OF ARTS ANT) SCIENCES. 6 I 



birch tree to feed on the seeds, then whirling away over the 

 mountain. From early September on, the White-wings were 

 present in the river valley in small numbers, but not in the 

 abundance seen on the higher parts of the mountains, above 

 2,500 feet. During the rest of September, both Red and White- 

 winged species were daily seen flying southward down the river 

 valley, but after the 20th I was unable to make further observa- 

 tions on them there. The cause of such an unusual incursion 

 is doubtless more or less complex. One factor may be the food 

 supply, of which there was that year an abundance, since 

 spruce, hemlock and birch trees bore heavily, and apparently 

 with unusual luxuriance. The exceedingly dry spring and 

 summer may have had an indirect influence in producing the 

 large crops of seeds in the case of these trees, for the pollen 

 would have escaped being wetted down, and thus a greater num- 

 ber of the fruiting parts may have been fertilized. 



