ALTITUDE OF THE MIGEATION FLIGHT 47 



ever, are performed by the birds at their ordinary everyday speed 

 of flight, and have nothing in common with the great and powerful 

 migration flight which is here under consideration, and which, as it 

 is displayed pre-eminently in Heligoland, proceeds at unknown 

 heights, with a tremendous velocity, and, for the most part, during 

 the dark hours of nieht. 



Though observations on the extreme height of the flight of 

 birds, so far as this can be ascertained directly by our power of 

 sight, are naturally very limited, our experience in this du-ection 

 nevertheless goes to show that birds can exist without difficulty in 

 strata of the air at such heights and of such a low density as neither 

 man nor any other warm-blooded creature could live in for any 

 length of time. Birds, therefore, must be organised in such a 

 manner as, on the one hand, to be uninfluenced by so considerable 

 a diminution of air-pressure as one meets with at heights from 

 25,000 to 30,000 feet, and, on the other hand, they must be able to 

 exist on the considerably reduced supply of oxygen obtainable in 

 strata of such rare density. Or, again, their respiratory apparatus 

 must be of such a nature as to be capable of abstracting from 

 these rarefied air-strata the amount of oxygen required by the blood, 

 as easily as it is able to remove it from the denser layers close to 

 the earth's surface. An organisation of this kind would give to 

 birds a completely distinct and unique position among all warm- 

 blooded animals. 



If then, we are obliged to assume the existence of a special 

 respiratory mechanism enabling birds to remain in strata of the 

 atmosphere beyond the reach of all other organised living beings, 

 how much more difficult is it to account for the means which render 

 them capable of flying in an atmosphere whose power of supporting 

 weight, i.e. buoyancy, is so considerably reduced. The first idea 

 which here suggests itself is that birds are capable of taking in 

 relatively large quantities of air, and of storing these for any desir- 

 able time, employing for this purpose not only the cavities of such 

 of their bones as are devoid of marrow, but more especially, and in 

 a considerably greater degree, the air-sacs which lie within the 

 thoracic and abdominal cavities, as well as between the outer inte- 

 gument and the body walls. Air-sacs of this latter kind are found, 

 so far as my own observations go, on all parts of the body devoid of 

 quill-feathers, and reach their largest extent on both sides of the 

 base of the neck, below the wings and behind the thighs. Anatomy 

 proves that all these air-sacs are connected with the lungs, and are 

 filled from the latter organs. Probably owmg to the possession of 

 these air-sacs, the flight of birds in the higher strata of the air is 

 so much facilitated that they are enabled to apply the muscular 



