330 Recent Literature. [f^^ 



registry of distance and direction by means of the semicircular canals. 

 It is simply a case of a home-loAdng animal away from home putting its 

 wits and senses and experiences together to get back to its home and in this 

 case these known resources are sufficient for the task. Why may not this 

 also," he reasonably asks, " be the true explanation of the manner in which 

 birds find their way on those greater pilgrimages we call migration?" 



The " foUow-the-leader theor\'" is accorded "a large element of proba- 

 bihty," for which he argues at considerable length. Thus he concedes 

 that "it seems reasonable to believe that the manner in ■n-hich it [migration] 

 is carried out, the Avay in which the path is followed, may find an adequate 

 explanation in the temporary leadership of some individual -n-ithin sight or 

 hearing of the others, who knows at least a fraction of the way by experi- 

 ence or who strikes out a safe path by means of landmarks." 



Under the second question "Why do Birds migrate?" various hypo- 

 theses are weighed, only to be found wanting. Decrease of temperatiire 

 does not satisfactorily explain fall migration, because "the fall migration 

 is largely completed before the weather becomes cold"; but scarcity of 

 food, dependent upon the low temperature of winter, is admittedly an im- 

 portant factor. 



The "premonition theory" of Brehm, which, he says, "at first thought 

 seems entirely fanciful," in reahty " contains a large element of probability," 

 since by \'irtue of their peculiar anatomical structure — large Imigs, pneu- 

 matic bones, and internal air sacs — they "are, to a remarkable degree, 

 li-\dng barometers, responding with great delicacy to changes in barometric 

 pressure." Yet "that birds can anticipate winter and as a result make 

 an effort to avoid its disastrous effects, is beyond demonstration and seems 

 quite unlikely." 



The "short day theorj^" also "suffers, as does many another, because of 

 a few obtrusive incontrovertible facts," as "the migration south begins 

 before the days are perceptibly shorter." In reality, however, this is 

 hardly true, even in the far north, whatever there may be in the theory. 



In this connection he again recurs to the " food supply theory," to which 

 he objects on the ground that "it must be admitted that a large per cent, 

 of migrating species leave for the south in the very height of the seed and 

 insect harvest." Yet, he adds, "upon the gromid of food supply, natural 

 selection would promptly eliminate those who did not go south and would 

 tend at the same time to favor the perpetuation of those who varied in 

 the direction of southern migrators' habits, whatever the cause of those 

 variations might be." What he intends the reader to infer from these 

 statements as to his position on the " food supply theory" is not very clear. 



The author now proceeds to consider the theories to account for the 

 spring migration, giving attention first to "the instinct theory," of which 

 he says: "That it is a bird's instinct to go north in the spring is no better 

 an explanation of the origin of migration than it is of how a bird finds its 

 way during migration" — a truism no one will question. Then are taken 



