83 



The above extracts will sufiBciently illustrate the fact that 

 while Herr Gatke's statement may be correct, that birds when on 

 their journeys approach nearest to the earth when winds from 

 the E., S.B., or E. prevail, it by no means follows that exten- 

 sive migration is not to be witnessed under entirely different 

 conditions, i.e., at times when the winds prevail from quarters 

 described as least favourable. It will also be gathered that winds 

 from B., S.B., or S., are not of necessity accompanied by move- 

 ments of birds, as one might imagine from the theories of the 

 author, such winds in whose opinion form the incentive to 

 exceptional migration from the far east. 



Herr Gatke makes some interesting remarks in connection with 

 the special organisation of birds ; which becomes a necessity — 

 if his theories as to the vast heights at which their migrations are 

 performed, are accepted as correct — in order to enable them to 

 sustain the cold they would naturally encounter, and to overcome 

 the difficulty of respiration, owing to the thinness of the air. 



He remarks (p. 47) : " Birds, therefore, must be organised in 

 such a manner as, on the one hand to be uninfluenced by so con- 

 siderable 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." The means 

 by which these difficulties are overcome are, in his opinion, the 

 possession of a system of air-sacs, which communicate with the 

 lungs. Setting aside for the moment the fact that, with the ex- 

 ception of certain highly specialised birds, which are at nearly all 

 times of the year, given to soaring about at great heights, and that 

 it still remains to be proved that the smaller species reaUy attain 

 such great altitudes during migration ; it will be interesting to 

 learn in what manner these air-sacs can be of such assistance to 

 birds in performing their annual flights. In connection with this 

 point, Herr Gatke remarks (p. 47) : " Probably owing 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 power of their instruments of flight almost exclu- 

 sively to the execution of their forward movements. This results 

 partly from the fact that by the filling of the air-sacs the volume 



