Natural History 403 



common to all birds, and to a large measure of uniformity in 

 general appearance among the nearly twenty thousand different 

 species which are known to science ; there are, it is true, wide differ- 

 ences in size, in coloration, and in manner of life, but there are no 

 gross divergences in form comparable to those found, for instance, 

 among mammals between the tiger and the goat, the kangaroo 

 and the elephant, or the bat and the whale. 



This distinctiveness and this uniformity may both be ac- 

 counted for in one word Flight. The whole body of the bird is 

 adapted to this habit of flying. The bird's skeleton is a wonderful 

 study from this point of view, but here it will suffice to mention the 

 external features. Flight has brought with it feathers, and these 

 are a unique feature: all birds have feathers, and nothing that is 

 not a bird possesses any trace of them. Furthermore, the func- 

 tion of flight has secured a virtual monopoly over the fore-limbs, 

 and it has thus brought two other striking adaptations in its train 

 a bird is of necessity a biped, walking on its two hind-limbs, 

 and its mouth has had to take the place of a hand, thus leading 

 to the evolution of a long flexible neck, and of a hard beak which 

 is often wonderfully adapted to the feeding habits of the 

 particular species. 



M 

 The Flight of Birds 



Birds are, of course, true heavier-than-air machines, and in 

 former days man used to strive to learn their secret for the pur- 

 poses of the flying-machines which his heart desired; but within 

 the last few years the main physical principles of the aeroplane 

 have become so familiar that we may perhaps reverse the process 

 by using them in the description of our present problem! Just 

 as gliders preceded aeroplanes, so gliding flight may, as we have 

 seen, have been the beginning of the mastery of the air in the 

 case of birds; and it is in gliding that the artificial machine and 

 the bird are most alike. In both cases advantage is taken of the 



