84 



of the bird is enlarged, and its specific weight considerably- 

 diminished, but also from the air taken in at any particular height 

 being warmed by the heat of the body, and considerably rarefied 

 in consequence, so that the contents of the air-saca are always 

 considerably lighter than the air which occupies surrounding 

 space." To the writer several fallacies appear in the foregoing 

 arguments. In the first place, any increase in the volume of the 

 bird must also mean increased resistance to its passage through 

 the air, and again, admitting that it is possible for a bird — though 

 it is very difficult to conceive how it can be accomplished — to so 

 regulate the action of its wings that nearly all the effort is 

 expended in furthering its forward movement, it seems that this 

 gain would also be counteracted by the loss of wing-power, owing 

 to the lessened momentum obtainable by the wings from the 

 surrounding rarified air. This is important when it is remem- 

 bered that in Herr Gatke's opinion a speed of anything from 100 

 up to 200 miles an hour is maintained by nearly all species of 

 birds throughout a flight of many hundred miles. 



Herr Gatke appears to hold the opinion, when he speaks of the. 

 air in the air-saes always being warmer than the surrounding 

 atniosphere, that their contents can be retained without change 

 for some considerable time. To take in a stock of cold air and 

 then to warm it by contact with the body means also to expand 

 it, and unless a constant change is going on it appears to the 

 writer that the retention of a supply of expanded air for any 

 length of time, without changing its identity, would be an abso- 

 lute inconvenience to a bird. Herr Gatke also appears to hold 

 the view that this warm air in the air-sacs means increased 

 buoyancy to a bird. But according to Dr. Drosier, practical 

 experiment has proved that in a pigeon weighing ten ounces, the 

 gain in this respect would only amount to the fraction of a grain. 

 In the opinion of the latter gentleman the air-sacs are a necessary 

 part f the respiratory apparatus of birds. He makes the further 

 statement that they are present in all species of birds, even such 

 as do not fly. However, it is only fair to add that Herr Gatke 

 gives only partial adherence to the theories he has advanced, for 

 he remarks (p. 48) : " More exact calculations based on physical 

 laws, have undoubtedly compelled us to recognise that this warm 



