THE CAUSE OF THE MIGRATORY MOVEMENT 147 



a second time to a danger which it had just managed to over- 

 come. Indeed, Palme'n too, although himself a supporter of the 

 theory of heredity, says on p. 269 of his book that birds which 

 have on some one previous occasion successfully got over an erratic 

 flight of this kind, during which they have endured hardships, 

 would hardly forget this experience, and therefore avoid it in 

 future. 



Erratic flights of this kind might possibly take place abundantly 

 every year ; but on account of the discomforts which accompany 

 them, it is by no means likely that they would be repeated by 

 particular individuals, so that it is difficult to see how flights of 

 this nature could ever pass into a habit. In fact it is highly 

 questionable whether an act performed but once in the course of 

 each year could ever become habitual at all, and if not, the 

 assumption of its hereditary transmission must also of necessity be 

 dismissed as untenable. 



In regard to both of these hypotheses, we would once more call 

 attention to what we have said in the preceding chapter in reference 

 to the migrations of certain nocturnal Lepidoptera, viz. that these 

 insects undertake but one migration in the course of their lives, 

 and die after it is accomplished, without producing offspring, 

 to which they might impart, either by hereditary transmission or 

 tradition, any of their migratory experiences. Notwithstanding 

 this, each successive generation performs its migrations with the 

 same unerring regularity and completeness. 



In unison with the above-named theories, it has further been 

 assumed that the capacity of flight of birds, which originally was 

 exercised within the modest limits of short daily excursions in 

 quest of food, attained its present astonishing development in con- 

 sequence of repeated migration flights. We are however equally 

 unable to accept this view, for it is impossible that a migratory act 

 of a transitory nature, and only repeated twice in the course of a 

 year, should so affect the whole organisation of a bird, increasing 

 its muscular powers to such an extent as to render it capable of 

 performing feats like that, for instance, of the Bluethroat, which 

 accomplishes its spring passage of sixteen hundred miles, from 

 Africa to Heligoland, in one uninterrupted flight ; or the still more 

 wonderful autumn migration of the Virginian Plover over a distance 

 of three thousand two hundred miles, executed without a stoppage, 

 from Labrador to North Brazil. No doubt, by continuous practice 

 day after day, the strength of the muscles, and their powers of 

 endurance, may be increased up to a certain degree, but transitory 

 efforts brought into play but once in every six months cannot per- 

 manently affect the organisation of an animal in this manner. 



