GENERAL FEATURES 37 



mal ; the dark spots and lines will be broader and more dense or 

 dusky, and the lighter ones will be sordid or dusky. But no ex- 

 cessive variation or deviation from the plan of the normal form 

 has ever been noticed, although the effort has many and many 

 times been made to create new forms by this method. 



So, also, by cold. Caterpillars raised in an ice-house, or in cold- 

 storage, where the temperature is kept down as low as possible, 

 will never develop any radical variation, but the butterflies will be 

 darker than they would have been if raised in a normal temper- 

 ature. 



§ 38. Migration of Butterflies. 



At times a so-called migration of a certain butterfly occurs, 

 when, for days together the air will be filled with millions of in- 

 dividuals of one species, all flying in the same direction, and ap- 

 parently never stopping to rest or to feed on flowers. These mi- 

 grations sometimes extend over a large territory, and continue 

 for a week or more. But little is known about them, really noth- 

 ing, in fact; and as to what the inducement is, or what the gain, 

 nothing is known, or even surmised. The flights are not always 

 in the same direction, though generally to the northward. I have 

 witnessed several such migrations, and all of the butterflies were 

 of the same kind, Pyrameis Cardui. One day, upon the top of 

 Grayback, at the altitude of 11,600 feet I encountered a migration 

 when the direction taken by the butterflies was due east ; they came 

 flying along up the slope of the mountain, a yard or two above 

 the ground, all going at the same rate of speed, say, six or seven 

 miles an hour, and upon reaching the top of the mountain, where 

 the crest dropped suddenly, instead of dropping down upon the 

 precipitous descent on the eastern side of the mountain, as might 

 have been expected, they kept straight on as far as the eye could 

 follow them, right up into the sky, and toward the moon, then in 

 full view in the eastern sky. 



In 1901, in May, a grand migration of Cardui occurred in 

 southern California, which lasted eleven days, and covered the 

 whole southwestern part of the State. In this case the butterflies 

 came up from Lower California, as I learned from correspond- 

 ents in that part of the country, and in the months following, the 

 region of Canada just north of the American line and east of the 

 Rocky Mountains, was reported to be full of Cardui, — that they 



