380 REPOKT ON THE MIGRATION OF BIRDS. 



the antlior of the "migrations". That conception has at all events its 

 authority as the personal opinion of the renowned author. It could 

 however be fully accepted only in case it. were founded upon a material 

 of communicated focts, and conclusions had been reached from these 

 in a strictly logical and conscientious manner. Because, however, these 

 conditions were wanting, a purely scientific examination of the views 

 given could be made only with difficulty or not at all. The discussion 

 could only take the form of a personal defense, wliich could at all events 

 somewhat clear up the question, dulled by the manner of the attack, but 

 could contribute little to the further elucidation of the problem itself. 



A little attention will suffice to show that the writer had neither 

 subscribed to narrow ''linear'' routes for "birds" in general, nor the 

 existence alone of coast and river ways. He maintained above all that 

 the individual species of birds do not move irregularly, and also that 

 each species is not obliged to follow a fixed direction in the sky. They 

 act according to the local and continental relations of the ground, and, 

 besides, each species according to its own nature. 



There are then also species whose routes go through narrow plains, 

 and which spread out here as far as the space permits. The migration 

 routes likewise often lead through regions where these species find 

 am])le scope and can choose suitable roads within wider l»ounds. The 

 migration lines of individuals and bauds group themselves then in 

 bundles, which, according to circumstances, branch out and run to- 

 gether again, and the individuals do not here confine themselves so 

 strictly to fixed places, as in the narrow bounded sections of the route. 

 In such wider roads, as may readily be conceived, a wider front is 

 also developed. Even our opponents seem to allow that here also cer- 

 tain obstacles situated on the flanks prevent the unlimited extension 

 of the individual routes. How many species of birds exist, which are 

 not at all dependent on the conformation of the ground and on the 

 resting places, can not be decided beforehand. 



Every species, every variety, and every form ditfering in the slight- 

 est respect must therefore, if possible, be treated separately. A com- 

 plex of species are to be examined together, which in the migration 

 agree somewhat geographically. Thus in certain respects a want of 

 material in some species can be to some extent supplied by analogies in 

 others. Accordingly an average treatment of heterogeneous species can 

 not be justified in a special scientific investigation of n)igration, and in 

 a region where the most dissimilar species are known to follow one and 

 the same direction or road, this surely has its cause in topographical 

 conditions which perhaps lie further and are only perceptible in the 

 whole mass. The importance of such highways to the problem of mi- 

 gration was discussed above. 



A "universal direction of migration," which perhaps all the sjjecies 

 of birds in the expanse of whole continents should follow, does not, 

 therefore, correspond to a well founded conception of the phenomenon 

 of migration. It is likewise evident that in aH exact scientific discus- 



