INTRODUCTION 15 



have a deadly enemy in the field-mice, which 

 destroy, it has been computed, no less than two- 

 thirds of their nests and combs. The mice in their 

 turn are destroyed by cats, Owls, Kestrels ; so that 

 in localities where the enemies of mice are common 

 the bees have more chance of multiplying, and the 

 flowers a correspondingly greater facility for 

 fertilisation. The abundance of clover in a district 

 may therefore depend upon the number of cats, 

 of Owls and Kestrels ! Take another instance. 

 Darwin has recorded some very curious effects 

 produced by the planting of several hundred acres 

 of Scotch fir on a large heath in Staffordshire. In 

 a quarter of a century the change produced in the 

 vegetation was very remarkable, plants having 

 appeared or disappeared in obedience to the altered 

 conditions, whilst many other organisms were un- 

 doubtedly similarly affected. One more instance 

 must suffice, and this we may quote from Darwin's 

 great work on The Origin of Species : " In several 

 parts of the world insects determine the existence 

 of cattle. Perhaps Paraguay offers the most 

 curious instance of this, for here neither cattle 

 nor horses nor dogs have ever run wild, though 

 they swarm southward and northward in a feral 

 state ; and Azara and Rengger have shown that this 



