378 REPORT ON THE MIGRATION OF BIRDS. 



the iudividual voices. The wliole thing must therein »re api)ear to us 

 as a complication, though a very interesting- one. 



Here also, then, an average answer is of no use, but the whole must 

 be closely analyzed. This however can not be attained by study of 

 the general migration. Just as a tangle of threads can only be loosened 

 by getting out the single threads, so out of the net-work of migratory 

 routes, that is out of the "highway," the separate road of each species 

 of bird must be picked out and followed. And this work pre-supposes 

 the knowledge of the last named, which however can only be acquired 

 where the migration assumes a simpler aspect. 



The existence of narrowly limited roads, ramilied in a characteristic 

 manner for each species, was felt beforehand by zoologists avIio, in their 

 investigations, depended upon the less productive climatological meth- 

 ods. Thus in admirable manner von Middendorff compared the routes 

 of sejiarate species of birds with the different forms of shrubs and trees 

 which were drawn on the paper. The leaves correspond to the nesting- 

 places, the branches are to compare with the routes, and the roots with 

 the winter quarters. On the map the shrubs would bear their foliage 

 not far from the roots drawn somewhat more to the south ; the trees — 

 likewise ramified, but of lofty growth — would take a higher place; and, 

 uuramified, some gigantic stalks would jut out, which on the map take 

 root in Egypt, but with their branches shade the coast regions of the 

 frozen sea, beyond the tree limits. 



With the indispensably necessary geographical determination of the 

 routes of separate species of birds, all reliable local faunas, as well as 

 smaller lists, must first of all be used as material for investigation, and 

 moreover, all notices of isolated discoveries of a species of bird in places 

 which the migration jjasses. Such isolated notices and minor works 

 are indeed the extreme roots of the inductive zoo-geographic inquiry. 

 Out of the entire ornithological literature therefore a comparison is to 

 be made of all accounts of the appearance of a species of bird. Coun- 

 try after country must make its contribution, and for each district a 

 separate conclusion must be drawn for the species in question, and the 

 result added to that of the next district. 



The writer's investigation of the migration of some birds breeding in 

 the Arctic regions, carried out on these principles, yielded certain re- 

 sults which need not be here considered in detail, but it may perhaps 

 be well to note some of the main points. 



During the migration, these birds do not by any means take any di- 

 rection they please, nor do they follow everywhere one and the same 

 direction, perhaps a "general migratory route." They rather follow 

 well defined, geographically bounded ways, whose bends depend pri 

 marily ujion the topographical relations of the regions. Other not more 

 accurately examined species of birds also act in the same or an analo- 

 gous manner; since their migrations, taking place almost everywhere, 

 are to be traced to extremely ramified routes. The routes of migration 

 may be grouped into several categories accordiiig to their character. 



