THEIR RELATION TO WEATHER 141 



succeeded in establishing itself in a faunal region defi- 

 nitely varying from that in which it started; but there 

 are indications also that it has reached its limit. 



A yet more striking case is found in the migration 

 of the cotton -boll weevil, a species indigenous south of 

 the line between Mexico and the United States, which 

 began its invasion into our territory somewhere about 

 1893 and has been annually extending its range through 

 the cotton-growing states since that time. So great a 

 variety of climatic conditions is now represented in 

 its distribution, that there seems to be no reason to 

 believe that its spread will be checked until it has 

 reached the faunal limits of its food plant; although 

 that period may be materially retarded by the quaran- 

 tine and other regulations now adopted by the cotton- 

 growing states. 



These are instances of natural spread from one 

 faunal region to one adjacent thereto, along the line 

 of the food plant. There are other cases where insects 

 have been accidentally introduced on trees, shrubs or 

 plants from one country to another, into which it could 

 not have spread naturally and where the climatic con- 

 ditions in the new home suited the species so much better 

 that injury became much more severe as specimens 

 became more numerous. An example of this character 

 is found in the recent introduction of the "white-fly" 

 of the Citrus from Florida to California, where the dry 

 climate exempts the insect from certain disease and 

 other checks favored by the more humid climate of the 

 eastern country. 



Leaving aside for the moment cases where migra- 

 tions have been from south to north when, under favor- 

 ing conditions insects from mild temperature regions 

 extended into normally more rigorous climates, it 

 sometimes happens that conditions reverse, and in- 



