Their  most  important  adaptations  are  those  connected
with  fitting  the  wings,  legs,  and  bills  for  locomotion  and  for
obtaining  food.  The  whole  structure  of  birds'  bodies  is
arranged  in  adaptation  to  fiying.  The  general  outline  of
the  body,  the  peculiar  structure  of  wings,  the  great  develop-
ment of  internal  air-sacs  connected  with  the  respiratory
organs  —  all  are  specialized,  with  reference  to  aerial  loco-
motion.
However,  adaptation  of  anterior  limbs  for  flying  is  not
limited  among  vertebrates  to  birds,  for  there  were  ancient
flying  reptiles  (Pterodactyls),  and  bats  are  flying  mammals.
The  so-called  "  flying  fishes  "  do  not  really  fly,  but  simply
use  their  large  fins  for  gliding  through  the  air  for  relatively
short  distances  when  they  leap  from  the  water.
Bats  and  birds,  then,  are  the  only  living  vertebrates
able  to  fly,  and  the  great  differences  in  their  structures
make  it  certain  that  they  have  developed  independently.
Only  the  birds  among  vertebrates  have  proved  perfectly
adapted  to  life  in  the  air.  That  they  are  well  fitted  is
shown  by  their  success  in  developing  more  species  than  any
other  group  of  vertebrates,  and  also  countless  numbers  of
individuals.
Biologists  who  have  studied  the  flight  of  birds  cannot  help
marveling  at  their  locomotor  mechanism.  It  is  simply
astounding  that  an  animal  as  large  as  a  homing  pigeon  can
fly  faster  than  a  limited  express  train,  and  average  such  speed
from  daylight  till  dark.  The  recent  attempts  at  perfecting
flying  machines  have  made  us  wonder  more  than  ever  at  the
flying  power  of  the  birds.
