XXXVI INTRODUCTION. 



When we alter the soil of a country by agricul- 

 tural operations, plants will follow, of their own 

 accord, the progress of man's improvement; and 

 wherever plants are introduced, animals are certain 

 to find their way thither. It would be difficult to 

 give a satisfactory account how this takes place. 

 Let brassicas be introduced into the most remote 

 valleys, which were formerly the receptacles for dry 

 heaths and furze, if they increase to any extent, 

 caterpillars will certainly be found in them; if 

 nettles are by any means introduced, the beautiful 

 butterflies which feed on them are sure to be found 

 there ; and as these again increase, insectivorous birds 

 will become resident on the spot. In confirmation of 

 this fact, Mr Loudon says, " Having made some oak 

 plantations, though only on a small scale, near my 

 residence, I have occasionally found therein Thecla 

 quercus, (Purple hair-streak Butterfly,) smdMilitcea 

 euphrosi/ne, (Pearl-bordered Fritillary,) insects which 

 previously had never been seen within some miles 

 of the spot. I have seldom planted the Athenian 

 poplar without finding it taken possession of by Sme- 

 rinthus populi, (Poplar Hawk Moth,) and Cerura 

 pinula, (Puss-Moth,) and by other less common 

 Phalanidce. The copious growth of broom in our 

 plantations induced, for several seasons, the appear- 

 ance of Phalcena spartiata, (the Broom Moth, 



