46 



for some reeison or other, in consequence of the temperature f. i., the enemies 

 of this species which threaten its larval life only occur in small numbers and 

 for this reason have not limited its multiplication. 



Now if this is observed on the same day, or on succeeding days, in 

 different places, it may easily give rise to the opinion that these butterflies 

 make long journeys, the more so, as sometimes a strong wind, which comes 

 up suddenly, can drive together butterflies flying in this manner, and carry 

 them far away, even across the sea. Turr also reproaches me with having 

 pretended that migrating butterflies cannot fly against the wind ; I meant a 

 strong wind. From my own notes it is apparent that they fly in the direction 

 of the wind as well as against it, but only if it be not too strong. They do 

 not fly on those days, just as litde as on rainy days; though at Batavia I 

 saw their swarms daily in November and December 1883, I did not notice 

 anything on such days. Against a strong wind no butterfly can make way, I have 

 often observed this, and if such a gust of wind attacks them from behind or 

 at the side when a lot of them are just flying about, it blows them together 

 in thick swarms and carries them far away, even across the sea, where they 

 generally perish, if they do not chance to be carried in this way to some 

 other countr}'. On the English and on the Dutch coasts this has often been 

 observed. In the same way individual butterflies are often carried away by 

 the wind to ships which are at a good distance from the land. And the proof 

 that this happens against their will is found in the circumstance which has 

 been stated more than once, that such butterflies arrive quite exhausted and 

 often all sit down at once on the shore or on the dunes to take a rest, as 

 well as the fact that such swarms as have come across the sea, not only 

 consist of one kind of insects, but of butterflies, dragon-flies, flies, Sphex wasps 

 together, insects which are not in the habit of flying together and which have 

 therefore evidently got mixed up together by the wind. 



What influence it is, that brings this about, is not yet known ; considering 

 it in connection with other observations about butterflies, whether they fly more 

 or less or at certain times, I think it probable that this influence is of a mete- 

 orological character, and that therefore, further researches on this subject will 

 particularly have to take account with the state of the barometer before, during 

 and after this phenomenon. 



This opinion which I have developed in detail in the latter of my two 

 above-mentioned essays, has since been supported by the essay of Prof Karl 

 Sajo, already mentioned, in which he also points out the signifiance of mete- 

 orological influences ; but especially by a remark of Gatke who, in consequence 

 of his observations about the migration of birds and insects made in Heligoland, 



