

Natural History 397 



armed with hook-like claws useful for taking hold at the end of 

 each jump and for more leisurely clambering at other times. The 

 crucial step in the evolution of the true bird stock, however, must 

 have been the acquisition of powers of real flight. At an early 

 stage the fore-limbs would be held out sideways during each leap, 

 and later the surface area would become enlarged by the develop- 

 ment of a fold of skin between each of these limbs and the body. 

 Later yet this fold would become still more important, and its 

 area would be still further increased by the transformation of its 

 covering scales into some primitive form of feather. Longer and 

 longer leaps would become possible, from branch to branch and 

 from tree to tree, as these aids to gliding flight improved. Finally, 

 the last great step would be taken when a beginning was made of 

 the active use of the primitive wings to prolong still further, until 

 at last indefinitely, the distances possible by leaping and gliding 

 alone. 



It is a curious history, this tale of the origin of birds. In the 

 first place we seem to see the earliest ancestors as a feeble reptilian 

 race driven from the ground and taking refuge among the 

 branches. There followed ages of arboreal life during which the 

 great adaptation of flight originated and was made perfect. Then 

 came a day when the new race of birds, fortified with the great 

 advantage of mastery of the air, spread abroad from the forests 

 to reconquer the ground-level, to find their bread upon the waters, 

 to cross the seas to distant isles, and to defy the rigours of climate 

 by their ability to "change their season in a night." So to-day 

 we have birds peopling the whole earth and filling every land with 

 the abundant beauty of their plumage and their song, and with 

 the immense wonder of their eager, spirited lives. 



2 



Flightless Birds 



It is a strange side-issue, too, to find that the priceless gift 

 of flight has not always been preserved. Over and over again 



