212 BUTTERFLIES 



one hesitates to put them, as to their cause, in the 

 same category as the seasonal movements of birds, 

 fishes and insects. One can only suppose that these 

 dragon-flies have a sense of the coming atmospheric 

 change ; that it is an impulse more sudden and violent 

 than that of migration, and inspires them with a 

 greater terror, and sends them flying hundreds of 

 miles over a parched waterless region at such a 

 speed that they are able to keep ahead of a wind 

 blowing as a rule at a rate of about seventy miles 

 an hour. 



Twice on the pampas I witnessed a great butterfly 

 migration : on both occasions it was the same insect, 

 a species of Vanessa, resembling our large tortoise- 

 shell, and the commonest as well as the hardiest of 

 all our butterflies. Both of these migrations occurred 

 in spring, about the middle of September, and the 

 direction was the same as with the birds arriving 

 and passing on to the south. They did not migrate 

 in clouds or masses, as in many other instances of 

 butterfly migration on record; like that one, for 

 instance, described by Darwin when the Beagle, off 

 the Patagonian coast, was in a cloud of white butter- 

 flies, so that the sailors cried out that it was 

 " snowing butterflies." 



The red Vanessa butterflies travelled close to the 

 surface, singly or in twos and threes together, passing 

 at intervals of a second or two, so that it was easy to 

 count them as they flew by. On the occasion of the 

 second migration I marked a space of a few feet, 

 staked at the sides, and counted all those passing 



