PREFACE. 



To make a dry subject entertaining, is impossible ; 

 but to render it, at any rate, readable, has been 

 my endeavour in the following pages. How far I 

 have succeeded in the experiment, it is not for me 

 to decide. 



It having been suggested, by several of my 

 friends, that it might be desirable to bring together 

 into a small compass some of the evidence on 

 Insect variation (with reference to external disturb- 

 ing causes) which my researches in the Madeira 

 Islands have supplied me with, I have been en- 

 couraged to do so : and I have added numerous 

 conclusions from other data also, which have from 

 time to time fallen in my way, so as to confer 



