70 



§ IV. Isolation ; and exposure to a stormy atmosphere. 



Having in the preceding pages touched upon the sub- 

 ject of insect variability, as the occasional result, to a 

 greater or less extent, of climatal and other iafluences ; 

 let us now proceed to consider the importance of a 

 certain physical condition, which mil be found, I believe, 

 on inquiry, to be accompanied by a more decided modi- 

 fying power than any which we have yet discussed. 



Every one who has examined the natural history of 

 islands, both in theory and practice, must be aware of 

 the many difficidties which have constantly to be en- 

 countered, before the several phsenomena can be satis- 

 factorily explaiaed. Laying aside those forms which 

 are manifestly endemic (the numerical proportion of 

 which usually accords with the distance from the nearest 

 mainland), again and again are we baffled by the near 

 resemblance of the various creatures to continental 

 types, — whilst the minute differences which they display, 

 |from them, are at the same time so permanently fixed, 

 ;that we are almost precluded, under the ordinary ac- 

 ceptation of a " species," from regarding the two as un- 

 doubted descendants of a common stock : and thus it is 

 that insular faunas have frequently been magnified, in 

 the novelties which they are supposed to contain, far 

 beyond what is right. A person however who looks to 

 the causes of things, and is prepared to recognize effects 

 where there are fair grounds for anticipating them, will 

 not be slow to perceive, that, in the small deviations 



