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4. Steering and Balancing Devices. — The tail and its feathers 

 (rectrices) is a rudder which may be used as well for vertical as for 

 lateral steering. Elevating the tail produces an upward slant, de- 

 pression a downward turning. Tilting from side to side gives lateral 

 steerage. Expanding the feathers like a fan, or closing them together, 

 increases or decreases the effectiveness of the rudder. Balancing 

 devices are used especially in soaring, when irregular wind currents 

 strike the outspread wings and tend to capsize the vessel. To equal- 

 ize irregularities of air pressure on the two wings the bird may de- 

 crease the surface of the wing by partially flexing it at elbow or shoul- 

 der, or by twisting the tip of the wing so as to spill off the excess wind. 

 Part of the stabilizing equipment consists of the flexible ends of the 

 feathers which bend upward and spill off the air, much after the fash- 

 ion of the ailerons on an aeroplane. In the bird no elaborate stabil- 

 izer is necessary, for each individual is an automaton, with an effective 

 system of balancing reflexes ever on the alert. 



Any more extensive discussion of the flight adaptations of the bird 

 would lead us into a technical exposition quite out of place in the 

 present volume. Enough has been presented to impress the reader 

 with the fact that almost all of the characters that distinguish a bird 

 from a reptile are fundamentally elements belonging to its flying 

 equipment. Unless therefore these characters were evolved in con- 

 nection with flight they are meaningless; for no other set of conditions 

 could have called forth this peculiar combination of characters. 



Birds and Reptiles Compared 



Apart from its flight adaptations the bird has been shown to be 

 extraordinarily reptile-like. The avian egg is essentially like that of 

 the reptile, both in size and in envelopes. The developmental history, 

 though much more rapid, as the result of higher temperatures, is 

 essentially reptilian. This speeding up of the developmental rate has 

 evidently been an important element in the evolution of the bird. 

 Like the reptile the bird's jaw consists of several bones and articulates 

 with the quadrate. The skull bones are not materially different from 

 those of the reptile; while the vertebrae, in their variable number 

 especially in the cervical region, and their lack of epiphyses, are rep- 

 tilian. The hind limbs and pelvic girdle are strikingly hke those of 

 some of the dinosaurs. The circulatory system is somewhat different 

 from that of any living reptile, but is the logical development of tend- 



