62 THE BIRDS OF HELIGOLAND 



poverty in oxygen, or the extremely low temperature which 

 prevails in those regions. If they experienced eyeu the least 

 discomfort during such flights — which frequently, as in the case 

 of the Condor, appear to be undertaken for mere pastime — they 

 would either abstain from them altogether, or, at any rate, 

 not extend them over such ample periods of time as they 

 actually do. 



Man is prompted by the thirst for knowledge to penetrate into 

 regions for which his physical adaptation, however much he may 

 be able to extend it, is no longer sufficient. Other beings, whose 

 whole life and actions are directed to the maintenance of the 

 individual and the species, are endowed with an organisation 

 corresponding to the simple purposes of their being and the vital 

 activities connected therewith, and each makes the most effective 

 use of the qualities and capacities with which it has been thus 

 provided. In the case of almost all, however, life ceases to be 

 possible in and beyond the domain of eternal snow. The one 

 exception to this rule we find, as already stated, in the case of birds. 

 They too, however, would not be able to nourish and propagate 

 themselves in regions bound in an unchanging state of icy frigidity ; 

 and hence, in their case, an additional condition of existence — the 

 migration flight — is brought into play. We have attempted in the 

 preceding pages to show that this flight proceeds at heights 

 raised far above our perceptive faculties, and it remains for 

 us to inquire what is the special purpose of this exceptional 

 phenomenon. 



In spite of isolated exceptions appearing to conti-adict our con- 

 clusions, this main purpose is, in the rirst place, to enable migrants 

 to elevate themselves to such strata of the air as, for the time being, 

 ofl'er them the most favourable conditions for their migration, and 

 hence make them independent of the numerous meteorological 

 disturbances prevalent, especially during the autumn months, in 

 strata of the atmosphere nearer to the earth's surface, and Avhich 

 might have the eftect of temporarily suspending for a considerable 

 period, or even entirely preventing, the migration of a species for 

 the time being. In the second place, the incomprehensible rapidity 

 of the migration flight, developed by so many species during their 

 long and uninterrupted journeys, and which they are obliged to 

 develop in crossing wide oceans, can probably onl}- be attained at 

 elevations where the atmosphere, by reason of its diminished 

 density, offers a considerably slighter resistance to their progress. 

 Undoubtedly with this wonderful phenomenon there are connected 

 many other physical questions, the settlement of which may 

 probably, for a long time yet, defy the most earnest investigation. 



