FLIGHTLESS BIRDS AND THEIR FATE. 71 



third and fourth generation have been called 

 upon to suffer, flight to them being impossible. 



In the course of the next few pages we shall 

 see how gradually changes of this kind come 

 about. In the domain of flight we have every 

 gradation of power and speed, from the arrow- 

 like velocity and skilled performance of the 

 swallow to the feeble flutter of the hoatzin, the 

 South American bird, whose whole life is spent 

 on low trees by the water-side. So far as is 

 known, this bird never descends to the ground, 

 and never takes a journey of more than 100 

 yards on the wing at a time. It is said to jump 

 from the highest point available, and then 

 gradually to descend until he alights upon the 

 desired bough, beyond and below. 



Ill that other bird world where wings are, 

 or have been, but flight is not, we shall see 

 similar gradations, but this time they will be 

 gradations of w/^perfection. We shall see the 

 wing growing smaller and smaller, and one by 

 one its bones fading away and its feathers 

 vanishing, till finally it will altogether cease 

 to be. We shall in some cases be able to de- 

 termine approximately the date at which this 

 phase of degeneracy began. 



It will be necessary here to refer once again to 

 certain points in the structure of the skeleton of 

 these flightless birds, and to compare these points 

 with similar others in those birds in which the 

 power of flight yet remains unimpaired. 



In the last, as already mentioned (p. 64), 

 the power of flight is largely commensurate 

 with the development of the breast muscles, 



