MUSEUM OF COMPARATIVE ZOOLOGY. 203 



the second the length of the tail, of the same specimens ; the third 

 column shows the difference in length between the tail and the win", 

 and the fourth column the amount of the difference between the two 

 extremes. In Mimus polyglottus the tail is usually one fourth to one 

 half an inch longer than the wing; but in many specimens the wings 

 and tail are equal, and in a small proportion the tail is shorter than the 

 wing. In the seven specimens of this species cited in the table, the 

 variation ranges from the tail being one fifth of an inch shorter than 

 the folded wing to one inch longer. In the three specimens which 

 agree in the length of the tail (4.35 in.), the variation in the 

 length of the folded wing ranges from 3.25 in. to 4.25 in., or is nearly 

 twenty-seven (2G.85) per cent. The larger specimen, however, is a 

 male, while the others are females ; but between the two females the 

 difference is over twenty-four (24.3) per cent. Similar differences 

 have been met with in various other species, but it has not been deemed 

 necessary to cite a larger list of examples. 



Variation in the Form of the Wing. — By the form of the wing is 

 meant its general outline when expanded, which is mainly deter- 

 mined by the relative length of the remiges. The form of the wing, 

 and especially the relative length of the different primary remiges, 

 has direct relation to the power of flight. In strong, swift-flying 

 birds, the outer primaries are the longest, giving a narrow pointed form 

 to the expanded wing, as in the swifts, the swallows, in Chordeiles, in 

 the Slernince and in most of the Procellaridce. In birds of medium 

 powers of flight, as in most of the true finches (Coccothranstince) and 

 Tardince, the Tyrannidce, the Sylvicolidce, etc., etc., the third, fourth, 

 and fifth primaries are the longest, the wing being less pointed and 

 broader. In species with low power of flight, as the Troglodytidce, 

 several genera of sparrows, the grouse, etc., the outer primaries are still 

 more reduced, the wing is much more rounded and shorter, and the 

 power of flight is in each case correspondingly less. In birds of the 

 first class, which live almost wholly on the wing, little variation is seen 

 in the relative length of the primaries. In those of the second and third 

 classes, slight variations affect in less degree the particular habits of 

 life, so that among the latter would be naturally expected the greatest 

 range of individual variation. 



Correlating with the variation in the form of the wing, as determined 

 by the relative length of the outer primaries to the length of the inner 



