the northernmost point made an erroneous observation, because 

 April 8 th is too early for Turdossin and if \ve substitute here the 

 date of the adjacent Zuberecz - - April 15 th - -'we shall find April 5'" 

 to be the mean figure in this case also. Consequently this method 

 may be used for the perception of errors also. 



This proves the great influence of height and geographical 

 latitude of the place or area of observation, as well as the fact 

 that it is not the power of flight that is decisive in migration, but 

 other factors, viz. as this method has already proved, meteorological 

 conditions and biological reasons, above all sexual impulse. 



The difference between 1890 and 1891 has already received 

 a meteorological explanation.* 



By the aid of this method 1 undertook to determine, - - as far 

 as the data at my disposal concerning the migration of the Hirundo 

 rustica would allow, - - at least the outlines of the interzonal features 

 of the migration in question. 



I began with Knysna in Africa, 320' S. L. and went as far as 

 Spitzbergen in Europe, 800'N. L. and the part extending East as 

 far as Kamtchatka. 



Knysna, ace. to Catalogue of Brit. Mus. . . . 2 nd January 

 Quilimane, 170'S. L. ace. to Reichenow . . 19 th 

 Victoria Nyanza 00' . . 2 nd March 



For Europe the earliest arrival was : 



Murcia, 380' N. L 15 th February 



Italy, mean date ace. to GIGLIOLIS' indications . 14 th March 

 Germany, mean date out of means .... 16 th 17 th April 



England, mean date 24 th 25 th April 



Lulea, mean date based on records of 21 years . 26- 27 May 

 Western Finland, ace. to Middendorff. . . .11 th June** 



From the acceptable data Murcia-Lulea it follows that the 

 chimney-swallow needs on an average 92 days to settle in this area,* 1 

 whereas, considering the velocity of her flight - - from 20, to 65 

 metres a second - - she should require less than 92 hours to cover 

 this distance : a direct proof that the progression is dependent upon 

 another factor, viz. the course of the isotherms, as HEGYFOKY later 



* Vide O. Herman: ,,Die Klemente des Vogelzuges in Ungarn etc. 1895". 

 *" For the whole series see ,Aquila" I. 1894. p. 1219. 



*** Rectified ultimately on the base of the observations of Gibraltar to 105 days; vide 

 ,Recensio" p. 21. 



