ON THE MIGRATION OF BIRDS. 243 



"miracles." They did not and still refuse to admit that it is 

 our knowledge which is at fault, a position not conducive to 

 experiment, leading to the declaration of judgments which are 

 void of positive foundation, and of any actual inductive basis. 



One of the most wonderful pictures of human intellect is 

 displayed to our eyes when we learn that the great German 

 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 the 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 temperature and food supply ; 

 that they meet before commencing migration ; that land-birds 

 move in a certain order, in two convergent lines — e. g. Cranes — 

 even 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 "rid- 

 dles," but only natural things in mind. 



But as in other fields, this period is followed by a time of 

 decadence, a natural consequence of departing from immediate 

 experience : the characteristic of this age of decadence is specula- 

 tion, which sought contributions from old authorities, and the 

 axioms established by them. 



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

 appurtenances ; the belief that the birds of passage do not leave 

 us at all, but spend the winter sleeping either at the bottom 

 of some water or in caverns or hollow trees. Even serious 

 natural scientists of the eighteenth century, as Geoffroy de St. 

 Hilaire (1772), still believed in winter sleep, and, indeed, pre- 

 tends to have seen with his own eyes sleeping Swallows, certainly 

 Bats. Your great countryman, E. Jenner, the benefactor of 



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