On the Migration of Birds. 165 



Emperor, Frederick II. (1194 — 1250), as auceps — i.e., 

 fowler and falconer — was induced to have recourse to the 

 method of experience and to follow Bacon's precepts long 

 before Bacon lived, and that his views on the migration of 

 birds were far better and more valid than those of many a 

 famous author of the nineteenth century. The Emperor 

 knew that birds pass from colder regions to warmer ones, 

 and vice versa ; that not all birds are birds of passage ; that 

 some of them pass only from the mountains to the valleys, 

 and vice versa ; that the movement is dependent upon tem- 

 perature and food supply ; that they meet before commencing 

 migration ; that land birds move in a certain order, in two 

 convergent lines — e.g., cranes — evon that the bird at the 

 head of the flight has to accomplish the hardest work and is 

 therefore relieved : all absolutely correct opinions. It is a 

 thousand pities that the concluding part of the manuscript 

 is lost, and that we are therefore not in a position to know 

 where, in the opinion of the Emperor Frederick, the birds 

 actually do pass ; but the words " warmer regions " are 

 sufficient to prove that the Emperor had no " riddles," but 

 only natural things in mind. 



But in this field, as in others, the above period was followed 

 by a time of decadence, a natural consequence of departing 

 from immediate experience : the characteristic of this age of 

 decadence was speculation, which laid under contribution old 

 authorities and the axioms established by them. 



Then arose the so-called immersion-theory with all its 

 appurtenances ; the belief that birds of passage do not 

 leave us at all, but spend the winter sleeping partly at the 

 bottom of some water, partly in caverns or hollow trees. 

 Even serious natural scientists of the eighteenth century, as 

 Geoffroy de St. Hilaire — 1772 — still believed in winter-sleep; 

 this writer, indeed, pretended to have seen with his own eyes 

 sleeping swallows, which must certainly have been bats. Your 

 great countryman, E. Jenner, the benefactor of mankind, had 

 to protest against this theory, as he did in 1824 ; yet this 

 misbelief has nevertheless prevailed in more or less obscure 

 writings until the present day. 



Even in the high-class works of famous authors of our 

 time, echoes of this mediseval belief in miracles are still 



