LOCUSTS AND GRASSHOPPERS (ACRIDIIDM}. 113 



There are Many Causes of Migration. 



The explanation of migration to be gathered from 

 the preceding remarks, is excessive multiplication. As 

 a single explanation of the phenomenon no better can 

 be offered ; for this is evidently the immediate or 

 special cause, and the others are mostly secondary, 

 or but consequences of this one ; for these move- 

 ments cannot truly be ascribed to any single cause. 

 Moreover, we come to recognize the fact that the 

 several influences bearing on migration fall into two 

 distinct categories, viz. immediate or special, and remote 

 or general. That certain climatic conditions, those of 

 heat and more than ordinary dryness, are in some way 

 necessary, or most favourable to the excessive increase, 

 and the origin of the migratory habit or instinct, may 

 be maintained. Wherever locusts are largely and 

 frequently developed, whether in the Eastern or Western 

 Continent, we find either extensive deserts or vast 

 treeless areas, and a climate arid or dry. These con- 

 ditions are believed to develop irritating or uneasy 

 sensations, which cause migration. But, plainly, there 

 are other exciting causes than the impulse simply to fly. 

 The immediate cause, increase, has been already con- 

 sidered. Annoyance from natural enemies probably often 

 proves a valid cause, as no one who has witnessed the 



