376 REPORT ON THE MIGRATION OF BIRDS. 



When the new impetus on the side of physicists and climatologists 

 became felt, the inquiry assumed adequate form. The arrival data 

 were considered like other climatological observations, and treated in 

 the same manner as the notices on temperature, air pressure, etc., with 

 the view of finding an expression for the climate of the corresponding 

 regions. The bird of passage was regarded as a complicated ajiparatus, 

 by the observation of which the climatological problems could be ex- 

 plained in certain relations. 



Zo(>logists however could not permit the bird and its migrations to 

 be regarded merely as a means to serve other purposes, but both con- 

 stituted independent problems in themselves, dependent only in certain 

 respects upon climate, etc. It was asked therefore how the question 

 was to be examined in this regard; and, as may readily be conceived, 

 methods were for the time being suggested which the climatologists 

 had already perfected. 



Consequently the "climatological" material of data about the arrival 

 and departure of the birds assumed the character of an avi-phenologi- 

 cal material. By its proper treatment, it was desired to fix the course of 

 the migration of the species of birds; and with this view first of all an 

 efibrt was made to determine its tiuie, in order to judge first by that of 

 the direction of the migration. Two means were now available: 



(1) Either data of arrival from one year only were to be used, because 

 only such can be compared with one another (Kessler, 1852, South 

 Eussia) ; 



(2) Or from as many years as possible, in order that the errors of 

 observation, unavoidable in any case, could be mutually corrected. 

 (V. Midden dorft', 1855, Russian Empire.) 



Both conditions were very difticult to fulfill, the former however, 

 easier than the latter. 



At all events, both methods offered a good view of the gradual prog- 

 ress of the bird species, as heretofore of that of the increase of tem- 

 perature and similar climatological i)henomena, but hardly anything 

 more. The conclusion that the migration takes place at right angles to 

 the isopipteses seemed justified; but, with regard to the direction of 

 the bird's migration to be ascertained, could not possibly prove more 

 exact than the premises themselves, the methods of inquiry, and the 

 material. 



As a reminder of the '-climatological" conception of the x)roblem 

 and of the citrresponding methods, something medium, something of 

 arithmetical mean values had to attach to the result, which however 

 did not exist in the phenomenon itself 



For the migration of birds is by no means a meteorological but a 

 biological phenomenon. The (piestion is about the advance of living 

 individuals, who move according to their own will. Their paths must 

 be investigated, and tiie result of this inquiry must be given much 

 more exactlv than the advance of the spring warmth. For, since the 



