107 



Seebohm's observations in the Yenesay, some four hundred miles 

 from its mouth, form undoubted proof of this fact. As he awaited 

 the break up of the frost at a post on the Arctic circle, he notes 

 that prior to the melting of the ice on that great river few birds 

 arrived. As soon, however, as this commenced migrants in small 

 numbers immediately appeared. Some too eager to advance had 

 to retrace their flight. We cannot think that any of these, in 

 their far-off winter quarters were aware of the state of the 

 weather some thousand or fifteen hundred miles to the north. 

 Undoubtedly they migrated to the verge of the frost at some 

 much earlier period, and had followed its daily retreat to the 

 north until his post of observation was reached, their journey being 

 governed by the break up of the ice, which in Mr. Seebohm's 

 opinion takes place at the rate of one hundred miles a day. 



The intrinsic weakness of Herr Gatke's theories on the 

 velocities maintained by birds during their migratory flights will 

 be apparent, when he has to call in their aid : — " The assistance of 

 other factors besides the mechanical instruments of motion with 

 which the bird is equipped " (p. 72), to make these accompUsh- 

 ments possible. His concluding words to this chapter are peculiar 

 and significant. They read as follows :— " In treating of the height 

 of the migration flight, we have considered in detail that birds, as 

 distinct from other warm-blooded creatures, are provided with a 

 respiratory mechanism enabling them to remain for any desirable 

 length of time in regions of the atmosphere so rare in density 

 and poor in oxygen, as must necessarily result from elevations 

 extending to 40,000 feet ; and we have further seen that they are, 

 in addition, provided with a very extensive system of air-sacs 

 which they are able to fill or empty at pleasure. . . . Their 

 sole purpose, therefore, is evidently to enable them to perform those 

 wonderful migrations — wonderful, both as regards the height at 

 which they proceed, and the velocity with which they are carried 

 out. If birds were restricted during their autumn and spring 

 migrations to the same low strata of the atmosphere in which 

 they move during the rest of the year, such of them as have to 

 perform their migratory journeys early in the spring or late in the 

 autumn would, in many cases, be obliged, in consequence of stress 

 of weather, to let the proper period of their migrations pass with- 



