126 Taverxer, The Origin of Migration. LJuly 



effected. One is the sudden irruption of a species, when it sud- 

 denly appears in numbers in a territory where it had been either 

 extremely fare, or entirely absent. Examples of this are to be 

 seen in the sudden occurrence of the Sand Grouse in Europe in 

 1888; the appearance of great flocks of Briinnich's Murre on 

 Lakes Ontario and Erie, 1894-97, and the great movements 

 occasionally noted in Lemmings. Of the underlying causes of 

 these strange migrations, whether they are due to inner psycho- 

 logical or outer physical phenomena, we are ignorant. These 

 strange overflows seem so erratic and abnormal in the light that 

 invasions of this kind do not succeed in forming permanent settle- 

 ments on the new grounds, that it would be reckless at present, to 

 use them as a basis for theorizing, until all other means fail. 



The second method is by a force exerted from within an estab- 

 lished range ; and the third, an attractive one acting from with- 

 out. These two, however antagonistic as they may superficially 

 seem, are, at root, one and the same. They are both caused by 

 differences in the desirability of two stations. One is caused by 

 a decrease in the desirability of a present, and the other by an 

 increase of the same quality in an adjoining territory. They are 

 but ratios of desirability, and can both be expressed by fractions 

 whose values depend upon the relative, not the numerical size of 

 their terms. If, then, attraction is but a phase of driving, and 

 birds cannot be driven from their haunts, we are forced to discard 

 all our present theories of geographical distribution and return to 

 that of special creation, or found our science upon the unknown 

 quantities of general irruption probably caused by psychological 

 disturbances of whose origin and intent we are ignorant. 



Ranges can be, have been, and in the course of time, must many 

 times have been, changed by necessity when the changes in con- 

 ditions occur slowly enough so that, though individuals may not, 

 the whole species might have advanced or retreated. In this 

 same manner, we know that even our forests have migrated back 

 and forth across the continent before the face of the glacial ice, 

 climbed mountains and descended valleys, though each individual 

 tree or plant remained rooted for life to the spot where it origi- 

 nally sprouted. If plants can and have done this, I see no reason 

 why birds could not also, as even in the most extreme case of local 



