INTRODUCTION. 



The manifestations of life in the organic connexion with animals 

 and in the influence on Nature, grouped in reference to time, place and 

 elevation, are inexhaustible, and form the „daily bread" of the very 

 greatest investigator, just as of him to whom only the Phenomena of 

 a very small district are assigned. 



But also within the parts into which we are obliged to divide 

 the question, there are problems which, only solvable by experience, 

 demand a methodical partition of work and consequently uniform 

 methodical treatment, since there is no other way of approaching the 

 essence of the problem. 



If we consider Birds from the point of view just expounded, 

 we can ask the question : Do Birds, compared with other animals, 

 either in respect to bodily structure or manifestations of life, or both 

 of these, possess peculiarities, which can form the object of special 

 research, being specially characteristic of the Bird and requiring special 

 treatment ? 



Among Birds, we do find such a peculiarity in the fact that a con- 

 siderable number of species, in accordance with the change of seasons, 

 undertake regular Migrations. They migrate from one zone to another 

 and thereby periodically avoid those influences of certain districts, which 

 are unfavourable to their propagation. Hence it follows that these Mig- 

 rations, as we understand them, signify a number of preliminary con- 

 ditions, which seem to go beyond the idea „animal", and even touch 

 upon the „mysterious" and „incomprehensible". 



For it is not the power of flight which, from an ornithological 

 point of view, needs special treatment, but the vast regular Migratory 

 Movement — Passage — through the zones, which from year to year 

 is regularly performed, and is repeated not only by the species, but 

 by the individual to the end of life. 



It is the deeper biological comprehension of this special Phenomenon 

 which in its course can only by a methodical distribution of labour, 

 by a uniform and strictly methodical treatment be brought closer to a 



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