 9 . Forked tails are of two kinds ; one which 
presents a single, the other a double, fork. There can be 
no doubt that this structure has great influence in faci- 
litating the evolutions of such birds as possess it; for 
we invariably find that fork-teiled birds are excellent 
flyers. Nevertheless, it must not be considered as 
necessarji to extreme swiftness, for although it is found 
in many swallows, one halt ot the family have the tail 
more or less even. The swifts, whose rapid flight is 
perhaps superior even to that of the swallows, have the 
tail perfectly even ; and there are comparatively few of 
the oceanic birds belonging to the Petrels and neigb- 
bouring genera — which rarely approach land — pos- 
sessing a forked tail. It is hardly necessary to say, that, 
by this structure, the two outer feathers are by far the 
longest, the others gradually shortening until the gra- 
dation terminates in the central or middle pair. The 
house swallow is a notable insUince of this form, but, 
perhaps, no two genera possess it in a higher degree, 
considering their size, than that of Psolurmtj among 
the night-jars (^Caprimulgidte'), and Milvulus among 
the tyrant fly-catchers. The scissar-tail {Giibernetes), 
which is almost equally retnarkable in this respect, as 
stated by Azara, is in the habit of opening and con- 
tracting its tail during flight, and has hence received a 
name from the Indians, likening the tail to a pair of 
scissars. It is unnecessiiry to cite further instances of a 
simply forked tail, particularly as they occur in nearly 
all natural groups, without having the peculiar charac- 
teristic of any one. It is nevertheless remarkable that 
not one instance of this structure occurs in the bcansorial, 
or climbing tribe, and very few even among the most aber- 
rant of the lldnoTCK. Doubly-forked tails, on the other 
hand, are very rare {fig- .11.). The most remarkable 
is that of a species of night-jar {Fualaruit hlfureatus) 
we discovered in Brazil ; in this, not only the two 
outer, but the two middle tail feathers are greatly 
elongated, the intervening lateral ones being gradu- 
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