254 PHENOMENA OF PLIGHT IN THE ANIMAL KINGDOM. 



Fiff. 10. 



equal amount of work, but in the different ways wbicli pertain to tlie 

 difference in their structural condition. 



If we study the form of the great pectoral muscle, or that whcih pro- 

 duces the downward stroke of the wing in different species of birds, we 

 shall see that this muscle presents much variation in form. In some 

 cases it is long and narrow, in others short and thick. We shall see 

 that this anatomical difference corresponds to important differences in 

 the character of liight. It is enough to observe the flight of a duck and 

 of a marsh harrier to be struck with the great difference in the move- 

 ments of the wings of these two birds. The duck elevates and depresses 

 its wings very much, describing with each of them an arc of more than 

 90°. The extent of these movements in the harrier, on the contrary, is 

 very small ; when it is observed in profile the point of wing is hardly 

 seen to pass below the shadow of its body. This difference in the type 

 of flight has struck some observers with such force that they have divided 

 the birds into rowers and sailors. The first are those which strike the air 

 with their wings in flying as a boatman strikes the water with his oar ; 

 the second class exposing the surtaces of their wings to the wind like the 

 sails of a ship, fly in a sort of passive condition, utilizing the currents of 

 the air in sustaining and directing themselves. We shall see hereafter 

 that there is a reality in this 

 distinction; at present w;e 

 shall only accept the incon- 

 testable fact that certain 

 species of birds impart a 

 great amplitude to the 

 movements of their wings, 

 while others move them 

 only through a very small 

 extent. 



I have dissected a wild 

 duck and a harrier to show 

 you the form of the pecto- 

 ral muscles. In the duck 

 the great pectoral is quite 

 long, while in the harrier it 

 is very short, but the trans- 

 verse section of the muscle 

 of the harrier is much larger 

 than that of the duck. If we 

 consider only the relative 

 length of the pectoral mus- 

 cles we see that it varies, 

 as it should do according 

 to the theory ,• that is, it is 

 greater or less, according to 

 the amplitude of the motion 

 which the bird executes in 

 flying 



Skeleton of the wing and sternum of a frigate-bird. 

 Tlie extreme brevity of the sternum is seen in pro- 

 liortion to the great length of the wing. 



But to what does this unequal development in the thickness of the 

 pectorals correspond"? It would appear to correspond to a greater 

 muscular exertion in the harrier than in the duck. How can this exer- 

 tion be demonstrated f If we compare the birds which have short and 

 thick pectorals with those which have long and thin ones, we see that 

 tho surface of the wings is very large in the former, while it is very 



